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Earlier it has been shown that extracellular glutathione peroxidase (GPx3) from human plasma is able to use cysteine (Cys-SH) instead of glutathione (GSH) as a thiol substrate. In the present study, the ability of rat plasma to utilize not only GSH, but also Cys-SH and homocysteine (Hcy-SH), in the thiol peroxidase reaction has been confirmed. The molar ratio between thiol and H2O2 in the catalyzed reaction was 2:1. The specific activity increased with fractionation of proteins. At a fixed thiol concentration of 0.23 mM, the saturation by H2O2 with vmax app of 100, 128, and 132 nmol H2O2 / s per 1 ml of plasma was found for DL-Cys-SH, L-GSH, and DL-Hcy-SH, respectively. Rank distributions of activities towards all three thiol substrates within plasma protein fractions are fully identical (the probability of random full coincidence was less than 0.01). The statistical analysis confirms that Cys-SH peroxidase, Hcy-SH peroxidase, and GSH peroxidase activities are closely associated with each other. The most probable outcome of this result is the ability of rat GPx3 to utilize all three thiols as substrates for oxidation. Probably, thiol peroxidase is a participant of formation of plasma cystine (Cys-SS-Cys) from Cys-SH in plasma. If the forms of Hcy exhibit different toxic effects, it can be suggested that thiol peroxidase regulates Hcy toxicity in hyperhomocysteinemia through Hcy-SH oxidation to homocystine (Hcy-SS-Hcy).

Thiol peroxidase activities in rat blood plasma determined with hydrogen peroxide and 5,5`-dithio-bis(2-nitrobenzoic acid)