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Mechanism-based inactivation of gastric peroxidase by mercaptomethylimidazole.

ABSTRACT: The mechanism of inhibition of gastric peroxidase (GPO) activity by mercaptomethylimidazole (MMI), an inducer of gastric acid secretion, has been investigated. Incubation of purified GPO with MMI in the presence of H2O2 results in irreversible inactivation of the enzyme. No significant inactivation occurs in the absence of H2O2 or MMI, suggesting the involvement of peroxidase-catalysed oxidized MMI (MMIOX.) in the inactivation process. The inactivation follows pseudo-first-order kinetics consistent with a mechanism-based (suicide) mode. The pseudo-first-order kinetic constants at pH 8 are ki = 111 microM, k(inact.) = 0.55 min-1 and t1/2 = 1.25 min, and the second-order rate constant is 0.53 x 10(4) M-1 x min-1. Propylthiouracil also inactivates GPO activity in the same manner but its efficiency (k(inact./ki = 0.46 mM-1 x min-1) is about 10 times lower than that of MMI (k(inact./ki = 5 mM-1 x min-1). The rate of inactivation with MMI shows pH-dependence with an inflection point at 7.3, indicating the involvement in the inactivation process of an ionizable group on the enzyme with a pKa of 7.3. The enzyme is remarkably protected against inactivation by micromolar concentrations of electron donors such as iodide and bromide but not by chloride. Although GPO oxidizes MMI slowly, iodide stimulates it through enzymic generation of I+ which is reduced back to I- by MMI. Although MMIOX. is formed at a much higher rate in the presence of I-, a constant concentration of I- maintained via the reduction of I+ by MMI, protects the active site of the enzyme against inactivation. We suggest that MMI inactivates catalytically active GPO by acting as a suicidal substrate.

SUBMITTER: Bandyopadhyay U

PROVIDER: S-EPMC1137657 | biostudies-other | 1993 Nov

REPOSITORIES: biostudies-other

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Project description:Humans are exposed to various hydrazine derivatives for therapeutic control of several diseases, and mammalian peroxidases are implicated in the oxidative metabolism of many drugs. The results presented here indicate that lacrimal-gland peroxidase is irreversibly inactivated in a mechanism-based way by phenylhydrazine, which acts as a suicidal substrate in the presence of H2O2. The pseudo-first-order kinetic constants for inactivation at pH 5.5 are Ki=18 microM, kinact=0.25 min-1 and tau50=2.75 min, with a second-order rate constant of 0.75x10(4) M-1.min-1. Approx. 27 mol of phenylhydrazine and 54 mol of H2O2 are required per mol of enzyme for complete inactivation. The pH-dependent inactivation kinetics indicate the involvement of an ionizable group on the enzyme with a pKa value of 5.4, protonation of which favours inactivation. SCN-, the plausible physiological electron donor of the enzyme, protects it from inactivation. Binding studies by optical difference spectroscopy indicate that phenylhydrazine interacts with the enzyme with a KD value of 60 microM, and its binding is prevented by the presence of SCN-. The enzyme is also protected by 5, 5-dimethyl-1-pyrroline N-oxide, a free-radical trap, suggesting the involvement of a radical species in the inactivation. ESR studies indicate the formation of a spin-trapped phenyl radical (aN=15.9G and abetaH=24.8G) generated on incubation of phenylhydrazine with the enzyme and H2O2. A 75% loss of the Soret spectrum is observed when the enzyme is completely inactivated. However, in the presence of the spin trap, spectral loss is prevented and the enzyme compound II is readily reduced to the native state by phenylhydrazine. The phenylhydrazine-inactivated enzyme reacts with H2O2 or CN- to form compound II or the cyanide complex with a characteristic spectrum, indicating that haem iron is protected from attack by the radical species. The inactivated enzyme binds SCN- with a KD value similar to that of the native enzyme (15+/-3 mM), suggesting that the donor-binding site remains unaffected. CD studies of the inactive enzyme show complete disappearance of the Soret band at 409 nm with the appearance of a new band at 275 nm. This indicates that the haem environment of the enzyme is perturbed in the inactive form. As benzene, the end product of phenylhydrazine oxidation, has no effect on the enzyme, we suggest that the phenyl radical formed by one-electron oxidation by catalytically active enzyme inactivates it by incorporation in the vicinity of its haem moiety. The data support the use of phenylhydrazine as a probe for structural and mechanistic analysis of the active site of the lacrimal-gland peroxidase.

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Mechanism-based inactivation of gastric peroxidase by mercaptomethylimidazole.

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