Physiological Substrate Concentrations Research Articles

Fatty acid oxidation and its impact on response of spontaneously hypertensive rat hearts to an adrenergic stress: benefits of a medium-chain fatty acid

The spontaneously hypertensive rat (SHR) is a model of cardiomyopathy characterized by a restricted use of exogenous long-chain fatty acid (LCFA) for energy production. The aims of the present study were to document the functional and metabolic response of the SHR heart under conditions of increased energy demand and the effects of a medium-chain fatty acid (MCFA; octanoate) supplementation in this situation. Hearts were perfused ex vivo in a working mode with physiological concentrations of substrates and hormones and subjected to an adrenergic stimulation (epinephrine, 10 microM). (13)C-labeled substrates were used to assess substrate selection for energy production. Compared with control Wistar rat hearts, SHR hearts showed an impaired response to the adrenergic stimulation as reflected by 1) a smaller increase in contractility and developed pressure, 2) a faster decline in the aortic flow, and 3) greater cardiac tissue damage (lactate dehydrogenase release: 1,577 +/- 118 vs. 825 +/- 44 mU/min, P < 0.01). At the metabolic level, SHR hearts presented 1) a reduced exogenous LCFA contribution to the citric acid cycle flux (16 +/- 1 vs. 44 +/- 4%, P < 0.001) and an enhanced contribution of endogenous substrates (20 +/- 4 vs. 1 +/- 4%, P < 0.01); and 2) an increased lactate production from glycolysis, with a greater lactate-to-pyruvate production ratio. Addition of 0.2 mM octanoate reduced lactate dehydrogenase release (1,145 +/- 155 vs. 1,890 +/- 89 mU/min, P < 0.001) and increased exogenous fatty acid contribution to energy metabolism (23.7 +/- 1.3 vs. 15.8 +/- 0.8%, P < 0.01), which was accompanied by an equivalent decrease in unlabeled endogenous substrate contribution, possibly triglycerides (11.6 +/- 1.5 vs. 19.0 +/- 1.2%, P < 0.01). Taken altogether, these results demonstrate that the SHR heart shows an impaired capacity to withstand an acute adrenergic stress, which can be improved by increasing the contribution of exogenous fatty acid oxidation to energy production by MCFA supplementation.

Interferon-alpha treatment and serotonergic dysfunction

Treatment with interferon-alpha in patients with a chronic hepatitis C (cHC) infection is frequently associated with depressive mood changes, possibly related to a modulation of the serotonergic system. Method: In a prospective controlled study biochemical measures included assays of 5-HT concentration, 5-HT uptake and MAO-B activity at a low physiological substrate concentration in platelets of 64 untreated HCV-infected patients were compared to 21 healthy controls. Furthermore 30 patients with a cHC were evaluated before and during IFN-α treatment. Results: While the serotonin platelet concentration did not differ between controls and HCV-infected patients, 5-HT-uptake and the MAO-B activity were significant lower in patients with HCV-infection (p<0.001). Interferon-treatment lead to an increase in 5-HT reuptake (p=0.01), to an increase of MAO-B activity (p<0.001) and to a decrease of serotonin concentration (p<0.01). Conclusion: Our data support the hypothesis of a serotonergic deficit in patients with chronic HCV-infection during interferon-alpha treatment.

Identification of potential regulatory sites of the Na+,K+-ATPase by kinetic analysis.

