Concentrations Of Guanine Nucleotides Research Articles

The Effect of Training on Erythrocyte Energy Status and Plasma Purine Metabolites in Athletes.

This study aimed to assess the changes in red blood cell (RBC) energy status and plasma purine metabolites concentration over a one-year training cycle in endurance-trained (EN; n = 11, 20–26 years), and sprint-trained (SP; n = 11, 20–30 years) competitive athletes in comparison to recreationally-trained individuals (RE; n = 11, 20–26 years). Somatic, physiological, and biochemical variables were measured in four training phases differing in exercise load profile: transition, general, specific, and competition. Significantly highest values of RBC adenylate energy charge (AEC; p ≤ 0.001), ATP-to-ADP and ADP-to-AMP ratios (p ≤ 0.05), and plasma levels of adenosine (Ado; p ≤ 0.05) were noted in the competition phase in the EN and SP, but not in the RE group. Significantly lowest plasma levels of adenosine diphosphate (ADP; p ≤ 0.05), adenosine monophosphate (AMP; p ≤ 0.001), inosine (Ino; p ≤ 0.001), and hypoxanthine (Hx; p ≤ 0.001) accompanied by higher erythrocyte hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity (p ≤ 0.001), were observed in the competition phase in both athletic groups. No significant alterations were found in the erythrocyte concentration of guanine nucleotides in any group. In conclusion, periodized training of competitive athletes’ results in a favorable adaptation of RBC metabolism. The observed changes cover improved RBC energy status (increased AEC and ATP/ADP ratio) and reduced purine loss with more efficient erythrocyte purine pool recovery (increased HGPRT activity and plasma levels of Ado; decreased Hx and Ino concentration).

Anti-Leukemia Effect of FF-10501-01, a Novel Inosine 5'-Monophosphate Dehydrogenase Inhibitor, in Acute Myeloid Leukemia

IntroductionInosine 5'- monophosphate dehydrogenase (IMPDH) plays a critical role in nucleotide synthesis by serving as a rate-limiting step for the de novo production of guanine from its precursors. Overexpression of IMPDH has been observed in both solid and hematologic malignancies. FF-10501-01 is a potent new competitive IMPDH inhibitor. In this study, we systematically investigated the anti-leukemia effect of FF-10501-01 in acute myeloid leukemia (AML) cell lines, including hypomethylating agent (HMA)-resistant derivative cells.MethodsThirteen leukemia cell lines were studied, including 5 parental AML cell lines and their HMA-resistant derivatives (MOLM13, SKM1, HL60, TF1, and U937) as well as three other AML cell lines (KG1, HEL, and OCI-AML3). Cell proliferation was determined using trypan blue analysis. Flow cytometry was performed to detect drug-induced apoptosis and cell cycle status. High-performance liquid chromatography (HPLC) was performed to detect the intracellular concentration of guanine nucleotides, with mycophenolic acid (MPA) treated cells used as positive control. We also studied the effect of guanosine supplementation on FF-10501-01-treated cells.ResultsTo understand whether FF-10501-01 was able to effectively limit AML cell proliferation, we subjected a variety of cell lines to 72 hours of FF-10501-01 treatment at various concentrations. We also included a large number of HMA-resistant cell lines in an effort to understand whether or not FF-10501-01 could be a useful secondary or complimentary treatment to HMA therapy. FF-10501-01 inhibited the proliferation of all 13 AML cell lines studied with 72 hours of treatment. The IC-50 of FF-10501-01 ranged between 4.3 and 144.5 µM. The IC50 values for HMA-resistant cells were all higher than those values in their HMA-sensitive counterparts, except SKM1, in which the HMA-sensitive line had a higher IC50 than the HMA-resistant SKM1 line. To further understand the mechanism by which FF-10501-01 effectively reduced cell numbers in these cell lines, we assessed the level of apoptosis in each line after FF-10501-01 exposure. FF-10501-01-induced apoptosis was observed in all of the studied cell lines in a dose-dependent manner except for the HMA-resistant TF-1 cell line. We then assessed whether FF-10501-01 affected cell cycle progression. This effect was highly variable. Increased numbers of cells in G1 phase and decreased numbers of cells in S phase were observed in MOLM13, SKM1, and TF-1 cell lines treated with less than 100 µM FF-10501-01. To understand the mechanistic effects of FF-10501-01, we performed rescue experiments with both HMA-resistant and HMA-sensitive MOLM13 and HL60 cells. Concurrent treatment with FF-10501-01 and guanosine in these cells partially rescued the antiproliferation effect of FF-10501-01. To further characterize the effect of FF-10501-01 on guanosine metabolism, we then performed HPLC experiments to analyze the levels of phosphoguanosine in treated MOLM13 and SKM1 cells. FF-10501-01 treatment effectively reduced the intracellular phosphoguanosine levels in both cell lines. This effect was seen for GMP, GDP, and GTP. We then sought to assess whether the combination of FF-10501-01 and HMAs could be effective in limiting MOLM13 and HL-60 cell proliferation, especially in their HMA resistant derivatives. The combination of HMA and FF-10501-01 showed little synergy beyond the effects of FF-10501-01 alone, regardless of HMA sensitivity, except in HMA-resistant HL-60 cells, in which FF-10501-01 showed moderate synergy with HMA. We further assessed the antiproliferative effect of FF-10501-01 in bone marrow blast samples taken from 3 AML patients. There was a minor dose-dependent antiproliferation effect seen in these samples, but this was not statistically significant. Notably, one patient showed a sharp increase in cell counts at the lowest concentration of FF-10501-01, but that sample’s cell numbers decreased more rapidly as FF-10501-01 concentration increased. This implies that FF-10501-01 treatment response may be related to cell proliferation.ConclusionsThe IMPDH inhibitor FF-10501-01 can produce potent anti-proliferative and apoptotic induction effects on AML cell lines, including HMA-resistant cell lines, through inhibition of de novo guanine nucleotide synthesis. These results indicate that FF-10501-01 might be a promising new therapeutic agent for AML. DisclosuresParadiso:Strategia Therapeutics, Inc.: Employment. Iwamura:FUJIFILM Corporation: Employment.

