Synergism between substrate and non-substrate thiols in peroxidase-oxidase reactions

Transient-state and steady-state kinetics of the oxidation of aliphatic and aromatic thiols by horseradish peroxidase

In the course of oxidation of thiols by peroxidases thiyl radicals are formed which are known to undergo several free-radical conjugative reactions, among others leading to hydrogen peroxide formation. The present paper for the first time presents a comparative transient-state and steady-state investigation of the reaction of 15 aliphatic and aromatic mono- and dithiols with horseradish peroxidase (HRP). Both sequential-stopped-flow spectrophotometric investigations of the reaction of HRP intermediates Compound I (k2) and Compound II (k3) with thiols and measurements of the overall thiol oxidation and the simultaneous oxygen consumption in the presence and absence of exogenously added hydrogen peroxide (10 microM) have been performed. With HRP as thiyl radical generator it was shown that three groups of thiols have to be distinguished: (i) Aromatic thiols (e.g. thiophenol, 2-mercaptopurine) were excellent electron donors of both Compounds (k2: 10(4)-10(7) M(-1) s(-1) and k3: 10(3)-10(6) M(-1) s(-1)); however, the overall reaction was shown to depend on addition of hydrogen peroxide, indicating insufficient peroxide regeneration by arylthiyl radicals. (ii) Aliphatic thiols which were extremely bad substrates (k3 < 10 M(-1) s(-1)) for HRP (e.g. homocysteine, glutathione) and/or have a pK(a,SH) > 9.5 (e.g. N-acetylcysteine, alpha-lipoic acid) were also shown to depend on exogenously added H2O2 to maintain the peroxidasic reaction, whereas (iii) with those thiols with rates of k3 between 11 and 1600 M(-1) s(-1) (e.g. cysteine, cysteamine, cysteine methyl ester, cysteine ethyl ester) and/or with a pK(a,SH) < 8 (penicillamine) thiol oxidation was independent of exogenously added hydrogen peroxide, indicating sufficient hydrogen peroxide regeneration.

Thiol-bearing compounds selectively inhibit protein kinase C-dependent oxidative events and proliferation in human T cells

The aminothiol cysteamine at 10(-5) to 10(-4) M concentrations inhibited both the proliferation of mitogenically stimulated human peripheral mononuclear cells and the phorbol myristate acetate-mediated oxidation of 2',7'-dichlorofluorescein within these cells. Both 2',7'-dichlorofluorescein oxidation and the proliferative response were maximally sensitive to cysteamine-induced inhibition during the first 2 h of mitogenic stimulation. This period of sensitivity indicates that cysteamine preferentially arrests cells transiting from G0 to G1 and is the first such demonstration, of an early cell cycle site of arrest for this compound. 2,3-Dimercapto-1-propane-sulfonic acid and WR 1065 were found to be more effective than cysteamine in attenuating T cell replication but not N-acetylcysteine. Aminothiols preferentially inhibited the intracellular oxidation of 2',7'-dichlorofluorescein, rather than the activity of protein kinase C, which initiates the oxidation, indicating that oxidative events are one of a number of crucial and independent events required for the successful transition through G0-G1. Since aminothiols affect both lectin and PMA/ionomycin-directed proliferation, these aminothiol-sensitive events may serve to integrate and regulate common pathways in T cell activation.

A fluorometric assay of acyl-CoA oxidase activity by a coupled peroxidatic reaction: elimination of interfering side reactions

We have developed a simple, reliable, and sensitive method for the assay of peroxisomal fatty acyl-CoA oxidase (FAO, EC 1.13.-) in subcellular fractions. It is based on a peroxidase-linked oxidation of 4-hydroxyphenylacetic acid to a fluorescent compound [M.S. Poosch and R.K. Yamasaki (1986) Biochim. Biophys. Acta 884, 585-593]. Our method eliminates the contribution of important interfering side reactions, notably those due to the presence of reducing agents, which function as competitive substrates to 4-hydroxyphenylacetic acid. Rapidly reacting thiol groups are of particular importance, notably CoASH present endogenously (e.g. in peroxisomes and mitochondria) or formed by enzymatic hydrolysis of acyl-CoA. Alkylation of the thiol compounds by N-ethylmaleimide eliminates this disturbing side reaction, and increases the amount of fluorescent product in the coupled peroxidatic reaction. The method is suitable for routine assay of FAO activity in a wide range of tissues, notably in those with a low specific peroxisomal beta-oxidation activity and/or a high content of reducing agents. As an example of this we have included data from rat heart peroxisomal fractions. The effect of alkylation of sulfhydryl groups in the incubation mixture also applies to other oxidase reactions based on H2O2-coupled peroxidatic reactions, if the oxidase itself does not contain functional sulfhydryl groups.

Scavenging of hypochlorous acid and of myoglobin-derived oxidants by the cardioprotective agent mercaptopropionylglycine

Mercaptopropionylglycine (MPG) has a marked cardioprotective action in several model systems of ischaemia-reoxygenation injury. Suggested mechanisms of action include scavenging of hydroxyl radical and of hypochlorous acid and reacting with an oxidant formed by reaction of myoglobin with H2O2, thereby slowing lipid peroxidation stimulated by myoglobin-H2O2 mixtures. This oxidant seems not to be singlet O2 or hydroxyl radical. Studies in vitro show that scavenging of hypochlorous acid is a feasible mechanism of cardioprotective action for MPG in vivo in ischaemia/reperfusion systems to which neutrophil-mediated injury contributes. However, the poor ability of MPG to inhibit lipid peroxidation stimulated by myoglobin/H2O2 mixtures and its ability to increase iron ion release from myoglobin in the presence of a large excess of H2O2 suggests that MPG is unlikely to protect the myocardium by interfering with oxidants produced by the myoglobin-H2O2 system.