Kinetic models are presented that allow the Na(+),K(+)-ATPase steady-state turnover number to be estimated at given intra- and extracellular concentrations of Na(+), K(+), and ATP. Based on experimental transient kinetic data, the models utilize either three or four steps of the Albers-Post scheme, that is, E(2) --> E(1), E(1) --> E(2)P (or E(1) --> E(1)P and E(1)P --> E(2)P), and E(2)P --> E(2), which are the major rate-determining steps of the enzyme cycle. On the time scale of these reactions, the faster binding steps of Na(+), K(+), and ATP to the enzyme are considered to be in equilibrium. Each model was tested by comparing calculations of the steady-state turnover from rate constants and equilibrium constants for the individual partial reactions with published experimental data of the steady-state activity at varying Na(+) and K(+) concentrations. To provide reasonable agreement between the calculations and the experimental data, it was found that Na(+)/K(+) competition for cytoplasmic binding sites was an essential feature required in the model. The activity was also very dependent on the degree of K(+)-induced stimulation of the reverse reaction E(1) --> E(2). Taking into account the physiological substrate concentrations, the models allow the most likely potential sites of short-term Na(+),K(+)-ATPase regulation to be identified. These were found to be (a) the cytoplasmic Na(+) and K(+) binding sites, via changes in Na(+) or K(+) concentration or their dissociation constants, (b) ATP phosphorylation (as a substrate), via a change in its rate constant, and (c) the position of the E(2)<==>E(1) equilibrium.

The interaction of FBPase with aldolase: a kinetic and fluorescence investigation on chicken muscle enzymes

Fructose-1,6-bisphosphatase (FBPase; EC 3.1.3.11) is strongly inhibited by AMP in vitro and, therefore, at physiological concentrations of substrate and AMP, FBPase should be completely inhibited. Desensitization of rabbit muscle FBPase against AMP inhibition was previously observed in the presence of rabbit muscle aldolase. In this study, we analysed the kinetics of an FBPase catalyzed reaction and interaction between chicken muscle FBPase and chicken muscle aldolase. The initial rate of FBPase reaction vs. substrate concentration shows a maximum activity at a concentration of 20 μM Fru-1,6P 2 and then decreases. Assuming rapid equilibrium kinetics, the enzyme-catalyzed reaction was described by the substrate inhibition model, with K s≈5 μM and K si≈39 μM and factor β≈0.2, describing change in the rate constant ( k) of product formation from the ES and ESS i complexes. Based on ultracentrifugation studies, aldolase and FBPase form a hetero-complex with approximately 1:1 stoichiometry with a dissociation constant ( K d) of 3.8 μM. The FBPase–aldolase interaction was confirmed via fluorescence investigation. The aldolase–FBPase interaction results in aldolase fluorescence quenching and its maximum emission spectrum shifting from 344 to 356 nm. The K d of the FBPase–aldolase complex, determined on the basis of fluorescence changes, is 0.4 μM at 25 °C with almost 1:1 stoichiometry. This interaction increases the I 0.5 for the AMP inhibition of FBPase threefold, and slightly affects FBPase affinity to magnesium ions, increasing the K a and Hill coefficient ( n). No effect of aldolase on the FBPase pH optimum was observed. Thus, the decrease in FBPase sensitivity to AMP inhibition enables FBPase to function in vivo thanks to aldolase.

High platelet-serotonin uptake activity is associated with a rapid response in depressed patients treated with amitriptyline

Based on the assumption that there is a biological basis behind the individual differences in the speed with which an antidepressant produces its therapeutic effects, we compared initial serotonin (5HT) uptake characteristics in platelets of rapid (2 weeks), slow (4 weeks) and non-responders in a group of 47 depressed patients who were treated with amitriptyline for at least 4 weeks. A response was defined as a reduction in the Hamilton Depression Rating Scale score of ≥50% from baseline. In 16 rapid responders, a significantly higher mean 5HT uptake efficiency (Vmax/Km) corresponded with a significantly higher 5HT uptake activity at a low, physiological substrate concentration in comparison with the 15 non-responders or the 16 slow responders (33.1±7.8 versus 25.5±7.7 versus 24.1±5.8 pMol [ 3H]-5HT/10 9 plat.×5 min, respectively). The findings indicate that pre-treatment 5HT uptake activity contributes to the individual variability in response time to amitriptyline treatment.