Anti-Leukemia Effect of FF-10501-01, a Novel Inosine 5'-Monophosphate Dehydrogenase Inhibitor, in Advanced Acute Myeloid Leukemia (AML) and Myelodysplastic Syndromes (MDS), Including Hypomethylating Agent (HMA) Failures

Inosine 5'- monophosphate dehydrogenase (IMPDH) is a rate-limiting enzyme that catalyzes de novo synthesis of the guanine nucleotide and is overexpressed in both hematologic and solid tumors. FF-10501-01 is a potent new competitive IMPDH inhibitor. We investigated the anti-leukemia effect of FF-10501-01 in AML cell lines and in a Phase 1 clinical study in advanced AML and MDS, including HMA failures.Thirteen leukemia cell lines were studied, including 5 parental AML cell lines and their HMA-resistant derivatives (MOLM13, SKM1, HL60, TF1, and U937), and 3 other AML cell lines (KG1, HEL, and OCI-AML3). Cell proliferation was determined using trypan blue analysis. Flow cytometry was performed to detect drug-induced apoptosis and cell cycle analysis. High-performance liquid chromatography (HPLC) was performed to detect the intracellular concentrations of guanine nucleotides. Mycophenolic acid-treated cells were used as positive control. Effect of guanosine supplement on FF-10501-01 treatment was evaluated. Within 72 hours of treatment, FF-10501-01 inhibited proliferation of all 13 AML cell lines. The IC50 of FF-10501-01 ranged between 4.3 and 144.5 µM. MOLM13 was the most sensitive leukemia cell line, whereas the decitabine-resistant TF1 cell line was the most resistant. FF-10501-01-induced apoptosis was observed in all cell lines. Increased numbers of cells in G1 phase and decreased numbers in S phase were observed in MOLM13, SKM1 and TF1 cell lines treated with <100 µM FF-10501-01. Decreased intracellular concentrations of guanine nucleotides were observed in MOLM13 and SKM1 cell lines treated with 3 to 30 µM of FF-10501-01 for 24 hours. Proliferation was partially rescued after 72 hours of treatment with 3 µM guanosine and FF-10501-01 in MOLM-13, HL60 cells and their HMA-resistant derivatives. No treatment synergy was observed with the combination of FF-10501-01 with HMAs in MOLM-14 and HL-60 or their HMA-resistant cell lines.In summary, FF-10501-01 produced potent anti-proliferative and apoptotic effects on AML cell lines through inhibition of de novo guanine nucleotide synthesis.In view of these pre-clinical findings, we performed a standard 3+3 dose-escalation Phase 1 trial to access the safety and clinical activity of FF-10501-01 in patients with advanced AML, MDS and chronic myelomonocytic leukemia (CMML). Eligibility criteria: age ³ 18 years, high risk MDS/CMML, AML with documented PD following previous therapy, AML ≥ 60 years of age and not a candidate for other therapy, adequate renal and hepatic function, and no known history of significant cardiac disease. Sixteen patients (12 AML, 4 MDS) have been enrolled in 5 dose cohorts (50 - 400 mg/m2 PO BID) for 14 days on/14 days off each 28-day cycle, including 8 M and 8 F. Median (range) values: age 75.3 yrs (59.1 - 88.6); bone marrow blasts for AML patients 40.5% (12 - 71), for MDS patients 10% (6 - 13), or 30% overall (6 - 71); and prior treatment regimens 2.5 (1 - 6). All patients relapsed from, or progressed on, prior HMAs. Mutations in FLT3, NPM1, GATA2, TET2, ASXL1, DNMT3A and/or MDM2 were present in 4/16 (25%) patients.The median number of FF-10501-01 cycles received to date is 1.5 (range 1 - 10). No DLTs or drug-related serious adverse events (AEs) have been observed and FF-10501-01 has been very well tolerated through 5 - 10 cycles. The most frequent drug-related AEs have been Gr 1-2 nausea, diarrhea and fatigue. Drug-related Gr 4 prolonged thrombocytopenia and Gr 4 prolonged neutropenia were reported in one patient at 200 mg/m2 BID.Two partial responses (PRs) have been achieved in 1 patient each at 50 and 100 mg/m2 BID after 3 cycles, 7 (50%) patients demonstrated long-term stable disease over 2 - 10 cycles, and 4 patients have remained on study drug through 5 - 10 cycles and are still ongoing. Updated safety and efficacy data, including PK/PD, will be presented at the meeting.FF-10501-01 is a promising new agent for the treatment of advanced AML and MDS. Preclinical activity was seen in multiple leukemia cell lines. In a Phase 1 trial, clinical activity with PRs, prolonged disease stabilization and a highly tolerable safety profile were observed. The Phase 2 expansion phase will be initiated soon. DisclosuresDiNardo:Novartis: Research Funding. Pemmaraju:Stemline: Research Funding; Incyte: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; LFB: Consultancy, Honoraria. Smith:Westat Corporation: Employment. Iwamura:FUJIFILM Corporation: Employment. Gipson:Strategia Therapeutics, Inc.: Employment. Rosner:Strategia Therapeutic, Inc.: Employment. Madden:Strategia Therapeutics, Inc.: Employment. Myers:Strategia Therapeutics, Inc.: Employment. Paradiso:Strategia Therapeutics, Inc.: Employment.

