Showed Saturation Kinetics Research Articles

Impact of Atmospheric Sulfur Deposition on Sulfur Metabolism in Plants: H2S as Sulfur Source for Sulfur Deprived Brassica oleracea L.

Abstract:Brassica oleracea L. was rather insensitive to atmospheric H2S: growth was only negatively affected at ≥0.4 μl I−1. Shoots formed a sink for H2S and the uptake rate showed saturation kinetics with respect to the atmospheric concentration. The H2S uptake rate was high in comparison with other species, which may reflect the high sulfur need of Brassica. The net uptake of sulfate by roots of hydroponically grown plants was substantially reduced after one week of exposure to 0.25 μl l−1 H2S, indicating that plants switched in part from sulfate to H2S as sulfur source for plant growth. Plants were sulfur deficient after two weeks of sulfur deprivation, illustrated by reduced growth, which was more pronounced for shoots than for roots, and in enhanced shoot dry matter content. The latter could for the greater part be attributed to enhanced levels of soluble sugars and starch. Sulfur deficiency was further characterized by a low pigment content, extremely low levels of sulfate and water‐soluble non‐protein thiols, and by enhanced levels of nitrate and free amino acids, particularly in the shoots. Furthermore, sulfur deficient plants contained a lower total lipid content in shoots, whereas its content in roots was unaffected. The level of sulfolipids was decreased in both roots and shoots. When sulfur deprived plants were exposed to 0.25 μl I−1 H2S for one week, all sulfur deficiency symptoms were abolished and growth was restored. Furthermore, plants were able to grow with 0.4 μl I−1 H2S as the sole sulfur source. Water‐soluble non‐protein thiol content was enhanced in both shoots and roots of H2S exposed plants, irrespective of the sulfate supply to the roots, whereas plants grown with H2S as sole sulfur source contained very low sulfate levels. The interaction between atmospheric and pedospheric sulfur nutrition in plants is discussed.

Identification of the 4-hydroxyphenylacetate transport gene of Escherichia coli W : construction of a highly sensitive cellular biosensor

The mechanism of uptake of 4-hydroxyphenylacetate (4-HPA) by Escherichia coli W. was investigated. The 4-HPA uptake was induced by 4-HPA, 3-hydroxyphenylacetate (3-HPA) or phenylacetate (PA) and showed saturation kinetics with apparent K t and V max values of 25 μM and 3 nmol/min per 10 9 cells, respectively. Transport of 4-HPA was resistant to N,N″-dimethylcarbodiimide (DCCD), but was completely inhibited by cyanide and 4-nitrophenol, and, to a lower extent, by arsenate and azide, suggesting that energy is required for the uptake process. Competition studies showed that 4-HPA uptake was inhibited by 3-HPA or 3,4-dihydroxyphenylacetate (3,4-DHPA) but not by 2-hydroxyphenylacetate (2-HPA), l-tyrosine or other structural analogues, indicating a narrow specificity of the transport system. We have demonstrated, using two experimental approaches, that the hpaX gene of the 4-HPA catabolic cluster, which encodes a protein of the superfamily of transmembrane facilitators, is responsible for 4-HPA transport. Aside from the aromatic amino acid transport systems, hpaX is the first transport gene for an aromatic compound of enteric bacteria that has been characterized. A highly sensitive cellular biosensor has been constructed by coupling the 4-HPA transport system to a regulatory circuit that controls the production of β-galactosidase. This biosensor has allowed us to demonstrate that the transport system performs efficiently at very low external concentrations of 4-HPA, similar to levels that would be expected to occur in natural environments.

Concentration dependence of sodium permeation and sodium ion interactions in the cyclic AMP-gated channels of mammalian olfactory receptor neurons.

