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Thiols as myeloperoxidase-oxidase substrates.

ABSTRACT: Nine low-Mr thiols were compared with regard to their ability to function as myeloperoxidase-oxidase substrates under conditions where no auto-oxidation of the thiols could be observed. The methyl and ethyl esters of cysteine were found to be about twice as active as cysteamine at pH 7.0, in terms of increased O2 consumption. Cysteine itself was poorly active, whereas glutathione, N-acetylcysteine and penicillamine were completely inactive as myeloperoxidase-oxidase substrates under these conditions. The structure-activity relationships indicated that both a free thiol and free amino group were required for peroxidase-oxidase activity, and also that a free carboxy group abolished activity. In analogy with cysteamine, the activities of both cysteine esters were inhibited by superoxide dismutase (less than 5 micrograms/ml) and by catalase and not by the hydroxyl-radical scavenger mannitol. In contrast with cysteamine, the activities of both cysteine esters were stimulated more than 2-fold by high concentrations (greater than 5 micrograms/ml) of superoxide dismutase. The activities of both cysteine esters exhibited broad pH optima at pH 7. A mechanism for the myeloperoxidase-oxidase oxidation of the cysteine esters is proposed, which is partly different from that previously proposed for cysteamine.

SUBMITTER: Svensson BE

PROVIDER: S-EPMC1149318 | biostudies-other | 1988 Jul

REPOSITORIES: biostudies-other

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Myeloperoxidase oxidation states involved in myeloperoxidase-oxidase oxidation of thiols.

Project description:The changes in the oxidation state of the leucocyte enzyme myeloperoxidase, induced by buffer and thiols, were studied with visible-light-absorption spectroscopy. It was concluded that phosphate buffer contains small amounts of H2O2 and that thiols, when added to buffer, induce the generation of minute amounts of superoxide radical anion. These minute amounts of reduced oxygen species are suggested to account for the initiation of myeloperoxidase-oxidase oxidation of thiols. Myeloperoxidase was found to be in its Compound III oxidation state during myeloperoxidase-oxidase oxidation of thiols. However, myeloperoxidase-mediated oxidation of thiols with concomitant O2 consumption can also occur with myeloperoxidase in its Compound II oxidation state. These studies indicate that the ferro and Compound III oxidation states may not be essential intermediates in myeloperoxidase-oxidase oxidation of thiols, but rather that the formation of the Compound III oxidation state retards the reaction.

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Engineering Klebsiella sp. 601 multicopper oxidase enhances the catalytic efficiency towards phenolic substrates.

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