Dynamin2 GTPase and Cortactin Remodel Actin Filaments

The large GTPase dynamin, best known for its activities that remodel membranes during endocytosis, also regulates F-actin-rich structures, including podosomes, phagocytic cups, actin comet tails, subcortical ruffles, and stress fibers. The mechanisms by which dynamin regulates actin filaments are not known, but an emerging view is that dynamin influences F-actin via its interactions with proteins that interact directly or indirectly with actin filaments. We show here that dynamin2 GTPase activity remodels actin filaments in vitro via a mechanism that depends on the binding partner and F-actin-binding protein, cortactin. Tightly associated actin filaments cross-linked by dynamin2 and cortactin became loosely associated after GTP addition when viewed by transmission electron microscopy. Actin filaments were dynamically unraveled and fragmented after GTP addition when viewed in real time using total internal reflection fluorescence microscopy. Cortactin stimulated the intrinsic GTPase activity of dynamin2 and maintained a stable link between actin filaments and dynamin2, even in the presence of GTP. Filaments remodeled by dynamin2 GTPase in vitro exhibit enhanced sensitivity to severing by the actin depolymerizing factor, cofilin, suggesting that GTPase-dependent remodeling influences the interactions of actin regulatory proteins and F-actin. The global organization of the actomyosin cytoskeleton was perturbed in U2-OS cells depleted of dynamin2, implicating dynamin2 in remodeling actin filaments that comprise supramolecular F-actin arrays in vivo. We conclude that dynamin2 GTPase remodels actin filaments and plays a role in orchestrating the global actomyosin cytoskeleton.

Functional effects of deleting the coiled-coil motif in Escherichia coli elongation factor Ts.

Elongation factor Ts (EF-Ts) is the guanine nucleotide-exchange factor for elongation factor Tu (EF-Tu) that is responsible for promoting the binding of aminoacyl-tRNA to the mRNA-programmed ribosome. The structure of the Escherichia coli EF-Tu-EF-Ts complex reveals a protruding antiparallel coiled-coil motif in EF-Ts, which is responsible for the dimerization of EF-Ts in the crystal. In this study, the sequence encoding the coiled-coil motif in EF-Ts was deleted from the genome in Escherichia coli by gene replacement. The growth rate of the resulting mutant strain was 70-95% of that of the wild-type strain, depending on the growth conditions used. The mutant strain sensed amino acid starvation and synthesized the nucleotides guanosine 5'-diphosphate 3'-diphosphate and guanosine 5'-triphosphate 3'-diphosphate at a lower cell density than the wild-type strain. Deletion of the coiled-coil motif only partially reduced the ability of EF-Ts to stimulate the guanine nucleotide exchange in EF-Tu. However, the concentration of guanine nucleotides (GDP and GTP) required to dissociate the mutant EF-Tu-EF-Ts complex was at least two orders of magnitude lower than that for the wild-type complex. The results show that the coiled-coil motif plays a significant role in the ability of EF-Ts to compete with guanine nucleotides for the binding to EF-Tu. The present results also indicate that the deletion alters the competition between EF-Ts and kirromycin for the binding to EF-Tu.

Guanine nucleotides, including GMP, antagonize kainate responses in Xenopus oocytes injected with chick cerebellar membranes.

