Kinetic analysis of the slow ionization of glutathione by microsomal glutathione transferase MGST1

An important aspect of the catalytic mechanism of microsomal glutathione transferase (MGST1) is the activation of the thiol of bound glutathione (GSH). GSH binding to MGST1 as measured by thiolate anion formation, proton release, and Meisenheimer complex formation is a slow process that can be described by a rapid binding step (K(GSH)d = 47 +/- 7 mM) of the peptide followed by slow deprotonation (k2 = 0.42 +/- 0.03 s(-1). Release of the GSH thiolate anion is very slow (apparent first-order rate k(-2) = 0.0006 +/- 0.00002 s(-)(1)) and thus explains the overall tight binding of GSH. It has been known for some time that the turnover (kcat) of MGST1 does not correlate well with the chemical reactivity of the electrophilic substrate. The steady-state kinetic parameters determined for GSH and 1-chloro-2,4-dinitrobenzene (CDNB) are consistent with thiolate anion formation (k2) being largely rate-determining in enzyme turnover (kcat = 0.26 +/- 0.07 s(-1). Thus, the chemical step of thiolate addition is not rate-limiting and can be studied as a burst of product formation on reaction of halo-nitroarene electrophiles with the E.GS- complex. The saturation behavior of the concentration dependence of the product burst with CDNB indicates that the reaction occurs in a two-step process that is characterized by rapid equilibrium binding ( = 0.53 +/- 0.08 mM) to the E.GS- complex and a relatively fast chemical reaction with the thiolate (k3 = 500 +/- 40 s(-1). In a series of substrate analogues, it is observed that log k3 is linearly related (rho value 3.5 +/- 0.3) to second substrate reactivity as described by Hammett sigma- values demonstrating a strong dependence on chemical reactivity that is similar to the nonenzymatic reaction (rho = 3.4). Microsomal glutathione transferase 1 displays the unusual property of being activated by sulfhydryl reagents. When the enzyme is activated by N-ethylmaleimide, the rate of thiolate anion formation is greatly enhanced, demonstrating for the first time the specific step that is activated. This result explains earlier observations that the enzyme is activated only with more reactive substrates. Taken together, the observations show that the kinetic mechanism of MGST1 can be described by slow GSH binding/thiolate formation followed by a chemical step that depends on the reactivity of the electrophilic substrate. As the chemical reactivity of the electrophile becomes lower the rate-determining step shifts from thiolate formation to the chemical reaction.     

Activation of rat liver microsomal glutathione transferase by limited proteolysis.

The activity of rat liver microsomal glutathione transferase is increased by limited tryptic proteolysis; the membrane-bound and purified forms of the enzyme are activated about 5- and 10-fold respectively. The cleavage sites that correlate with this activation were determined by amino acid sequence analysis to be located after Lys-4 and Lys-41. Differences in the relative extent of cleavage at these two sites did not consistently affect the degree of activation. Thus the data support the conclusion that cleavage at either site results in activation. The trypsin-activated enzyme was compared with the form activated with N-ethylmaleimide, which modifies Cys-49. These two differently activated forms were found to have similar kinetic parameters, which differ from those of the unactivated enzyme. The relatedness of the two types of activation is also demonstrated by the observation that microsomal glutathione transferase fully activated by N-ethylmaleimide is virtually resistant to further activation by trypsin. This is the case despite the fact that the N-ethylmaleimide-activated enzyme is much more susceptible to trypsin cleavage at Lys-41 than is the untreated enzyme. The latter observation indicates that activation with N-ethylmaleimide is accompanied by a conformational change involving Lys-41.     

Kinetic properties of microsomal UDP-glucuronyltransferase. Regulation by metal ions

Treatment of microsomes with EDTA abolishes the stimulation of glucuronidation produced by UDP-N-acetylglucosamine. Addition of divalent metal ions to EDTA-treated microsomes restores the sensitivity of UDP-glucuronyltransferase to UDP-N-acetylglucosamine. Regulation of the activity of this enzyme by UDP-N-acetylglucosamine depends, therefore, on the presence of divalent metal ions. In addition, divalent metals increase the rate of glucuronidation of p-nitrophenol at V_max. The data indicate that metals are essential for the efficient function of UDP-glucuronyltransferase.     