Effect of ursodeoxycholic acid on methionine adenosyltransferase activity and hepatic glutathione metabolism in rats: Rodriguez-Ortigosa CM, Cincu RN, Sanz S, et al., Supplemental Bile Salts: The Good, the Bad Gut 2002;50:701–6

Both bile salts and glutathione participate in the generation of canalicular bile flow. In this work, Rodriguez-Drtigosa et al. investigated the effect of different bile salts on hepatic glutathione metabolism. Using the isolated and perfused rat liver, the authors studied hepatic glutathione content, metabolism, and catabolism of this compound in livers perfused with taurocholate, ursodeoxycholate, or deoxycholate. They found that in livers perfused with ursodeoxycholate, levels of glutathione and the activity of methionine adenosyltransferase (an enzyme involved in glutathione biosynthesis) were significantly higher than in livers perfused with other bile salts. In ursodeoxycholate-perfused livers, methionine adenosyltransferase showed a predominant tetrameric conformation, which is the isoform with highest activity at physiological concentrations of substrate. In contrast, the dimeric form prevailed in livers perfused with taurocholate or deoxycholate. The hepatic activities of γ-glutamylcysteine synthetase and γ-glutamyltranspeptidase, enzymes involved, respectively, in biosynthetic and catabolic pathways of glutathione, were not modified by bile salts. Ursodeoxycholate specifically enhanced methionine adenosyltransferase activity and hepatic glutathione levels. As glutathione is a defensive substance against oxidative cell damage, the authors’ observations provide an additional explanation for the known hepatoprotective effects of ursodeoxycholate.

Post-translational regulation of glucose-6-phosphate dehydrogenase activity in (pre)neoplastic lesions in rat liver.

Glucose-6-phosphate dehydrogenase (G6PD; EC 1.1.1.49) is the key regulatory enzyme of the pentose phosphate pathway and produces NADPH and riboses. In this study, the kinetic properties of G6PD activity were determined in situ in chemically induced hepatocellular carcinomas, and extralesional and control parenchyma in rat livers and were directly compared with those of the second NADPH-producing enzyme of the pentose phosphate pathway, phosphogluconate dehydrogenase (PGD). Distribution patterns of G6PD activity, protein, and mRNA levels were also compared to establish the regulation mechanisms of G6PD activity. In (pre)neoplastic lesions, the V(max) of G6PD was 150-fold higher and the K(m) for G6P was 10-fold higher than in control liver parenchyma, whereas in extralesional parenchyma, the V(max) was similar to that in normal parenchyma but the K(m) was fivefold lower. This means that virtual fluxes at physiological substrate concentrations are 20-fold higher in lesions and twofold higher in extralesional parenchyma than in normal parenchyma. The V(max) of PGD was fivefold higher in lesions than in normal and extralesional liver parenchyma, whereas the K(m) was not affected. Amounts of G6PD protein and mRNA were similar in lesions and in extralesional liver parenchyma. These results demonstrate that G6PD is strongly activated post-translationally in (pre)neoplastic lesions to produce NADPH.

Substitution of Cysteine for Selenocysteine in the Catalytic Center of Type III Iodothyronine Deiodinase Reduces Catalytic Efficiency and Alters Substrate Preference

Human type III iodothyronine deiodinase (D3) catalyzes the conversion of T(4) to rT(3) and of T(3) to 3, 3'-diiodothyronine (T2) by inner-ring deiodination. Like types I and II iodothyronine deiodinases, D3 protein contains selenocysteine (SeC) in the highly conserved core catalytic center at amino acid position 144. To evaluate the contribution of SeC144 to the catalytic properties of D3 enzyme, we generated mutants in which cysteine (D3Cys) or alanine (D3Ala) replaces SeC144 (D3wt). COS cells were transfected with expression vectors encoding D3wt, D3Cys, or D3Ala protein. Kinetic analysis was performed on homogenates with dithiothreitol as reducing cofactor. The Michaelis constant of T(3) was 5-fold higher for D3Cys than for D3wt protein. In contrast, the Michaelis constant of T(4) increased 100-fold. The D3Ala protein was enzymatically inactive. Semiquantitative immunoblotting of homogenates with a D3 antiserum revealed that about 50-fold higher amounts of D3Cys and D3Ala protein are expressed relative to D3wt protein. The relative substrate turnover number of D3Cys is 2-fold reduced for T(3) and 6-fold reduced for T(4) deiodination, compared with D3wt enzyme. Studies in intact COS cells expressing D3wt or D3Cys showed that the D3Cys enzyme is also active under in situ conditions. In conclusion, the SeC residue in the catalytic center of D3 is essential for efficient inner-ring deiodination of T(3) and in particular T(4) at physiological substrate concentrations.

