Oxidative inactivation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase and subunit cross-linking involve different dithiol/disulfide centers.

Rat liver microsomal 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (HMGR) is extremely sensitive to oxidative inactivation by low concentrations (micromolar) of glutathione disulfide (GSSG) even in the presence of millimolar concentrations of glutathione (GSH). Inactivation involves the formation of an intramolecular protein-SS-protein disulfide in thiol/disulfide redox equilibrium with the reduced, active enzyme (Cappel, R.E., and Gilbert, H.F. (1988) J. Biol. Chem. 263, 12204-12212). In the absence of dithiothreitol, HMGR oxidation has been previously shown to cross-link the microsomal enzyme into a covalent dimer (Ness, G.C., McCreery, M.J., Sample, C.E., Smith, M., and Pendelton, L.C. (1985) J. Biol. Chem. 260, 12391-12393). Examination of the extent of HMGR cross-linking and residual HMGR activity in microsomes equilibrated with glutathione redox buffers establishes that inactivation and cross-linking result from oxidation of different dithiol pairs. The thiol/disulfide oxidation potential (K(ox)) for the oxidative inactivation of HMGR, E(SH)2,active + GSSG<-->E(S-S)inactive + 2 GSH, is 0.67 +/- 0.07 M. However, the equilibrium constant for HMGR cross-linking, E(SH)2,monomer + GSS<-->E(S-S)dimer + 2 GSH, is 0.19 +/- 0.02 M, significantly lower than that for inactivation (p < 0.001). Because of the significantly different oxidation potentials and the lack of a linear relationship between cross-linking and inactivation, the two processes must involve two different sets of vicinal dithiols. HMGR becomes 5-10-fold more difficult to oxidize in the presence of saturating levels of the substrate, HMG-CoA. Both inactivation and cross-linking exhibit significantly lower oxidation potentials in the presence of this substrate, 0.072 +/- 0.01 and 0.047 +/- 0.007 M, respectively. The decrease in oxidation potential caused by substrate binding is observed for both inactivation and cross-linking, showing that both processes are affected by the binding of substrate to the enzyme. The dithiols involved in HMGR subunit cross-linking are 2-3-fold more difficult to oxidize than the dithiols that affect the enzyme activity. Thus, the observation of partial cross-linking of HMGR in vivo would imply that conditions are sufficiently oxidizing to result in significant enzyme inactivation. The extreme thermodynamic sensitivity of HMGR to oxidative inactivation and cross-linking in glutathione redox buffers that span the physiological redox state implies that thiol/disulfide redox state changes could provide a mechanism for regulating the activity and/or stability of this enzyme.

Assessment of the antioxidant/prooxidant status of murine skin following topical treatment with 12-O-tetradecanoylphorbol-13-acetate and throughout the ontogeny of skin cancer. Part II: Quantitation of glutathione and glutathione disulfide.

Tissue glutathione (GSH) and glutathione disulfide (GSSG) contents were quantitated in the skins of female SENCAR mice following the topical application of 12-O-tetradecanoylphorbol-13-acetate (TPA), and in the skin tumors generated by an initiation-promotion protocol. Total epidermal GSHt (GSH + GSSG) and GSSG contents were not reproducibly and significantly altered 0.5, 4 or 24 h after one or four topical applications of 1 microgram TPA, relative to the values obtained in age-matched, solvent-treated mice. Similar findings held for dermal GSHt at all times of analyses, and for dermal GSSG contents 0.5 and 4 h after TPA application. However, dermal GSSG contents were slightly elevated 24 h after TPA application. The GSHt and GSSG contents of skins initiated with 10 nmol 7,12-dimethylbenz[a]anthracene (DMBA) and harvested 17, 29 and 37 days after the cessation of chronic treatment with acetone (14 weeks, twice a week) were comparable to the values measured in age-matched, non-treated skins. In contrast, GSHt contents of papillomas harvested 17, 29 and 37 days after the cessation of chronic treatment with 1 microgram TPA (14 weeks, twice a week) were 2- to 4-fold greater than the values measured in non-treated mice, and DMBA-initiated, acetone-promoted mice, and the non-tumorous tissue adjacent to the papillomas. Comparable changes did not occur in papilloma GSSG contents. GSHt contents in squamous cell carcinomas (SCC) were twice the values measured in papillomas and 5- to 8-fold greater than the values measured in non-treated skins, and the non-tumorous tissue adjacent to SCC. Similarly, GSSG contents in SCC were elevated multifold relative to papillomas, non-treated skin and the non-tumorous tissue adjacent to SCC. Epidermal cell suspensions prepared by the trypsin-flotation procedure retained less than 2% of their original GSHt content and had reduced GSHt/GSSG ratios. Collectively these studies suggest that (i) if promoting doses of TPA induce oxidative stress in murine epidermis, it cannot be detected by measurements of GSH/GSSG; (ii) the antioxidant capacity of epidermal cells prepared by the trypsin-flotation procedure is severely compromised; and (iii) GSHt contents progressively increase during skin tumor ontogeny.