Kinetic characterization of thiolate anion formation and chemical catalysis of activated microsomal glutathione transferase 1

Microsomal glutathione transferase 1 (MGST1) displays the unique ability to be activated, up to 30-fold, by the reaction with sulfhydryl reagents, e.g., N-ethylmaleimide. Analysis of glutathione (GSH) thiolate formation, which occurs upon mixing activated MGST1 with GSH, reveals biphasic kinetics, where the rapid phase dominated at higher GSH concentrations. The kinetic behavior suggests a two-step mechanism consisting of a rapid GSH-binding step (K(D)(GSH) approximately 10 mM), followed by slower formation of thiolate (k(2) approximately 10 s(-1)). The release rate (or protonation of the enzyme GSH thiolate complex) of GS(-) was slow (k(-2) = 0.016 s(-1)), consistent with overall tight binding of GSH. Electrophilic second substrates react rapidly with the E*GS(-) complex, and again, a two-step mechanism is suggested. In comparison to the unactivated enzyme [Morgenstern et al. (2001) Biochemistry 40, 3378-3384], the mechanisms of GSH thiolate formation and electrophile interaction are similar; however, thiolate anion formation is enhanced 30-fold in the activated enzyme, contributing to an increased k(cat) (3.6 s(-1)). Interestingly, in the activated enzyme, thiolate formation and proton release from the enzyme are not strictly coupled, because proton release (as well as k(cat)) was found to be approximately 4 times slower than GSH thiolate formation in an unbuffered system. Solvent kinetic isotope effect measurements demonstrated a 2-fold decrease in the rate constant (k(2)) for thiolate formation and k(cat) (in the reaction with 1-chloro-2,4-dinitrobenzene) for both unactivated and activated MGST1. This indicates that thiolate formation contributes to k(cat) for the activated enzyme, as suggested previously for unactivated MGST1. The stoichiometry of thiolate formation, proton release, and burst kinetics suggested utilization of one GSH molecule per enzyme trimer.     

Kinetic properties and solubilization of microsomal cholesterol ester hydrolase from rat liver

Some kinetic properties of the microsomal cholesterol ester hydrolase (CEH) have been examined in rat liver. The reaction was linear with time up to 60 min and with enzyme concentration up to 0.3 mg/mL, and a pH optimum of 6.7 for enzyme activity was observed. Cholesterol esterase exhibited the following apparent kinetic constants: Km, 68.88 microM and Vmax, 33 Units/mg protein. Cholesteryl palmitate was hydrolyzed to a much greater extent than cholesteryl oleate by the enzyme. Product inhibition with cholesterol and palmitic acid was not apparent; however, oleic acid added to the system reduced markedly microsomal CEH activity. The present paper also reports the solubilization of cholesteryl palmitate hydrolase from the microsomal fraction by pretreating it with Triton X-100, sodium deoxycholate, and sodium dodecylsulfate. All ionic and non-ionic detergents tested are capable of making the enzyme soluble, and maximal effects were found at higher concentrations of detergents although the esterase activity was strongly inhibited. Triton X-100 was found to be more effective than sodium deoxycholate and sodium dodecylsulfate in enzyme and protein solubilization. When the direct effects of detergents on CEH activity were studied, progressive concentration-dependent inhibitions were observed.     

Kinetic properties of the Ca2+-accumulation system of a rat liver microsomal fraction.

1. By using Ca-EGTA buffers, the Km for Ca2+ uptake into rat liver heavy microsomes (microsomal fraction) was found to be 0.2 microM free Ca2+. 2. In the absence of oxalate, these vesicles accumulate about 20 nmol of Ca2+/mg of protein. Efflux of Ca2+ from the vesicles is much faster at pH 7.6 than at pH 6.8, but does not apparently show saturation kinetics or any stringent requirement for external ions. 3. The steady-state distribution of Ca2+ between the microsomes and the medium in the presence of ATP and the absence of oxalate is dependent on Ca2+ load. When the vesicles are loaded to 50% capacity, the external free Ca2+ concentration is 70 nM. 4. The affinity of heavy microsomes for Ca2+ is such that is seems likely that they has a dominant role in the determination of cytoplasmic free Ca2+ concentrations.     