Weak Strand Displacement Activity Enables Human DNA Polymerase β to Expand CAG/CTG Triplet Repeats at Strand Breaks

Using synthetic DNA constructs in vitro, we find that human DNA polymerase beta effectively catalyzes CAG/CTG triplet repeat expansions by slippage initiated at nicks or 1-base gaps within short (14 triplet) repeat tracts in DNA duplexes under physiological conditions. In the same constructs, Escherichia coli DNA polymerase I Klenow Fragment exo(-) is much less effective in expanding repeats, because its much stronger strand displacement activity inhibits slippage by enabling rapid extension through two downstream repeats into flanking non-repeat sequence. Polymerase beta expansions of CAG/CTG repeats, observed over a 32-min period at rates of approximately 1 triplet added per min, reveal significant effects of break type (nick versus gap), strand composition (CTG versus CAG), and dNTP substrate concentration, on repeat expansions at strand breaks. At physiological substrate concentrations (1-10 microm of each dNTP), polymerase beta expands triplet repeats with the help of weak strand displacement limited to the two downstream triplet repeats in our constructs. Such weak strand displacement activity in DNA repair at strand breaks may enable short tracts of repeats to be converted into longer, increasingly mutable ones associated with neurological diseases.

CD26/DPPIV signal transduction function, but not proteolytic activity, is directly related to its expression level on human Th1 and Th2 cell lines as detected with living cell cytochemistry.

CD26/DPPIV is a cell surface glycoprotein that functions both in signal transduction and as a proteolytic enzyme, dipeptidyl peptidase IV (DPPIV). To investigate how two separate functions of one molecule are regulated, we analyzed CD26 protein expression and DPPIV enzyme activity on living human T-helper 1 (Th1) and Th2 cells that express different levels of CD26/DPPIV. DPPIV activity was specifically determined with the synthetic fluorogenic substrate ala-pro-cresyl violet and CD26 protein expression was demonstrated with an FITC-conjugated CD26-specific antibody. Fluorescence of liberated cresyl violet (red) and FITC (green) was detected simultaneously on living T-cells using flow cytometry and spectrofluorometry. Th1 cells expressed three- to sixfold more CD26 protein than Th2 cells. The signal transduction function of the CD26/DPPIV complex, tested by measuring its co-stimulatory potential for proliferation, was directly related to the amount of CD26 protein at the cell surface. However, DPPIV activity was similar in both cell populations at physiological substrate concentrations because of differences in K(m) and V(max) values of DPPIV on Th1 and Th2 cells. Western blotting and zymography of Th1 and Th2 whole-cell lysates demonstrated similar patterns. This study shows that two functions of one molecule can be controlled differentially.

Effect of ursodeoxycholic acid on methionine adenosyltransferase activity and hepatic glutathione metabolism in rats.