The dependence of currents through the cyclic nucleotide-gated (CNG) channels of mammalian olfactory receptor neurons (ORNs) on the concentration of NaCl was studied in excised inside-out patches from their dendritic knobs using the patch-clamp technique. With a saturating concentration (100 microM) of adenosine 3',5'-cyclic monophosphate (cAMP), the changes in the reversal potential of macroscopic currents were studied at NaCl concentrations from 25 to 300 mM. In symmetrical NaCl solutions without the addition of divalent cations, the current-voltage relations were almost linear, reversing close to 0 mV. When the external NaCl concentration was maintained at 150 mM and the internal concentrations were varied, the reversal potentials of the cAMP-activated currents closely followed the Na+ equilibrium potential indicating that PCl/PNa approximately 0. However, at low external NaCl concentrations (< or = 100 mM) there was some significant chloride permeability. Our results further indicated that Na+ currents through these channels: (i) did not obey the independence principle; (ii) showed saturation kinetics with K(m)s in the range of 100-150 mM and (iii) displayed a lack of voltage dependence of conductance in asymmetric solutions that suggested that ion-binding sites were situated midway along the channel. Together, these characteristics indicate that the permeation properties of the olfactory CNG channels are significantly different from those of photoreceptor CNG channels.

Stimulatory effect of DIMBOA on NADH oxidation catalysed by horseradish peroxidase

2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) effectively enhanced the rate of NADH oxidation catalysed by horseradish peroxidase isoenzyme C (HRPC), a reaction that generates H 2O 2. The effect showed saturation kinetics with increasing DIMBOA concentrations and the extrapolated maximum rate gave an activating factor of 246 for this hydroxamic acid. After 80% of the NADH had been oxidized, DIMBOA had converted into a species that showed absorption bands centered at 430 and 345 nm. A product with the same absorption properties was formed upon oxidation of DIMBOA with H 2O 2. The V max for the latter reaction was 3.3 times lower than the maximum rate of NADH oxidation in the presence of DIMBOA. Ferulic and p-coumaric acids were also stimulators of NADH oxidase activity of HRPC, giving saturating kinetics. Maximum rates obtained for these effectors give activating factors of 35 and 1170, respectively. The DIMBOA analogue 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) that lacks the methoxy substitution in the aromatic ring was only 16% efficient in stimulating NADH oxidase as compared with DIMBOA at the 0.1 mM level. On the other hand, the benzoxazolinones 6-methoxy-benzoxazolin-2-one (MBOA) and benxoxazolin-2-one (BOA) had negligible effect on the rate of NADH oxidation and did not alter the activating effect of DIMBOA over this reaction. © 1997 Elsevier Science Ltd. All rights reserved

Catalytic Cyclophanes. Part XI. A flavo‐thiazolio‐cyclophane as a biomimetic catalyst for the preparative‐scale electro‐oxidation of aromatic aldehydes to methyl esters

AbstractFlavo‐thiazolio‐cyclophane 6 was prepared on a gram scale by an 18‐step synthesis (Schemes 3 and 4). This pathway involved the very efficient preparation of bromo‐cyclophane 32 (37% yield over 13‐steps), which can be readily modified to create various multiply functionalized receptors. This bromide 32 was subsequently converted into the corresponding boronic acid and connected to the 7‐bromoflavin 10 (Scheme 2) via Suzuki coupling to give flavo‐cyclophane 36. The thiazolium unit was then introduced after quaternization of the tertiary amino groups of 36. Flavo‐thiazolio‐cyclophane 6, with both prosthetic groups attached in proximity to the well‐defined cyclophane binding site, is a functional model for the enzyme pyruvate oxidase. In basic methanolic solution, 6 catalyzes the oxidation of aromatic aldehydes to their corresponding methyl esters. Cyclophane 6 shows saturation kinetics, and the turnover number calculated for the oxidation of naphthalene‐2‐carbaldehyde to methyl naphthalene‐2‐carboxylate (kcat = 0.22 s−1) is one of the highest reported for an artificial enzyme. Control experiments showed that the catalytic advantages of 6 result from the macrocyclic binding and reaction site as well as from the covalent attachment of both cofactors to this site. The catalytic cycle is completed by electrochemical re‐oxidation of the reduced flavin moiety at a low working electrode potential (‐ 0.3 V vs. Ag/AgCl), and up to ca. 100 catalytic cycles can be performed on a preparative scale, The intramolecular nature of the electron transfer from the active aldehyde intermediate to the flavin is particularly conducive to the oxidation of unreactive aldehydes.