Inhibition of CDP-DG: inositol transferase by inostamycin.

Inostamycin, a novel microbial secondary metabolite, inhibited [3H]inositol and 32P1 incorporation into phosphatidylinositol (PtdIns) induced by epidermal growth factor (EGF) in cultured A431 cells, the IC50 being 0.5 micrograms/ml, without inhibiting macromolecular synthesis. The drug inhibited cellular inositol phosphate formation only when it was added at the same time as labeled inositol. It was found to inhibit in vitro CDP-DG:inositol transferase activity of the A431 cell membrane, the IC50 being about 0.02 micrograms/ml. It did not inhibit tyrosine kinase, PtdIns phospholipase C, or PtdIns kinase. Therefore, inhibition of PtdIns turnover by inostamycin must be due to the inhibition of CDP-DG:inositol transferase. Thus, inostamycin is a novel inhibitor of CDP-DG:inositol transferase.     

Inhibition of rat testicular microsomal steroidogenesis by oxytocin and metyrapone

Capillary gas chromatographic 'steroid profiling' has been utilised to separate and quantify the metabolites (derivatized as methyloximes and/or trimethylsilyl ethers) formed from pregnenolone after incubation with rat testicular microsomes. A wide range of steroid metabolites was found, indicating that both the 5-ene and 4-ene pathways of testosterone biosynthesis were operating, as well as 16 alpha-hydroxylation, 20 beta-reduction and the formation of several C19 steroids (the 16-androstenes). At the concentration used, Metyrapone markedly inhibited 16 alpha- and 17-hydroxylation and side-chain cleavage of 17-hydroxylated C21 steroids. 16-Androstene production was also markedly inhibited and the formation of other metabolites was affected to lesser extents. Oxytocin abolished the formation of all C21 and C19 metabolites of pregnenolone.     

UDP-glucuronosyl transferase and the conjugation of benzo(a)pyrene metabolites to DNA.

Addition of UDP-glucuronic acid to microsomal incubations containing benzo(a)pyrene caused a dose-dependent conjugation of principally quinone and phenol metabolites. Total benzo(a)pyrene oxidation was also stimulated with a maximum increase at 2 nM UDPGA. In the presence of calf thymus DNA, UDPGA caused a 2.7-fold increase in benzo(a)pyrene diol-oxide modification of DNA, as analyzed by Sephadex LH-20 chromatography. Maximum DNA modification by diol-oxides occurred at a UDPGA concentration which gave the highest level of free benzo(a)pyrene 7,8-dihydrodiol; likewise, the amount of DNA adduct derived from benzo(a)pyrene phenols declined in parallel with levels of free phenol metabolites. The UDPGA-induced increase in benzo(a)pyrene oxidation and concomitant increase in diol-oxide modification of DNA is consistent with removal of product inhibition by glucuronide conjugation of an inhibitory benzo(a)pyrene metabolite.     

Kinetic independence of the subunits of cytosolic glutathione transferase from the rat.

The steady-state kinetics of the dimeric glutathione transferases deviate from Michaelis-Menten kinetics, but have hyperbolic binding isotherms for substrates and products of the enzymic reaction. The possibility of subunit interactions during catalysis as an explanation for the rate behaviour was investigated by use of rat isoenzymes composed of subunits 1, 2, 3 and 4, which have distinct substrate specificities. The kinetic parameter kcat./Km was determined with 1-chloro-2,4-dinitrobenzene, 4-hydroxyalk-2-enals, ethacrynic acid and trans-4-phenylbut-3-en-2-one as electrophilic substrates for six isoenzymes: rat glutathione transferases 1-1, 1-2, 2-2, 3-3, 3-4 and 4-4. It was found that the kcat./Km values for the heterodimeric transferases 1-2 and 3-4 could be predicted from the kcat./Km values of the corresponding homodimers. Likewise, the initial velocities determined with transferases 3-3, 3-4 and 4-4 at different degrees of saturation with glutathione and 1-chloro-2,4-dinitrobenzene demonstrated that the kinetic properties of the subunits are additive. These results show that the subunits of glutathione transferase are kinetically independent.     