Both bile salts and glutathione participate in the generation of canalicular bile flow. In this work, we have investigated the effect of different bile salts on hepatic glutathione metabolism. Using the isolated and perfused rat liver, we studied hepatic glutathione content, and metabolism and catabolism of this compound in livers perfused with taurocholate, ursodeoxycholate, or deoxycholate. We found that in livers perfused with ursodeoxycholate, levels of glutathione and the activity of methionine adenosyltransferase (an enzyme involved in glutathione biosynthesis) were significantly higher than in livers perfused with other bile salts. In ursodeoxycholate perfused livers, methionine adenosyltransferase showed a predominant tetrameric conformation which is the isoform with highest activity at physiological concentrations of substrate. In contrast, the dimeric form prevailed in livers perfused with taurocholate or deoxycholate. The hepatic activities of gamma-glutamylcysteine synthetase and gamma-glutamyltranspeptidase, enzymes involved, respectively, in biosynthetic and catabolic pathways of glutathione, were not modified by bile salts. Ursodeoxycholate specifically enhanced methionine adenosyltransferase activity and hepatic glutathione levels. As glutathione is a defensive substance against oxidative cell damage, our observations provide an additional explanation for the known hepatoprotective effects of ursodeoxycholate.

The effects of fatty acyl CoA esters on the formation of prostaglandin and arachidonoyl-CoA formed from arachidonic acid in rabbit kidney medulla microsomes

Under physiological conditions, small amounts of free arachidonic acid (AA) are released from membrane phospholipids, and cyclooxygenase (COX) and acyl CoA synthetase (ACS) competitively act on this fatty acid to form prostaglandins (PGs) and arachidonoyl-CoA (AA-CoA). We have previously shown that palmitoyl-CoA (PA-CoA) shifts AA away from the COX pathway into the ACS pathway in rabbit kidney medulla at a low concentration of AA (5 μM, close to the physiological concentration of substrate). In the present study, we investigated the effects of stearoyl (SA)-, oleoyl (OA)- and linoleoyl (LA)- CoAs on the formation of PG and AA-CoA from 5μM AA in rabbit kidney medulla microsomes. The kidney medulla microsomes were incubated with 5μM [14C]-AA in 0.1 M-Tris/HCl buffer (pH 8.0) containing cofactors of COX (reduced glutathione and hydroquinone) and cofactors of ACS (ATP, MgCl2and CoA). After incubation, PG (as total PGs), AA-CoA and residual AA were separated by selective extraction using petroleum ether and ethyl acetate. SA- and OA-CoAs increased AA-CoA formation with a reduction of PG formation, as well as PA-CoA. On the other hand, LA-CoA decreased formation of both PG and AA-CoA. These results suggest that fatty acyl CoA esters can be regulators of PG and AA-CoA formation in kidney medulla under physiological conditions.

1α,25-Dihydroxy-3-epi-vitamin D 3 a physiological metabolite of 1α,25-dihydroxyvitamin D 3: Its production and metabolism in primary human keratinocytes

Recent studies of metabolism using pharmacological substrate concentrations of 1α,25-dihydroxyvitamin D 3 [1α,25(OH) 2D 3] in several tissues including primary cultures of human keratinocytes, bovine parathyroid cells and bone cells led to the identification of 1α,25-dihydroxy-3-epi-vitamin D 3 [1α,25(OH) 2-3-epi-D 3] as a major natural metabolite of 1α,25(OH) 2D 3. In the present study, we demonstrate that human keratinocytes incubated with 25-hydroxy[26,27- 3H] vitamin D 3 produce 1α,25(OH) 2-3-epi-D 3 along with 1α,25(OH) 2D 3. The production of 1α,25(OH) 2-3-epi-D 3 is also identified in human keratinocytes incubated with physiological substrate concentrations of 1α,25(OH) 2D 3. Unlike 24-hydroxylase, the major enzyme involved in the further metabolism of 1α,25(OH) 2D 3 in human keratinocytes, the enzyme(s) responsible for the production of 1α,25(OH) 2-3-epi-D 3 is constitutive and is not inhibited by ketoconazole. It is also noted that 1α,25(OH) 2-3-epi-D 3 is further metabolised in human keratinocytes into several as yet unidentified metabolites, the production of which is inhibited to a great extent by SDZ 89-443, an inhibitor of 24-hydroxylase. This finding indicates that the 24-hydroxylase like in the case of 1α,25(OH) 2D 3, also plays a major role in the metabolism of 1α,25(OH) 2-3-epi-D 3. The results obtained from the metabolism studies performed in parallel among 25OHD 3, 1α,25(OH) 2D 3 and 1α,25(OH) 2-3-epi-D 3 indicate that 1α,25(OH) 2-3-epi-D 3 and its metabolites exhibit higher metabolic stability. In summary, we demonstrate for the first time that 1α,25(OH) 2-3-epi-D 3 is a physiological metabolite of 1α,25(OH) 2D 3 in human keratinocytes. Also, 1α,25(OH) 2-3-epi-D 3 is further metabolised in human keratinocytes mainly through the activity of 24-hydroxylase. Furthermore, our finding of the relative metabolic stability of 1α,25(OH) 2-3-epi-D 3 and especially its metabolites when compared to 1α,25(OH) 2D 3 and its metabolites provides an important explanation for its previously observed potent inhibitory effect on keratinocyte growth in spite of its low affinity to vitamin D receptor.