Caco-2 cell line used as an in vitro model to study cadmium accumulation in intestinal epithelial cells.

109Cd uptake was studied using the highly differentiated TC7 clone of Caco-2 cells as a model of human enterocyte function. Intracellular accumulation of 0.3 microM 109Cd involved a rapid and a slow uptake phase, which resulted in complete equilibration (t(1/2) = 17.3 +/- 1.3 min) with an apparent in-to-out distribution ratio (alphae) of 11.6 +/- 0.8. The amplitude of the rapid phase (U0) and the rate of the slow phase (V) were similarly reduced in the less differentiated PF11 clone, but comparable alphae values were observed at equilibrium. In both clones, the t(1/2) and alphae values increased and decreased, respectively, upon addition of unlabeled Cd to the uptake media. In TC7 cells, 109Cd uptake at 1 min (U1) was unaffected by Ca concentrations four order of magnitude in excess, but both U0 and V demonstrated similar sensitivities to unlabeled Cd, Zn and sulfhydryl-reactive agents. Only U0 disappeared when EDTA was present in the wash solutions. U1 showed saturation kinetics and the data were found compatible with a model assuming rapid initial Cd binding and transport through a unique transport protein (Km = 3.8 +/- 0.7 microM). Cd efflux kinetics demonstrated partial reversibility in EDTA-containing solutions, suggesting that the taken up Cd might be both tightly and loosely bound to intracellular binding sites. However, the displacement of 109Cd measured at 65 min failed to reveal this heterogeneity: the data were found compatible with a model equation assuming the presence of one class of high-capacity high-affinity binding sites. We conclude that a slow-transport fast-intracellular binding mechanism of Cd uptake best accounts for these results and that Cd transport most likely involves a carrier-type of protein unrelated to Ca absorption.

1400W Is a Slow, Tight Binding, and Highly Selective Inhibitor of Inducible Nitric-oxide Synthase in Vitro and in Vivo

N-(3-(Aminomethyl)benzyl)acetamidine (1400W) was a slow, tight binding inhibitor of human inducible nitric- oxide synthase (iNOS). The slow onset of inhibition by 1400W showed saturation kinetics with a maximal rate constant of 0.028 s-1 and a binding constant of 2.0 microM. Inhibition was dependent on the cofactor NADPH. L-Arginine was a competitive inhibitor of 1400W binding with a Ks value of 3.0 microM. Inhibited enzyme did not recover activity after 2 h. Thus, 1400W was either an irreversible inhibitor or an extremely slowly reversible inhibitor of human iNOS with a Kd value </= 7 nM. In contrast, inhibition of human neuronal NOS and endothelial NOS (eNOS) was relatively weaker, rapidly reversible, and competitive with L-arginine, with Ki values of 2 microM and 50 microM, respectively. Thus, 1400W was at least 5000-fold selective for iNOS versus eNOS. This selectivity was similar to that observed in rat aortic rings, in which 1400W was greater than 1000-fold more potent against rat iNOS than eNOS. Finally, 1400W was greater than 50-fold more potent against iNOS than eNOS in a rat model of endotoxin-induced vascular injury. Thus, the potency and selectivity of 1400W inhibition of iNOS both in vitro and in vivo were far greater than of any previously described iNOS inhibitor.

Iron uptake and intracellular metal transfer in mycobacteria mediated by xenosiderophores.