Injection of chick cerebellar membranes, rich in kainate binding sites, into Xenopus oocytes resulted in the structural integration of chick membrane patches into the oocyte plasma membrane that could be easily identified by specific immunofluorescent staining. Application of kainate to the oocyte perfusion medium, under voltage-clamp conditions, induced dose-dependent (EC50 = 87+/-14 microM) inward currents, confirming the functional incorporation to the oocyte of kainate-driven channels. Responses to kainate were consistently nondesensitizing and strongly potentiated by cyclothiazide, suggesting the selective involvement of alpha-amino-3-hydroxy-5-methyl-4isoxazolepropionate (AMPA)-preferring receptors. Binding experiments with (S)-[3H]AMPA confirmed the presence in the chick membrane preparation of low-affinity AMPA receptors (K(D) = 278 nM) amounting to <2% of the total population of kainate binding sites. A tenfold concentration of guanine nucleotides, with different degrees of phosphorylation, blocked the responses to 100 microM kainate by approximately 90%. In the case of GMP, additional concentration-inhibition studies yielded an IC50 of 180+/-11 microM. Our results illustrate the apparent failure of kainate-binding proteins to form functional channels, even when maintaining their own native membrane environment, and confirm the antagonistic behavior of guanine nucleotides, including GMP, toward glutamate receptors, in agreement with previous results of ligand-binding experiments and, more interestingly, with the marked neuroprotective effects of some guanine nucleotides in different excitotoxicity experimental paradigms.

Influence of ageing on functional recovery and guanine nucleotide levels of the heart following cold cardioplegic arrest.

The effect of age on metabolism and mechanical recovery of the heart after cardioplegic arrest is important, but remains a relatively unexplored subject. In this study, functional recovery and nucleotide levels were compared in the heart at different ages subjected to prolonged hypothermic cardioplegic arrest. Three different age groups of rats: 1 (A); 4 (B); and 16 months (C) were perfused in working mode and subjected to cardioplegic arrest (St. Thomas' No. 1) and ischemia for 4 h at 4 degrees C, followed by reperfusion for 35 min. Cardiac function (cardiac output and aortic pressure) was recorded before and after ischemia. Another series of hearts in all three age groups underwent 5 min of normoxic perfusion to obtain pre-ischemic baseline metabolite concentrations. Hearts were freeze-clamped at the end of each experiment and used for determination of nucleotide and creatine metabolites by HPLC. The post-ischemic recovery (% of the pre-ischemic value) of the cardiac power was 48.9 +/- 7.8% for group A, which was significantly higher than the functional recovery of group B (24.1 +/- 3.5%) or C (21.4 +/- 4.7%, P < 0.05, respectively). There was no difference in ATP or the total adenine nucleotide or creatine metabolite concentrations between the three age groups. In contrast, both GTP and the total guanine nucleotide concentration was highest in A (P < 0.05). Total guanylate pool was 1.52 +/- 0.10 1 micromol/g dry wt. in A, as compared to B (1.05 +/- 0.04) or C (1.12 +/- 0.04). NAD was significantly higher in B (4.1 +/- 0.1. P < 0.05), when compared to A (3.6 +/- 0.1) and C (3.8 +/- 0.1). Best post-ischemic functional recovery after cardioplegic arrest was observed in the 1-month-old hearts (A) and was associated with highest guanine nucleotide concentration; preservation of guanine nucleotide pool in the youngest hearts may be an important mechanism for improved cardioprotection due to the important role of GTP in signalling pathways.

Allosteric regulation of the binding of [ 3H]acetylcholine to m2 muscarinic receptors

Muscarinic receptors of the m2 subtype expressed in Chinese hamster ovary cells were labeled with [ methyl- 3H]acetylcholine([ 3H]ACh), and the rate of dissociation in the presence and absence of several compounds known to exert allosteric effects on labeled antagonist binding was observed. At 25 °, [ 3H]ACh bound to the receptors with a K d of 1.2 nM and dissociated with a half-time of 1.6 min. This binding was sensitive to appropriate concentrations of guanine nucleotide and the muscarinic antagonist N-methylscopolamine (NMS). Gallamine, tetrahydroaminoacridine, physostigmine, obidoxime, and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8) all inhibited the binding of [ 3H]ACh and all slowed the rate of dissociation of [ 3H]ACh in a concentration-dependent manner. However, the nature of some of the allosteric effects differed from previous studies that used other labeled ligands. In particular, TMB-8, which is very effective in slowing the dissociation of the antagonist [ 3H]NMS, had much weaker effects on the dissociation of [ 3H]ACh. Furthermore, TMB-8 was able to partially reverse the stronger effects of gallamine on the dissociation of [ 3H]ACh, consistent with the possibility that TMB-8 and gallamine share a common site on the receptor. In summary, the binding of ACh to muscarinic receptors is subject to allosteric regulation, and assays using [ 3H]ACh may be especially useful in the evaluation of potential allosteric regulators of muscarinic systems.