The Role of Pyruvate Ferredoxin Oxidoreductase in Pyruvate Synthesis during Autotrophic Growth by the Wood-Ljungdahl Pathway

Pyruvate:ferredoxin oxidoreductase (PFOR) catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA and CO(2). The catalytic proficiency of this enzyme for the reverse reaction, pyruvate synthase, is poorly understood. Conversion of acetyl-CoA to pyruvate links the Wood-Ljungdahl pathway of autotrophic CO(2) fixation to the reductive tricarboxylic acid cycle, which in these autotrophic anaerobes is the stage for biosynthesis of all cellular macromolecules. The results described here demonstrate that the Clostridium thermoaceticum PFOR is a highly efficient pyruvate synthase. The Michaelis-Menten parameters for pyruvate synthesis by PFOR are: V(max) = 1.6 unit/mg (k(cat) = 3.2 s(-1)), K(m)(Acetyl-CoA) = 9 micrometer, and K(m)(CO(2)) = 2 mm. The intracellular concentrations of acetyl-CoA, CoASH, and pyruvate have been measured. The predicted rate of pyruvate synthesis at physiological concentrations of substrates clearly is sufficient to support the role of PFOR as a pyruvate synthase in vivo. Measurements of its k(cat)/K(m) values demonstrate that ferredoxin is a highly efficient electron carrier in both the oxidative and reductive reactions. On the other hand, rubredoxin is a poor substitute in the oxidative direction and is inept in donating electrons for pyruvate synthesis.

Kelley-Seegmiller syndrome due to a unique variant of hypoxanthine-guanine phosphoribosyltransferase: reduced affinity for 5-phosphoribosyl-1-pyrophosphate manifested only at low, physiological substrate concentrations

A male child, who presented at the age of 3.5 years with acute renal failure, was diagnosed as having partial deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT; EC 2.4.2.8). The underlying HPRT mutation was unique in that the specific activity of HPRT in erythrocyte and in fibroblast lysates was normal, but the rate of uptake of hypoxanthine into nucleotides of intact cultured fibroblasts was markedly reduced (23% of normal). The low functioning of HPRT in the intact fibroblasts was associated with decreased utilization of endogenously generated hypoxanthine and with decreased utilization of the cosubstrate 5-phosphoribosyl-1-pyrophosphate (PRPP). The non-utilized hypoxanthine was excreted into the incubation medium. The accumulation of PRPP was indicated by the 2.3-fold increase in the rate of uptake of adenine into intact cell nucleotides and by the 7.5-fold enhancement of the rate of de novo purine synthesis. Kinetic studies of HPRT activity in fibroblast lysates revealed reduced affinity of the enzyme for PRPP (apparent K m 500 μM in comparison to 25 μM in control lysates), manifested in low activity at low (physiological), but not at high PRPP concentrations. The apparent K m for hypoxanthine was normal (23 μM in comparison to 14.2 μM in control lysates). With allopurinol treatment, our patient has had no problems since presentation, and is developing normally at 5 years of age.