Growth promotion was tested using M. smegmatis wild type strain, an exochelin-deficient mutant, and M. fortuitum employing a broad variety of xenosiderophores including hydroxamates, catecholates and alpha-hydroxy carboxylic acids. The experiments revealed that utilization of siderophore-bound iron is substrate specific suggesting high-affinity siderophore receptor and transport systems. Concentration-dependent uptake of a selected xenosiderophore (fericrocin) in M. smegmatis showed saturation kinetics and uptake was inhibited by respiratory poisons. In situ Mössbauer spectroscopy of ferricrocin uptake in M. smegmatis indicated rapid intracellular reductive removal of the metal excluding intracellular ferricrocin accumulation. The ultimate intracellular iron pool is represented by a compound (delta = 0.43 mm s-1, delta EQ = 1.03 mm s-1) which has also been found in many other microorganisms and does not represent a bacterioferritin, cytochrome or iron-sulfur cluster. By contrast, iron uptake via citrate-a compound exhibiting a very low complex stability constant-involves ligand exchange with mycobactin. Mycobactin has merely a transient role. The ultimate storage compound is an E. coli-type bacterioferritin, in which over 90% of cellular iron is located.

Arylimido complexes of ruthenium(IV) porphyrins

Treatment of [Ru(tbpp)O 2 ] [H 2 tbpp = 5,10,15,20-tetrakis(p-tert -butylphenyl)porphyrin] with SiMe 3 Cl gave [Ru(tbpp)Cl 2 ] in good yield. Reaction of [Ru(tbpp)Cl 2 ] with para-substituted anilines NRH 2 (R = p-XC 6 H 4 where X = Me, H, Cl or I) afforded the first arylimidoruthenium(IV) complexes [Ru(tbpp)(NR)]. These are paramagnetic with µ eff ca. 2.8 µ B and display 1 H NMR spectra that are typical for paramagnetic ruthenium(IV) porphyrins. The cyclic voltammograms of [Ru(tbpp)(NR)] exhibit reversible Ru V –Ru IV and Ru IV –Ru III couples. Treatment of [Ru(tbpp)(NR)] with Ag I or Ce IV afforded the imidoruthenium(V) complex [Ru(tbpp)(NR)] + . The complexes [Ru(tbpp)(NR)] underwent imido-group transfer reactions with tertiary phosphines to give [Ru(tbpp)(PR′ 3 ) 2 ] and RNPR 3 ′. The reduction of [Ru(tbpp)(NR)] by PMe 2 Ph shows saturation kinetics, in which the rate is first order in [Ru IV ]. The mechanism proposed for the Ru-mediated imido transfer involves reversible binding of phosphine to Ru IV and rate-limiting intramolecular imido-group transfer. The first-order rate constant (k 1 ) and phosphine binding constant (K) and for the reduction of [Ru(tbpp)(NC 6 H 4 Me-p)] by PMe 2 Ph at 25.0 °C in toluene solution were determined to be (6.86 ± 0.19) × 10 -4 s -1 and (23.6 ± 6.5) × 10 3 mol dm -3 , respectively. The activation enthalpy (ΔH ‡ ) and entropy (ΔS ‡ ) for the above reaction are 125 ± 1 kJ mol -1 and 113 ± 21 J K -1 mol -1 , respectively. For the reduction of para-X-substituted arylimido complexes [Ru(tbpp)(NC 6 H 4 X-p)] by tertiary PMe 2 Ph the rate decreases in the order X = I > H ≈ Cl > Me. The imido transfer from [Ru V (tbpp)(NC 6 H 4 Me-p)] to PMe 2 Ph is about 60 times faster than that from [Ru IV (tbpp)(NC 6 H 4 Me-p)].

Polysaccharides as drug carriers: biodisposition of fluorescein-labeled dextrans in mice.

The biodisposition of fluorescein-labeled dextrans (FDs) with different molecular weights (MW = 4-500 kDa) was systematically examined in mice. After intravenous injection of FDs at a dose of 120 mg/kg, the levels of FDs in the blood circulation and in the various organs were measured fluorometrically. FDs with a molecular weight lower than 20 kDa showed poor hepatic distribution (2.1-3.7% of dose/g tissue) due to their rapid elimination from the blood circulation. FDs with higher molecular weights were appreciably distributed in the liver (18.9-24.0% of dose/g tissue) and accumulated there over a long period, whereas the FD levels in the other organs were almost negligible a few days after injection. The hepatic mean residence time of FDs ranged from 22.5 to 28.1 d. Partial depolymerization of FDs which accumulated in the liver was observed within 10 d after administration. The hepatic uptake clearance of FDs was decreased with an increase in molecular weight. A marked molecular weight dependency was also seen in the urinary and fecal excretions of FDs. An appreciable dose-dependency was demonstrated in the hepatic uptake of FDs (MW = 40 kDa), as well. The amount of hepatic uptake as a function of dose showed saturation kinetics and was analyzed by a Michaelis-Menten type equation. The apparent values of K(m) (dose) and Vmax (hepatic level) estimated were 116 +/- 5 mg/kg and 1.10 +/- 0.05 mg/g tissue, respectively.