Evidence for the presence of G-proteins, adenylyl cyclase and phospholipase C activities in lymphatic smooth muscle cell membranes

In plasma membranes derived from bovine mesenteric lymphatic smooth muscle cells, guanine nucleotide and forskolin stimulated adenylyl cyclase (AC) activity in a concentration-dependent manner, indicative of the presence of the stimulatory G-protein G, linked to AC. There was no significant enzyme inhibition by low concentrations of guanine nucleotide and no effect on basal or guanine nucleotide-stimulated activity following pertussis toxin treatment of cells, suggesting the absence of G, linked to inhibition of AC. Furthermore, there was no effect of adrenaline, isoprenaline or clonidine on basal or forskolin-stimulated activities, nor was there any specific binding of the β-adrenoceptor ligand [ 125I]cyanopindolol to membranes, suggesting that catecholamine receptors do not modulate AC activity in these membranes. Pertussis toxin-mediated ADP ribosylation of membrane proteins and Western immunoblotting analysis revealed the presence of G-protein subunits of G αi2, G αq, G α11 and G β1. In experiments designed to identify a possible effector enzyme for these G-proteins, membranes were screened with a range of antibodies raised against phospholipase C (PLC) β, γ and δ isozymes. Though no evidence was obtained by Western blotting for any of these proteins, PLC activity was concentration-dependently stimulated by Ca 2+, but not by AlF 4-, GTP[S], or purified G βγ subunits. Finally, no specific binding to membranes of the α 1-adrenoceptor ligand [ 3H]prazosin or the α 2-adrenoceptor ligand [ 3H]yohimbine was obtained. In conclusion, this study provides evidence for a G s-dependent stimulation of AC, and for the presence of G i2 and G q/11, which do not appear to regulate a PLC activity also identified in lymphatic smooth muscle cell membranes. Furthermore, neither AC nor PLC appear to be associated with catecholamine receptors.

Guanine nucleotide-induced enhancement of affinity of dopaminergic membrane receptors of nerve tissues ofLymnaea stagnalis for agonists

Reception of labeled dopamine [7,83H] DA (hereafter, DA) and of a D1 receptor agonist, [3H] SKF 38393, in membranes from nerve tissues of the fresh-water lunged mollusc,Lymnaea stagnalis, was investigated. The presence of 10−6 to 10−5 M of guanine di- and triphosphate as well as of their nonhydrolizable analogs amplified the binding of agonists to membrane DA receptors, especially after EGTA addition. Replacement of EGTA with EDTA partly suppressed the binding amplification effect. Higher concentrations of guanine nucleotides (10−3 to 10−4 M) inhibited the binding of DA and of its agonists. The GDPβS-dependent stimulation of agonist binding was found not to be induced by subunits of GTP-binding proteins (G proteins), immunologically similar to β1-, β2-, and Goα-subunits of G protein in vertebrates. Membrane phosphorylation by a catalytic subunit of cAMP-dependent protein kinase fully inhibited the stimulating effect of guanine nucleotides on the agonist binding to DA receptors and markedly depressed the DA-dependent GTPase activity.

Inhibition of SDC25 C-domain-induced guanine-nucleotide exchange by guanine ring binding domain mutants of v-H-ras.

Guanine-nucleotide exchange is the reaction that controls the activation of H-ras p21. This reaction is stimulated by the guanine-nucleotide exchange factor. In this study we show that H-ras p21 harboring guanine ring binding domain (the conserved NKXD motif) mutations, such as N116I or K117E, are potent inhibitors of H-ras p21 guanine-nucleotide exchange reaction promoted by SDC25C (Saccharomyces cerevisiae SDC25 C-domain gene product), a guanine-nucleotide exchange factor. The inhibition is due to the formation of a stable but catalytically inactive complex consisting of the H-ras mutant and SDC25C. By examining the interaction of v-H-ras(N116I) or v-H-ras(K117E) with SDC25C at different concentrations of guanine-nucleotide, we demonstrate that the mechanism of SDC25C-promoted guanine-nucleotide exchange proceeds through the following pathway. First, SDC25C binds to H-ras and forms an intermediate H-ras.SDC25C complex; subsequently, an incoming guanine-nucleotide dissociates the complex, releasing SDC25C from H-ras and causes guanine-nucleotide exchange. This mechanism is similar to the one proposed for Escherichia coli elongation factor Ts-catalyzed guanine-nucleotide exchange.

NADH Oxidase Activity of Plasma Membranes of Soybean Hypocotyls Is Activated by Guanine Nucleotides.