Inhibition of glutathione disulfide reductase by glutathione.

Rat-liver glutathione disulfide reductase is significantly inhibited by physiological concentrations of the product, glutathione. GSH is a noncompetitive inhibitor against GSSG and an uncompetitive inhibitor against NADPH at saturating concentrations of the fixed substrate. In both cases, the inhibition by GSH is parabolic, consistent with the requirement for 2 eq. of GSH in the reverse reaction. The inhibition of GSSG reduction by physiological levels of the product, GSH, would result in a significantly more oxidizing intracellular environment than would be realized in the absence of inhibition. Considering inhibition by the high intracellular concentration of GSH, the steady-state concentration of GSSG required to maintain a basal glutathione peroxidase flux of 300 nmol/min/g in rat liver is estimated at 8-9 microM, about 1000-fold higher than the concentration of GSSG predicted from the equilibrium constant for glutathione reductase. The kinetic properties of glutathione reductase also provide a rationale for the increased glutathione (GSSG) efflux observed when cells are exposed to oxidative stress. The resulting decrease in intracellular GSH relieves the noncompetitive inhibition of glutathione reductase and results in an increased capacity (Vmax) and decreased Km for GSSG.

The effects of NADPH and 3-hydroxy-3-methylglutaryl-CoA on the thiol/disulfide redox behavior of rat liver microsomal 3-hydroxy-3-methylglutaryl-CoA reductase.

Microsomal 3-hydroxy-3-methylglutaryl-CoA reductase isolated from the livers of rats fed a diet containing cholestyramine (HMGR-C) is oxidized to a protein-SS-protein disulfide via a thermodynamically favorable thiol/disulfide exchange in glutathione redox buffers which approach the normal in vivo redox poise. In the presence of either substrate (NADPH or 3-hydroxy-3-methylglutaryl-CoA), the equilibrium thiol/disulfide redox behavior of HMGR-C is substantially different than that observed in the absence of substrates or in the presence of both substrates. NADPH present during redox equilibrium in a glutathione redox buffer decreases the equilibrium constant for formation of the protein-SS-protein disulfide (Kox,i) from 0.55 +/- 0.07 M to 0.18 +/- 0.02 M and increases the Kox,m for formation of an inactive protein-SS-glutathione mixed disulfide from less than 1 to 6 +/- 1. The presence of 3-hydroxy-3-methylglutaryl-CoA during redox equilibrium has a similar effect, decreasing the Kox,i for protein-SS-protein disulfide formation to 0.10 +/- 0.02 M and increasing the Kox,m for protein-SS-glutathione mixed disulfide formation to 3.8 +/- 0.9. A three-state model is developed which describes the simultaneous accumulation of protein-SS-protein and protein-SS-glutathione mixed disulfides at redox equilibrium with glutathione redox buffers. Because of the different redox behavior of the free and substrate-liganded forms of the enzyme, addition of 3-hydroxy-3-methylglutaryl-CoA or NADPH to HMGR-C at redox equilibrium results in increased reduction and activation of the enzyme.

The effects of mevinolin on the thiol/disulfide exchange between 3-hydroxy-3-methyglutaryl-coenzyme A reductase and glutathione.

The feeding of mevinolin plus cholestyramine to rats results in the production of a form of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR-CM) having thiol/disulfide redox properties different from those of 3-hydroxy-3-methylglutaryl-CoA reductase isolated from animals which had been given only cholestyramine (HMGR-C). The second-order rate constant for the inactivation of HMGR-CM by GSSG is 7-fold slower than for HMGR-C, while the second-order rate constant for the reactivation of oxidized enzyme by GSH is 100-fold slower. However, in the presence of saturating concentrations of both substrates, the rate constants for thiol/disulfide exchange are similar for both forms of the enzyme. HMGR-CM behaves as if a protein-glutathione mixed disulfide having a Kox of 27 +/- 4 is formed at equilibrium. In contrast, HMGR-C has previously been shown to form a protein-protein disulfide (Cappel, R. E., and Gilbert, H. F. (1988) J. Biol. Chem. 263, 12204-12212). Both forms of the enzyme are more difficult to oxidize thermodynamically in the presence of saturating levels of both substrates. For HMGR-CM, NADPH alone has no effect on the equilibrium constant for oxidation, but hydroxymethylglutaryl-CoA alone makes the enzyme approximately twice as difficult to oxidize. Under physiological conditions, HMGR-CM is thermodynamically more difficult to oxidize than HMGR-C. HMGR-C can be converted to HMGR-CM by in vitro treatment with mevinolinate. A direct or indirect interaction of mevinolin with HMGR-C results in some persistent, as yet undefined, structural alteration which inhibits the formation of a protein-SS-protein disulfide upon oxidation by glutathione disulfide.