In vitro evaluation of iodine-125-labeled monoclonal antibody (MAb 425) in human high-grade glioma cells.

Human high-grade glioma cell lines (A1207, U-87MG, U-373MG, and F39) with high levels of epidermal growth factor receptor (EGF-R) expression were incubated for 2-48 h with 1 microCi/ml of the EGF-R-specific 125I-MAb 425 and measured for surface-bound, cytoplasmic, and nuclear radioactivity. The A1207 and U-373MG cell lines showed the highest surface-bound radioactivity with 215.9 +/- 8.7 nCi (30 h) and 287.8 +/- 23.2 nCi (24 h)/10(6) cells, respectively, whereas the U-87MG and the F39 cell lines bound significantly less antibody (48.8 +/- 5.4 nCi [48 h] and 31.1 +/- 0.7 nCi [24 h]). Surface-bound antibody was efficiently internalized into the cytoplasm. The U-373MG, U-87MG, and A1207 cell lines achieved 19.8% +/- 2.1 internalization of the surface-bound antibody in contrast to > 40% for the F39 cell line. Only the A1207 cell line showed significant nuclear radioactivity. There was no correlation between the reported EGF-R number and amount of antibody bound or internalized. We conclude that binding and uptake of the 125I-MAb 425 is specific for human glioma cells and shows saturation kinetics independent of receptor density.

Inhibition of the efflux of glutathione S-conjugates by plant polyphenols

The formation of dinitrophenylglutathione (DNP-SG) in human colon adenocarcinoma cells was identified and quantified by an HPLC-UV method, following exposure to 1-chloro-2,4-dinitrobenzene (CDNB) at 10 ° for 40 min. The rate of efflux of DNP-SG at 37 ° likewise, was measured by monitoring the DNP-SG content in the extracellular medium. Among the polyphenols examined for their action on DNP-SG export, butein was the most potent inhibitor with an IC 50 value of 15 μM. The others, in order of decreasing potencies, were quercetin, tannic acid, 2′-hydroxychalcone, 2-hydroxychalcone and morin, all of which have IC 50 values in the micromolar range. These polyphenols did not affect the ATP or the glutathione content of the cells. Mg 2+-ATPase extracted from the plasma membrane of the cells was activated by DNP-SG in a concentration-dependent manner, and the reaction showed saturation kinetics with K m and V max values of 110 μM and 12.3 nmol/min/mg protein, respectively. However, the six polyphenols mentioned above had negligible effects on the Mg 2+-ATPase activity, suggesting that this was probably not the target of their inhibitory action. Probenecid, p-trifluoromethoxy-phenylhydrazone (FCCP) and chlorambucil also showed varying degrees of inhibition of the export of DNP-SG.

Kidney N-Acetylgalactosamine (GalNAc)-1-phosphate Kinase, a New Pathway of GalNAc Activation