The activity of an auxin-stimulated NADH oxidase of the plasma membrane of hypocotyls of etiolated soybean (Glycine max Merr.) seedlings responded to guanine and other nucleotides, but in a manner that differed from that of enzymes coupled to the classic trimeric and low molecular weight monomeric guanine nucleotide-binding proteins (G proteins). In the presence and absence of either auxin or divalent ions, both GTP and GDP as well as guanosine-5[prime]-O-(3-thiotriphosphate) (GTP-[gamma]-S) and other nucleoside di- and triphosphates stimulated the oxidase activity over the range 10 [mu]M to 1 mM. GTP and GTP-[gamma]-S stimulated the activity at 10 nM in the absence of added magnesium and at 1 nM in the presence of added magnesium ions. Other nucleotides stimulated at 100 nM and above. The NADH oxidase was stimulated by 10 [mu]M mastoparan and by 40 [mu]M aluminum fluoride. Neither cholera nor pertussis toxins, tested at a concentration sufficient to block mammalian G protein function, inhibited the activity. Guanosine 5[prime]-O-(2-thiodi-phosphate) (GDP-[beta]-S) did not stimulate activity, suggesting that the stimulation in response to GDP may be mediated by a plasma membrane nucleoside diphosphate kinase through conversion of GDP to GTP. Auxin stimulation of the NADH oxidase was unaffected by nucleotides at either high or low nucleotide concentrations in the absence of added divalent ions. However, pretreatment of plasma membranes with auxin increased the apparent affinity for nucleotide binding. This increased affinity, however, appeared not to be the mechanism of auxin stimulation of the oxidase, since auxin stimulation was similar with or without low concentrations of guanine nucleotides. The stimulation by nucleotides was observed after incubating the membranes with 0.1% Triton X-100 prior to assay. The results suggest a role of guanine (and other) nucleotides in the regulation of plasma membrane NADH oxidase that differs from the interactions with G proteins commonly described for animal models.

Kinetic characterization of guanine-nucleotide-induced exocytosis from permeabilized rat mast cells.

We have measured the time course of secretion of hexosaminidase from rat mast cells permeabilized (in simple buffered NaCl solutions) in response to guanine nucleotides [GTP or guanosine 5'-[gamma-thio]triphosphate (GTP[S])] and Ca2+. In these experiments, ATP was excluded from the system (and the cells were pretreated with metabolic inhibitors). For cells permeabilized in the absence of Mg2+ but in the presence of Ca2+, secretion commences promptly in response to addition of GTP; when Mg2+ (2 mM) is provided, secretion commences after an extended delay, much higher concentrations of GTP are required, and the final extent of secretion is decreased. Ongoing secretion due to GTP and Ca2+ is abruptly terminated by addition of Mg2+ to cells initially stimulated in its absence. In contrast, although Mg2+ has no effect on the sensitivity to the non-hydrolysable analogue GTP[S], its absence does nevertheless cause delays in the onset of secretion triggered by the addition of GTP[S] to cells initially permeabilized in the presence of Ca2+ (micromolar range, again in the absence of ATP). However, exocytosis from cells triggered with Ca2+ after permeabilization in the presence of high concentrations of GTP[S] is instantaneous. The delays due to triggering by GTP[S] have GTP[S]-concentration-dependent and -independent components. The guanine-nucleotide-concentration-dependent component is expressed as an extended duration of delay as the concentration of GTP[S] is decreased, and may reflect the binding of GTP[S] to GE. The concentration-independent component is manifested as a limiting delay which cannot be further diminished by increasing the guanine nucleotide concentration. The duration of the limiting delay is sensitive to the identity of the stimulating nucleotide (GTP < GTP[S] < p[NH]ppG) and may reflect the time taken for an activating conformational change to occur after binding. Since both components of the delays are abolished by the presence of Mg2+, both the binding of guanine nucleotide and the activation of GE appear to be Mg(2+)-dependent. We therefore conclude that nucleotide binding, activation and the GTPase activity of GE are strongly dependent on Mg2+, in common with the same three processes in Gs and Gi.

Calcium inhibition of cardiac adenylyl cyclase. Evidence for two distinct sites of inhibition

Increasing the free calcium concentration from 10 −8 M to 10 −4 M inhibited cardiac sarcolemmal adenylyl cyclase activated by the addition of 5×10 −4 M forskolin or 1×10 −4 M GTP or Gpp(NH)p. The calcium inhibition curve in the presence of all three activators was shallow and best fit by a two site model of high affinity (< 1.0 μM) and low affinity (0.1 mM). Gpp(NH)p appeared to decrease the sensitivity of adenylyl cyclase to inhibition by calcium at the high affinity site. Similar inhibition constants were obtained with each of the activators. Calmodulin content of native freeze-thaw vesicles was 76.2 ± 14.2 ng/mg. Treatment of the vesicles with 1 mM EGTA to remove calmodulin significantly reduced calmodulin content to 19.7 ± 1.35 ng/mg. This treatment had no significant effect on the calcium inhibition profile. Increasing free calcium to 3×10 −6 M was shown to have no effect on the EC 50 estimated for either Gpp(NH)p or forskolin but did slightly increase the EC 50 estimated for Mg 2+ in the presence of maximal concentrations of elther activator. Nevertheless, maximally stimulating concentrations of Mg 2+ were unable to overcome calcium inhibition. Pretreatment of sarcolemmal membranes with pertussis toxin was shown to have no significant effect on calcium inhibition of adenylyl cyclase. The results suggest that the overall inhibitory action of calcium was most likely calmodulin independent and involved a direct interaction with the catalytic subunit at two distinct sites of high and low affinity. At the low affinity site calcium most likely competes with Mg 2+ for an allosteric divalent cation binding site. Inhibition at the high affinity site was non-competitive with respect to Mg 2+, forskolin and guanine nucleotide concentration. The high affinity site may mediate feedback inhibition of cardiac adenylyl cyclase in vivo.