Metabolism of [3alpha-3H] 25-hydroxyvitamin D2 in kidneys isolated from normal and vitamin D2-intoxicated rats.

With the availability of A-ring labelled 25OHD2, [3alpha-3H] 25OHD2, we have performed the present study to examine the metabolism of 25OHD2 using physiological substrate concentrations in perfused kidneys isolated from both normal and vitamin D2-intoxicated rats. Our results indicate that [3alpha-3H] 25OHD2 is metabolized into both 24(S),25,28-trihydroxyvitamin D2 [24(S),25,28(OH)3D2] and 24(R),25,26-trihydroxyvitamin D2 [24(R), 25,26(OH)3D2], and the amounts of these two metabolites produced in the kidney of vitamin D2-intoxicated rat were about 3-5 times higher than those produced in the kidney of normal rat. Similar results were also obtained with rat kidney homogenates incubated with [3alpha-3H] 25OHD2. Furthermore, we noted that the production of both 24(S),25,28(OH)3D2 and 24(R),25,26(OH)3D2 in the kidney homogenates of vitamin D2-intoxicated rats increased with the time of incubation and then subsequently decreased. The decrease in both 24(S),25,28(OH)3D2 and 24(R),25,26(OH)3D2 coincided with an increase in the fraction of total radioactivity distributed in the aqueous phase of the kidney homogenates. This finding suggested the possibility of further metabolism of 24(S),25,28(OH)3D2 and 24(R), 25,26(OH)3D2 into polar water-soluble metabolite(s). We then measured the radioactivity in the aqueous phase of kidney homogenates of both normal and vitamin D2-intoxicated rats incubated with [3alpha-3H] 25OHD2. It was noted that the amount of radioactivity in the aqueous phase of kidney homogenates of vitamin D2-intoxicated rats is higher than that present in the aqueous phase of kidney homogenates of normal rats. Thus, our study provides evidence for the first time for the formation of both 24(S),25,28(OH)3D2 and 24(R),25, 26(OH)3D2 under physiological conditions, and the possibility of their further metabolism into as yet unidentified polar water-soluble metabolite(s). As the formation of all these metabolites is increased in the kidney of vitamin D2-intoxicated rats when compared to normal rats, it appears that the increased rate of metabolism of 25OHD2 during hypervitaminosis D2 plays a significant role in the deactivation of 25OHD2.

Immunoassay protocol for quantitation of protein kinase activities

Quantitation of at least two orders of magnitude of kinase enzyme concentration is achieved with detection of less than 0.1 U/well of src kinase activity (Fig. 3). A comparison between a sequential protocol, in which biotinylated peptide substance is captured prior to incubation with the kinase enzyme, and a simultaneous protocol, in which peptide capture and the kinase reaction proceed concurrently, demonstrates that the simpler simultaneous protocol provides similar detection sensitivity. these have also been demonstrated with 0.1 microM peptide substrate in a protein kinase A assay.5 Quantitation of protein kinase activity with chemiluminescent detection has been demonstrated with several different protein kinases, including both tyrosine and serine/threonine kinases.5 An immunoassay format provides high sensitivity and can be performed under conditions that most closely mimic physiological substrate and ATP concentrations with chemiluminescent detection. This assay format is also automated easily for use in high-throughput screening.