A new enzyme that phosphorylates GalNAc at position 1 to form GalNAc-alpha-1P was purified approximately 1275-fold from the cytosolic fraction of pig kidney, and the properties of the enzyme were determined. The kinase is quite specific for GalNAc as the phosphate acceptor and is inactive with GlcNAc, ManNAc, glucose, galactose, mannose, GalN, and GlcN. This enzyme is clearly separated from galactokinase by chromatography on phenyl-Sepharose. The GalNAc kinase has a pH optimum between 8.5 and 9.0 and requires a divalent cation in the order Mg2+ > Mn2+ > Co2+, with optimum Mg2+ concentration at approximately 5 mM. The enzyme was most active with ATP as the phosphate donor, but slight activity was observed with ITP, acetyl-P, and phosphoenolpyruvate. Enzyme activity was highest in porcine and human kidney and porcine liver, but was low in most other tissues. Cultured HT-29 cells also had high activity for this kinase. The purified enzyme fraction was incubated with azido-[32P]ATP, exposed to UV light, and run on SDS gels. A 50-kDa protein was labeled, and this labeling showed saturation kinetics with increasing amounts of the probe and was inhibited by unlabeled ATP. Although the most purified GalNAc kinase preparation still had two bands that labeled with ATP, maximum labeling of the 50-kDa protein, but not the 66-kDa band, was coincident with maximum GalNAc kinase activity on a column of DEAE-Cibacron blue. On Sephacryl S-300, the native enzyme has a molecular mass of 48-51 kDa, indicating that the active kinase is a monomer. The product of the reaction was characterized as GalNAc-alpha-1-P by various chemical procedures.

Pravastatin transport across the hepatocyte canalicular membrane requires both ATP and a transmembrane pH gradient.

Hepatic excretion of non-bile acid organic anions is reported to be ATP-dependent and a defect of this transport has been reported in congenitally jaundiced rats, animal models of human Dubin-Johnson syndrome. To investigate the effect of the transmembrane pH gradient on hepatocyte canalicular membrane transport of ATP-dependent organic anions, uptake of pravastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase-inhibiting organic anion, by hepatocyte canalicular membrane vesicles was observed in the presence or absence of transmembrane pH gradients. Uptake was assessed by a rapid filtration technique. ATP-dependent pravastatin uptake was stimulated in the presence of a transmembrane pH gradient (in > out) in Sprague-Dawley (SD) rats. Uptake was dependent on both pravastatin and ATP concentrations and showed saturation kinetics. After intravenous injection of [14C]-pravastatin (0.3 mumol), 81% of the dose was excreted in the bile within 35 min in SD rats, whereas only 20% was excreted in the bile in Eisai hyperbilirubinuria rats. ATP and the pH gradient also co-stimulated the uptake of pravastatin in Eisai hyperbilirubinuria rats, although the K(m) was much higher and Vmax was much lower than corresponding values in SD rats. This coincided well with the marked reduction in vivo biliary excretion of pravastatin in jaundiced rats.

A dopamine transporter in human erythrocytes: modulation by insulin.

Red blood cells are capable of transport and accumulation of catecholamines. The aim of this paper is to characterize the catecholamine transport system in the human red blood cell and in particular that of dopamine. Dopamine, noradrenaline and adrenaline enter the red blood cell by a similar process, which shows saturation kinetics with Vmax values of 0.54 +/- 0.12, 0.48 +/- 0.08 and 0.63 +/- 0.13 mumol (1 cells)-1 min-1, respectively, and K(m) values of 15.62 +/- 1.19, 5.81 +/- 1.19 and 12.00 +/- 2.97 nM, respectively. Observations based on the dependence of dopamine influx on the transmembrane H+ gradient, and the effect of transport inhibitors such as DMA (dimethyl-amiloride), DIDS (4,4'-diisothiocyanatostilbene 2,2'-disulphonic acid), reserpine, GBR 12909 (1-(2-(di(4-fluoro-phenyl)-methoxy)-ethyl)-4-(3-phenylpropyl)piperazine) , GBR 12935 (1-(2-(diphenyl-methoxy)-ethyl)-4-(3-phenyl-propyl)piperazine), and cyanine suggest that catecholamine transport is not mediated by the Na(+)-H+ exchanger, the anion exchanger or a system similar to that responsible for dopamine uptake in either synaptosomes or the proximal tubule. However, choline inhibits the influx of dopamine with an IC50 value of 17 microM and stimulates the efflux of dopamine with a K(m) value of 8.20 microM. These results strongly suggest that dopamine is transported by the choline exchanger previously reported to be present in red blood cells. Probenecid inhibits dopamine uptake with an IC50 of 0.63 microM. The presence of insulin receptors in human red blood cells, and the relationship between insulin and catecholamine levels in the plasma led us to investigate the effect of insulin on catecholamine transport. In fasting subjects, dopamine, adrenaline and noradrenaline influxes were higher than in fed subjects. Furthermore, the addition of exogenous insulin to red blood cells from fasting subjects significantly reduced the influx of catecholamines while no effect was observed when insulin was added to red blood cells obtained from fed subjects. The present study shows that catecholamines, and in particular dopamine, are transported in red blood cells via an exchanger which is possibly the choline transport system. The activity of this transporter is regulated by insulin. These results support a role for red blood cells as a storage pool for circulating catecholamines.