Characterization of Melatonin Receptors in the Rat Suprachiasmatic Nuclei: Modulation of Affinity with Cations and Guanine Nucleotides*

We have characterized melatonin (MT) receptors in the rat suprachiasmatic nuclei by using quantitative autoradiography in vitro at equilibrium conditions for a picomolar affinity site. Binding of the MT agonist 2-[125I]iodomelatonin (30 pM to 6 nM) was saturable, of high affinity (Kd, 52.8 pM), of high specificity, and to a single class of sites (binding capacity, 16.5 fmol/mg protein). However, by shortening the washing time for bound and free ligand, we were able to demonstrate an additional low affinity form of this receptor (Kd, 761 pM; binding capacity, 72.2 fmol/mg protein). Micromolar concentrations of guanine nucleotides and millimolar concentrations of monovalent cations (Na+ and Li+) dose-dependently and specifically inhibited agonist binding at 22 C. Saturation studies revealed that this was due to a decrease in receptor affinity in both cases. Ca+2 promoted high affinity agonist binding, as omission of this cation decreased the affinity of the suprachiasmatic MT receptor. These results suggest coupling of the rat suprachiasmatic MT receptors to guanine nucleotide-binding regulatory protein(s). Moreover, our data demonstrate specific modulation of the affinity of these receptors with cations.

Characterization of Melatonin Receptors in the Rat Area postrema: Modulation of Affinity with Cations and Guanine Nucleotides

We have localized and characterized the binding of the melatonin agonist, 2-[125I]iodomelatonin, in the rat area postrema (AP), by using quantitative autoradiography in vitro. At equilibrium conditions, Scatchard analysis revealed saturable high-affinity binding to a single class of sites (Kd 45.9 +/- (SE) 6.0 pM and Bmax 30.8 +/- 4.6 fmol/mg protein, n = 4 experiments with a total of 18 rats). Melatonin and 6-hydroxymelatonin were potent displacers of 2-[125I]iodomelatonin binding in the AP (IC50 20 and 500 pM, respectively) while N-acetylserotonin exhibited only a modest potency (IC50 25 nM). Micromolar concentrations of guanine nucleotides dose-dependently and specifically inhibited agonist binding at 22 degrees C. Saturation studies revealed that this was due to a decrease in binding affinity. Divalent cations (4 mM CaCl2 or 2 mM MgCl2) had no detectable effect on the affinity of the binding site, whereas physiological concentrations of NaCl significantly decreased the binding affinity. These results demonstrate specific high-affinity binding sites for 2-[125I]iodomelatonin in the rat AP and suggest coupling of these putative receptors to guanosine nucleotide binding regulatory protein(s).

Muscarinic acetylcholine receptor regulates phosphatidylcholine phospholipase D in canine brain

The hydrolytic activity of phosphatidylcholine phospholipase D in the synaptosomes from canine brain was examined using a radiochemical assay with 1,2-dipalmitoyl-sn-glycerol-3-phosphoryl[3H]choline as the exogenous substrate. The involvement of G protein(s) in regulation of this enzyme was demonstrated by a 2- to 3-fold stimulation of the basal activity (4.81 +/- 0.44 nmol choline released/mg protein/h) with guanosine 5'-(3-O-thiol)triphosphate (GTP gamma S), guanyl-5'-yl-(beta, gamma-methylene)diphosphonate, aluminum fluoride, or cholera toxin. The stimulation of phospholipase D hydrolytic activity by GTP gamma S was inhibited by 2 mM guanosine 5'-(2-O-thiol)diphosphate. GTP gamma S at the maximum stimulatory concentration (10 microM) had an additive effect on the maximum cholera toxin stimulation of phospholipase D activity. However, the reverse was not true, thus indicating the possibility that more than one G protein may be involved. Furthermore, cholinergic agonists, including acetylcholine, carbachol, and muscarine, were able to increase the phospholipase D hydrolytic activity at low but not maximally stimulatory concentrations of guanine nucleotide. These cholinergic stimulations were antagonized by atropine, a muscarinic blocker. In addition, O-tetradecanoylphorbol 13-acetate, a protein kinase C activator, was able to stimulate the hydrolytic activity of phospholipase D more than 300% in the presence of 0.2 microM GTP gamma S. However, in the absence of GTP gamma S, stimulation was less than 60%. Our results not only indicate that the receptor-G protein-regulated phospholipase D may be directly responsible for the rapid accumulation of choline and phosphatidic acid in the central nervous system but also reveal that muscarinic acetylcholine receptor-G protein-regulated phospholipase D is a novel signal transduction process coupling the neuronal muscarinic receptor to cellular responses.