The lecithin-cholesterol acyl transferase activity of rat intestinal lymph.

The lecithin-cholesterol acyl transferase (LCAT) activity in rat mesenteric lymph was examined as a possible source of chylomicron cholesteryl ester. Lymph activity was only 2-3% of rat serum activity. Removal of d less than 1.006 lipoproteins increased lymph LCAT activity, but only to 6-8% of that of serum. Relative to total cholesterol in the d greater than 1.08 g/ml fractions, lymph LCAT activity in lymph from fasting rats was less than serum, but in lymph from nonfasting rats the ratio LCAT/HDL-cholesterol reached levels greater than serum, suggesting a contribution of enzyme from the gut. Both LCAT activity and HDL concentration in mesenteric lymph increased during feeding. Subfractions of lymph that inhibited serum LCAT were: chylomicrons, VLDL, chylomicron lipid, VLDL apoprotein, and HDL apoprotein. In the rat, the low LCAT activity of mesenteric lymph was in part due to the low enzyme concentration present, and the activity was apparently lowered further by lipid-rich lipoproteins that inhibited the reaction. Enzyme inhibition due to the apoprotein fractions of lipoproteins is probably minor in the rat in vivo.     

Protection of cells from oxidative stress by microsomal glutathione transferase 1

Rat liver microsomal glutathione transferase 1 (MGST1) is a membrane-bound enzyme that displays both glutathione transferase and glutathione peroxidase activities. We hypothesized that physiologically relevant levels of MGST1 is able to protect cells from oxidative damage by lowering intracellular hydroperoxide levels. Such a role of MGST1 was studied in human MCF7 cell line transfected with rat liver mgst1 (sense cell) and with antisense mgst1 (antisense cell). Cytotoxicities of two hydroperoxides (cumene hydroperoxide (CuOOH) and hydrogen peroxide) were determined in both cell types using short-term and long-term cytotoxicity assays. MGST1 significantly protected against CuOOH and against hydrogen peroxide (although less pronounced and only in short-term tests). These results demonstrate that MGST1 can protect cells from both lipophilic and hydrophilic hydroperoxides, of which only the former is a substrate. After CuOOH exposure MGST1 significantly lowered intracellular ROS as determined by FACS analysis.     

CDP-diglyceride:inositol transferase from rat liver. Purification and properties.

CDP-diglyceride:inositol transferase, which catalyzes the final step of the de novo synthesis of phosphatidylinositol, was solubilized by sodium cholate from microsomes prepared from rat liver and purified by ammonium sulfate fractionation, sucrose density gradient centrifugation, and DEAE-cellulose column chromatography. Addition of phospholipid during the purification and the assay procedures prevented irreversible loss of the enzyme activity to some extent. The resulting preparation was nearly homogeneous as judged by polyacrylamide gel electrophoresis. The recovery of the purified enzyme from the microsomal fraction was 3 to 3.3% with respect to activity and 0.12% with respect to amount of protein. The molecular weight of the enzyme was estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to be 60,000. The purified enzyme required exogenous phospholipds for its activity. Various phospholipid classes activated the enzyme rather nonspecifically. The Km for myo-inositol was 2.5 X 10(-3) M and that for CDP-diglyceride was 1.7 X 10(-4) M. The pH optimum was 8.6. The enzyme required Mm2+ or Mg2+ for activity. The optimal concentration of Mn2+ for activation was 0.5 mM, while the activity in the presence of Mg2+ increased up to 20 mM. The enzyme was inhibited by thiol-reactive reagents. There was a competition for inositol by inosose-2 but not by scyllitol.     