Kinetic characterization of antibody-catalyzed insertion of a metal ion into porphyrin.

The insertion of a copper (II) ion into mesoporphyrin by a monoclonal catalytic antibody has been investigated kinetically by measuring the increase in Soret absorbance due to the production of copper (II)-mesoporphyrin. The initial rate of the reaction showed saturation kinetics as a function of the mesoporphyrin concentration, while it increased linearly with an increase in the copper (II) concentration. Based on observations, a scheme for the reaction was proposed: mesoporphyrin binds to the antibody to form a complex, and copper (II) binds to the complex to yield copper (II)-mesoporphyrin. Kinetic parameters for the respective steps were estimated, and the thermodynamic parameters were calculated. The binding of mesoporphyrin to the antibody was endothermic and entropically driven. This implies that hydrophobic interactions are an important factor in the binding. Free energy profiles for the antibody-catalyzed and uncatalyzed reactions were drawn by use of the obtained thermodynamic parameter values. The results demonstrate that the rate acceleration by, the antibody is ascribable to transition-state stabilization, and suggest that the structure of mesoporphyrin in the complex is more distorted than that of free mesoporphyrin.

Adeno-Associated Virus Type 2 Binds to a 150-Kilodalton Cell Membrane Glycoprotein

Early events, including the identification of the cellular receptor, have not yet been described for adeno-associated virus (AAV) infection. In this study, the binding characteristics of AAV-2 to human cells were examined in two different assays. In a liquid-phase assay, in which binding of biotinylated virus to cells in suspension was measured, AAV-2 showed specific binding to four different permissive cell lines: HeLa S3, 293, D6, and KB cells. In contrast, AAV-2 binding to erythrocytes or to the nonpermissive cell line UT-7/Epo was negligible. AAV-2 binding showed saturation kinetics. Both binding and infectivity of AAV-2 were abolished by trypsin treatment of cells, with significant recovery of bindings after 8 hr of culture, suggesting that virus attachment occurs through a protein that can be regenerated on the cell surface. In a second, virus overlay assay, we assessed binding of [35S]methionine-labeled AAV-2 to membrane proteins that had been transferred to nitrocellulose after electrophoretic separation. In this assay, virus attachment was shown to a 150-kDa protein. This protein was present in membranes from the AAV-2 permissive cell lines but not detected in membranes from erythrocytes or UT-7/Epo cells. Enzymatic deglycosylation studies suggested that N-linked glycans are required for AAV-2 binding. A 150-kDa glycoprotein might serve as the cellular receptor for AAV-2.

Species differences in hepatic bile acid uptake: Comparative evaluation of taurocholate and tauroursodeoxycholate extraction in rat and rabbit

Dose-response curves for taurocholate and tauroursodeoxycholate were performed in rat and rabbit livers to get more insight into species differences in the hepatic bile acid uptake. The bile acids showed saturation kinetics in both animals, the V max in rat being higher than in rabbit and the K m being lower in the rat than in the rabbit for both the bile acids, with no significant difference in the hepatic cells morphometric parameters. Therefore, it is possible that differences in the kinetic parameters are related to the number and, to a lesser extent, to the affinity of the transporters on the sinusoidal plasma membranes.

Molecular recognition by cholesterol esterase of active site ligands: structure-reactivity effects for inhibition by aryl carbamates and subsequent carbamylenzyme turnover.