Bovine mitochondrial protein synthesis elongation factors: Identification and initial characterization of an elongation factor Tu-elongation factor Ts complex

Animal mitochondrial protein synthesis factors elongation factor (EF) Tu and EF-Ts have been purified as an EF-Tu. Ts complex from crude extracts of bovine liver mitochondria. The mitochondrial complex has been purified 10,000-fold to near homogeneity by a combination of chromatographic procedures including high performance liquid chromatography. The mitochondrial EF-Tu.Ts complex is very stable and cannot be dissociated even in the presence of high concentrations of guanine nucleotides. No guanine nucleotide binding to this complex can be observed in the standard nitrocellulose filter binding assay. Mitochondrial EF-Ts activity can be detected by its ability to facilitate guanine nucleotide exchange with Escherichia coli EF-Tu. The EF-Tumt exhibits similar levels of activity on isolated mammalian mitochondrial and E. coli ribosomes, but displays minimal activity on Euglena gracilis chloroplast 70 S ribosomes and has no detectable activity on wheat germ cytoplasmic ribosomes. In contrast to the bacterial EF-Tu and the EF-Tu from the chloroplast of E. gracilis, the ability of the mitochondrial factor to catalyze polymerization is not inhibited by the antibiotic kirromycin.

Kinetics of Activation of Phospholipase C by P2Y Purinergic Receptor Agonists and Guanine Nucleotides

Membranes prepared from [3H]inositol-labeled turkey erythrocytes express a phospholipase C that is markedly stimulated by stable analogs of GTP (Harden, T. K., Stephens, L., Hawkins, P. T., and Downes, C. P. (1987) J. Biol. Chem. 262, 9057-9061). We now report that P2-purinergic receptor-mediated regulation of the enzyme occurs in the membrane preparation. The order of potency of a series of ATP and ADP analogs for stimulation of inositol phosphate formation, i.e. 2-methylthioadenosine 5'-triphosphate (2MeSATP) greater than adenosine 5'-O-(2-thiodiphosphate) greater than adenosine 5'-O-(3-thiotriphosphate) greater than ATP greater than 5'-adenylyl imidodiphosphate approximately ADP greater than alpha, beta-methyleneadenosine 5'-triphosphate greater than beta, gamma-methyleneadenosine 5'-triphosphate, was consistent with that for the P2Y-purinergic receptor subtype. Agonist-stimulated effects were completely dependent on the presence of guanine nucleotide. Activation of phospholipase C by guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) occurred with a considerable time lag. The rate of activation followed first order kinetics and was markedly increased by increasing concentrations of a P2Y receptor agonist; in contrast, the rate of activation at a fixed agonist concentration was independent of guanine nucleotide concentration. Addition of guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) prior to addition of agonist and GTP, 5'-guanylyl imidodiphosphate (Gpp(NH)p), or GTP gamma S blocked in a concentration-dependent manner the stimulatory effect of guanine nucleotide. GDP beta S, added subsequent to preactivation of membranes with 2MeSATP and GTP gamma S or Gpp(NH)p had only small inhibitory effects on the rate of inositol phosphate production observed over the subsequent 10 min. In contrast, addition of GDP beta S to GTP-preactivated membranes resulted in a rapid return of enzyme activity to the basal state within 60 s. Taken together, the data are consistent with the idea that P2Y receptor activation increases the rate of exchange of GTP and GTP analogs for GDP on the relevant guanine nucleotide regulatory protein. Once the active enzymic species is formed, hydrolysis of guanine nucleotide reverts the enzyme to the inactive state.

3H]-8-cyclopentyl-1,3-dipropylxanthine binding to A1 adenosine receptors of intact rat ventricular myocytes.

The purpose of the present study was the identification of A1 adenosine receptors in intact rat ventricular myocytes, which are thought to mediate the negative inotropic effects of adenosine. The adenosine receptor antagonist [3H]-8-cyclopentyl-1,3-dipropylxanthine was used as radioligand. Binding of the radioligand to intact myocytes was rapid, reversible, and saturable with a binding capacity of 40,000 binding sites per cell. The dissociation constant of the radioligand was 0.48 nM. The adenosine receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine, "xanthine amine congener," and theophylline were competitive inhibitors with affinities in agreement with results obtained for A1 receptors in other tissues. Competition experiments using the adenosine receptor agonists R-N(6)-phenylisopropyladenosine, 5'-N-ethylcarboxamidoadenosine, and S-N(6)-phenylisopropyladenosine gave monophasic displacement curves with Ki values of 50 nM, 440 nM, and 4,300 nM, which agreed well with the GTP-inducible low affinity state in cardiac membranes. The low affinity for agonists was not due to agonist-induced desensitization, and correlated well with the corresponding IC50 values for the inhibition of cyclic AMP accumulation by isoprenaline. It is suggested that only a low affinity state of A1 receptors can be detected in intact rat myocytes due to the presence of high concentrations of guanine nucleotides in intact cells.