Functional and structural membrane topology of rat liver microsomal glutathione transferase

The membrane topology of rat liver microsomal glutathione transferase was investigated by comparing the tryptic cleavage products from intact and permeabilized microsomes. It was shown that lysine-4 of microsomal glutathione transferase is accessible at the luminal surface of the endoplasmic reticulum, whereas lysine-41 faces the cytosol. These positions are separated by a hydrophobic stretch of 25 amino acids (positions 11–35) which comprises the likely membrane-spanning region. Reaction of cysteine-49 of the microsomal glutathione transferase with the charged sulfhydryl reagent DTNB (2,2′-dithiobis(5-nitrobenzoic acid))) in intact microsomes further supports the cytosolic localization of this portion of the polypeptide chain. The role of two other potential membrane-spanning/associated segments in the C-terminal half of the polypeptide chain was examined by investigating the association of the protein to the membrane after trypsin cleavage at lysine-41. Activity measurements and Western blot analysis after washing with high concentrations of salt, as well as after phase separation in Triton X-114, indicate that this portion of the protein also binds to the membrane. It is also shown that cleavage of the purified protein at Lys-41 and subsequent separation of the fragments obtained yields a functional C-terminal polypeptide with the expected length for the product encompassing positions 42–154. The location of the active site of microsomal glutathione transferase was investigated using radiolabelled glutathione together with a second substrate. Since isolated rat liver microsomes do not take up glutathione or release the glutathione conjugate into the lumen, it can be concluded that the active site of rat liver microsomal glutathione transferase faces the cytosolic side of the endoplasmic reticulum.     

Effect of nucleotides on UDP-N-acetylglucosamine pyrophosphatase and N-acetylglucosaminyltransferase activities in microsomal membranes.

Rat liver microsomes solubilized by incubating with lysolecithin or Triton X-100 showed very active UDP-N-acetylglucosamine pyrophosphatase activity leading to the hydrolysis of the substrate into N-acetylglucosamine-P and N-acetylglucosamine. ATP, GTP, CDPcholine, and CDPglucose exerted a considerable inhibitory effect on the solubilized membrane pyrophosphatase activity. CDPcholine and CDPglucose, in addition, appeared to stimulate the transfer of N-acetylglucosamine into endogenous and exogenous acceptor proteins. Evidence is also presented of an inhibitory effect of ATP (and to some extent GTP) on N-acetylglucosaminyltransferase activity. This inhibitory effect of ATP and GTP became clearly evident when the pyrophosphatase activity in the membranes was virtually eliminated in the presence of CDP-choline and CDPglucose. The effect of ATP and GTP on the solubilized membrane enzymes indicated that the inhibition of pyrophosphatase activity alone did not determine the rate of transfer of sugar to protein. The results also suggested that the UDP-N-acetylglucosamine pyrophosphatase and N-acetylglucosaminyltransferase activities were controlled independently and the effect of each nucleotide on these enzymes should, therefore, be carefully evaluated to understood its role in glycopolymer biosynthesis. Also, a possible role of choline and its derivatives in glycoprotein synthesis is discussed.     

Inhibition of rat testis microsomal 3beta-hydroxysteroid dehydrogenase activity by tributyltin

In this study, we have examined the effects of a range of organotin compounds (mono-, di-, tributyltin, mono-, di-, trioctyltin) on the activities of rat testis microsomal 3beta-hydroxysteroid dehydrogenase (3beta-HSD), 17-hydroxylase (17-OHase) and 17beta-hydroxysteroid dehydrogenase (17beta-HSD). 17-OHase activity was inhibited by more than 50% compared with the control rate by 59 microM tributyltin (TBT) but other organotin compounds showed no inhibition. 17beta-HSD activity was unaffected by all organotins tested. 3beta-HSD was inhibited by monooctyltin (81 microM) and by TBT at all concentrations tested in a dose-dependent manner, with almost complete loss of activity at TBT concentrations of 12 microM. The mechanism of inhibition of 3beta-HSD was investigated in kinetic analysis with 0-12 microM TBT. Three rat testis microsomal preparations were incubated with dehydroepiandrosterone as the steroid substrate ranging from 1 to 10,000 nM. Tributyltin was primarily a competitive inhibitor of 3beta-HSD activity, causing an increase in the value of the K(m(app)). However, the mechanism was not entirely competitive as while there was an increase in K(m(app)), a decrease in the V(max(app)) was also observed with increasing concentrations of TBT. Slope and intercept replots demonstrated that the K(i)((app)) from slope replots was around 2.7 microM whereas the K(i)((app)) value from intercept replots was around 30 microM. When compared with the K(m(app)) for 3beta-HSD of around 0.42 microM, TBT could be an effective inhibitor of this enzyme.