Interactions of mammalian pancreatic cholesterol esterases from pig and rat with a family of aryl carbamates CnH2n+1NHCOOAr [n = 4-9; Ar = phenyl, p-X-phenyl (X = acetamido, bromo, fluoro, nitro, trifluoromethyl), 2-naphthyl, 2-tetrahydronaphthyl, estronyl] have been investigated, with an aim of delineating the ligand structural features which lead to effective molecular recognition by the active site of the enzyme. These carbamates inhibit the catalytic activity of CEase by rapid carbamylation of the active site, a process that shows saturation kinetics. Subsequent slow decarbamylation usually leads to full restoration of activity, and therefore aryl carbamates are transient inhibitors, or pseudo-substrates, of CEase. Structural variation of carbamate inhibitors allowed molecular recognition in the fatty acid binding and steroid binding loci of the extended active site to be probed, and the electronic nature of the carbamylation transition state to be characterized. Optimal inhibitory activity is observed when the length of the carbamyl function is n = 6 and n = 7 for porcine and rat cholesterol esterases, respectively, equivalent to eight- and nine-carbon fatty acyl chains. In contrast, inhibitory activity increases progressively as the partial molecular volume of the aromatic fragment increases. Hammett plots for p-substituted phenyl-N-hexyl carbamates indicate that the rate-determining step for carbamate inhibition is phenolate anion expulsion. Effects of the bile salt activator taurocholate on the kinetically resolved phases of the pseudo-substrate turnover of aryl carbamates were also studied. Taurocholate increases the affinity of the carbamate for the active site of cholesterol esterase in the reversible, noncovalent complex that precedes carbamylation and increases the rate constants of the serial carbamylation and decarbamylation steps. Structural variation of the N-alkyl chain and of the aryl fused-ring system provides an accounting of bile salt modulation of the fatty acid and steroid binding sites, respectively. In that pseudo-substrate turnover of aryl carbamates proceeds by a three-step mechanism that is analogous to that for rapid turnover of lipid ester substrates, these investigations illuminate details of ligand recognition by the extended active site of cholesterol esterase that are prominent determinants of the substrate specificity and catalytic power of the enzyme.

Synthesis of sn-1,2-diacylglycerols by monoacylglycerol acyltransferase from Manduca sexta fat body.

The pathway for the synthesis of sn-1,2-diacylglycerol stimulated by the action of adipokinetic hormone (AKH) in the insect fat body is unknown. Previous results from this laboratory suggested that the hydrolysis of stored triacylglycerol to sn-2-monoacylglycerol followed by the stereospecific acylation of sn-2-monoacylglycerol catalyzed by a monoacylglycerol-acyltransferase (MGAT) could be the major route of AKH-stimulated sn-1,2-diacylglycerol synthesis. Thus, MGAT might represent a key enzyme of this pathway. In this study we characterized the MGAT activity from the Manduca sexta fat body. The activity, which was assayed by acylation of 2-monoolein using radioactive labeled palmitoyl-CoA, was found to be primarily a microsomal enzyme. The products of the acylation of 2-monoolein were 1,2-diacylglycerol (40-50%), 1,3-diacylglycerol (20-30%), and triacylglycerol (30-40%). The presence of triacylglycerol as a product revealed the presence of diacylglycerol-acyltransferase activity in the fat body microsomes. The pH optimum of MGAT activity was 7.0, and the dependence of the activity on the concentration of 2-monoolein showed saturation kinetics. An endogenous MGAT activity, which represented 20% of the maximal activity observed with added substrate, was detected. Optimal concentrations of palmitoyl-CoA ranged between 0.10-0.20 mM. The specific activity of MGAT, measured under optimal conditions, was about 0.6 nmol DG formed/min-mg protein. MGAT activity was greatest with 2-monoolein, and lower activity was observed when a saturated 2-monoacylglycerol was employed. The activity observed with sn-1-monoacylglycerol was lower than that observed with sn-2-monoacylglycerol. AKH did not stimulate MGAT activity, suggesting that either the enzyme is not under hormonal regulation or the monoacylglycerol pathway is not involved in the AKH-stimulated production of sn-1,2-diacylglycerol in the M. sexta fat body.