Thiol/disulfide exchange between 3-hydroxy-3-methylglutaryl-CoA reductase and glutathione. A thermodynamically facile dithiol oxidation.

In glutathione redox buffers, rat liver, microsomal 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase rapidly equilibrates between a reduced, active form and an oxidized, inactive form. At pH 7.0, 37 degrees C, the second order rate constant for inactivation of the reduced enzyme by GSSG is 1700 +/- 200 M-1 min-1, approximately 20-fold faster than the reaction of GSSG with a typical, unhindered thiol of pKa 7.7. High concentrations of GSH or lower concentrations of dithiothreitol restore the activity of the oxidized enzyme. The oxidation of the enzyme by GSSG is only 30-fold slower in the presence of saturating levels of both substrates. The incomplete inhibition of thiol/disulfide exchange by substrates can lead to significant changes in the activity of the enzyme during the assay when glutathione is present. At redox equilibrium, both in the absence and presence of substrates, the activity of the enzyme depends on the quantity [GSH]2/[GSSG], suggesting that the redox transition involves the formation of a protein-SS-protein disulfide. The equilibrium constant for the reaction HMGRred + GSSG in equilibrium HMGRox + 2 GSH is 0.55 +/- 0.07 M in the absence of substrates and 0.20 +/- 0.02 M in the presence of saturating levels of both substrates. Thus, HMG-CoA reductase is very sensitive to dithiol oxidation both kinetically and thermodynamically. Significant changes in the oxidation state and activity of this enzyme could be expected to result from normal changes in the thiol/disulfide oxidation state of the cellular glutathione redox buffer.

Thiol/disulfide redox equilibrium between glutathione and glycogen debranching enzyme (amylo-1,6-glucosidase/4-alpha-glucanotransferase) from rabbit muscle.

Rabbit skeletal muscle glycogen debranching enzyme is inactivated in a kinetically biphasic manner by GSSG at pH 8.0. The rapid phase results in the loss of 30% activity, while the slower phase leads to total enzyme inactivation. Both the glucosidase and the transferase activities of the enzyme are inhibited by GSSG. The inactivation by disulfides is fully and rapidly reversed in a biphasic manner by reduction with excess reduced dithiothreitol or GSH. After a fast initial recovery of 70% of the initial activity, the remaining 30% of the activity is recovered more slowly. Equilibration of the enzyme with a redox buffer of GSH and GSSG shows a monophasic equilibration of the activity. The ratio of GSH/GSSG where the enzyme is 50% active (R0.5) is 0.06 +/- 0.03. The R0.5 does not vary significantly with the total concentration of glutathione species suggesting formation of protein-SSG mixed disulfides. The ratios of the observed second-order rate constants for GSSG inactivation and GSH reactivation do not lead to a correct value of the observed thiol/disulfide oxidation equilibrium constant. Although the enzyme has sulfhydryl groups, the oxidation of which leads to activity changes, the kinetic and thermodynamic resistance to oxidation suggests that the enzyme is not likely to be subject to regulation by thiol/disulfide exchange in vivo.

Cooperative behavior in the thiol oxidation of rabbit muscle glycogen phosphorylase in cysteamine/cystamine redox buffers.

Glycogen phosphorylase a and b are irreversibly inactivated by oxidation with the disulfide cystamine. The mechanism is complex and involves oxidation of at least two classes of sulfhydryl groups. The oxidation of one or more of the first class of 4 +/- 1 sulfhydryl groups is reversible, but the equilibrium constant for the oxidation is so unfavorable (1 X 10(-4)) that the micromolar concentrations of cysteamine released stoichiometrically with enzyme oxidation are sufficient to prevent complete oxidation even in the presence of 100 mM cystamine. The rapid phase of inactivation of phosphorylase b, which is first order in cystamine (k = 2.9 +/- 0.3 M-1 min-1), is followed by the oxidation of 5 +/- 1 groups in an irreversible process that is second order in cystamine concentration (k = 3.9 +/- M-2 min-1). Similar behavior is observed for phosphorylase a, although the behavior is complicated by association/dissociation equilibrium. The second-order dependence of the rate of irreversible inactivation on cystamine concentration is interpreted in terms of a "cooperative" model in which a rapidly reversible thermodynamically unfavorable equilibrium oxidation of one or more sulfhydryl groups must precede the irreversible oxidation of one or more additional sulfhydryl groups. The thiol/disulfide oxidation equilibrium constant for the initial reversible reaction is estimated to be at least 10(4) less favorable than that for the reversible oxidation of phosphofructokinase.