The regulation of oestrone sulphate formation in breast cancer cells

The formation of oestrone sulphate has been examined in MCF-7 (oestrogen receptor positive, ER+) and MDA-MB-231 (ER negative, ER−) breast cancer cells. Using intact cell monolayers and a physiological substrate concentration, progesterone (1 μM) and dexamethasone (1 μM) both increased oestrone sulphate formation in MCF-7 cells. In MDA-MB-231 cells, dexamethasone, but not progesterone, increased conjugate formation. A number of growth factors, cytokines and human serum albumin (HSA), which have previously been found to regulate oestrogen synthesis, were also examined for their ability to regulate oestrone sulphate formation. In MCF-7 cells epidermal growth factor, acidic and basic fibroblast growth factors, insulin-like growth factor-type I and insulin all stimulated oestrone sulphate formation. The cytokines, tumour necrosis factor α (TNFα) and interleukin-1β, also increased conjugate formation in the ER+ cells, as did HSA. In contrast, in MDA-MB-231 cells TNFα was without effect and HSA inhibited oestrone sulphate formation. The ability to modulate oestrone sulphate formation in ER+ cells may be an important mechanism to limit the availability of oestrogen to interact with the ER.

Oscillations in activities of enzymes in pancreatic islet subcellular fractions induced by physiological concentrations of effectors.

Glucose, the most potent insulin secretagogue, stimulates insulin secretion by aerobic glycolysis, but other secretagogues stimulate insulin release exclusively by mitochondrial metabolism. It is well known that in the intact pancreatic beta-cell, either kind of secretagogue can induce oscillations in metabolism (e.g., glycolysis, ATP/ADP, NAD(P)/NAD(P)H ratios) that occur with a periodicity similar to oscillations in membrane electrical potential and insulin secretion. In this study, pancreatic islet cytosol or mitochondrial fractions were incubated in the presence of physiological concentrations of substrates. Repeated additions of physiological effectors caused oscillations in the activities of the three enzymes studied. Succinate dehydrogenase activity in islet mitochondrial extracts was made to oscillate by adding oxaloacetate (5 micromol/l) to inhibit the enzyme. The enzyme was reactivated by adding acetyl-CoA (3 micromol/l), which combines with oxaloacetate in the citrate synthase reaction and lowers the concentration of oxaloacetate, thus beginning another oscillation. Pyruvate kinase activity was made to oscillate by adding fructose bisphosphate (10 micromol/l). Fructose bisphosphate was degraded to triose phosphates fairly rapidly, and, as it was degraded, there was a parallel decrease in pyruvate kinase activity. The enzyme was reactivated and made to oscillate with subsequent additions of fructose bisphosphate. The mitochondrial glycerol phosphate dehydrogenase was made to oscillate by adding EGTA to chelate calcium, which activates the enzyme. When the concentration of free calcium was raised to >0.1 micromol/l by adding more calcium, the activity of the enzyme increased. Repeated additions of chelator and calcium caused the enzyme activity to oscillate. The results with these three enzymes and physiological concentrations of naturally occurring effectors raise the possibility that the activities of not only these enzymes but of numerous enzymes oscillate in vivo in response to levels of allosteric effectors and substrates. If this is the case, pacemaker activity may result from complex effects distributed across multiple regulatory sites in both the cytosol and mitochondria, rather than from a single enzyme acting as a primary pacemaker.

Dependence of H2O2 formation by rat heart mitochondria on substrate availability and donor age.

We have examined the substrate specificity and inhibitor sensitivity of H2O2 formation by rat heart mitochondria. Active H2O2 production requires both a high fractional reduction of Complex I (indexed by NADH/NAD+ + NADH ratio) and a high membrane potential, delta psi. These conditions are achieved with supraphysiological concentrations of succinate. With physiological concentrations of NAD-linked substrates, rates of H2O2 formation are much lower (less than 0.1% of respiratory chain electron flux) but may be stimulated by the Complex III inhibitor antimycin A, but not by myxothiazol. Addition of Mn2+ to give 10 nmol/mg of mitochondrial protein enhances H2O2 production with all substrate combinations, possibly by repleting mitochondrial superoxide dismutase with this cation. Contrary to previously published work, no increased activity of H2O2 production was found with heart mitochondria from senescent (24 month) rats, relative to young adults (6 month).