STUDIES ON BIOLOGICAL OXIDATIONS

Toxicity of aromatic disulphides. I. Generation of superoxide radical and hydrogen peroxide by aromatic disulphides in vitro

The aromatic thiol, thiophenol, is readily autoxidized at neutral pH in a reaction which generates superoxide radical and hydrogen peroxide. The oxidation product, diphenyl disulphide, may be reduced back to thiophenol by glutathione and in the presence of an excess of the latter thiol a reduction/autoxidation cycle for generation of 'active oxygen' species is established. The autoxidation reaction is strongly catalysed by haematin; haemoglobin is also an effective mediator of 'active oxygen' generation from the diphenyl disulphide/glutathione couple, being oxidized to methaemoglobin in the process. Certain derivatives of diphenyl disulphide also generate superoxide radical and hydrogen peroxide in the presence of glutathione, although the rate of the reaction is strongly influenced by the nature of the substituent groups. Among the ring-substituted derivatives of diphenyl disulphide investigated, the rate of 'active oxygen' production decreased in the order 4-amino greater than 2-amino greater than 4-methyl greater than unsubstituted greater than 4-nitro greater than 2-carboxyl; little reaction was detected with the homologous compound, dibenzyl disulphide.

pH Dependence of the inhibition of yeast glyoxalase I by porphyrins

A number of porphyrin derivatives have been found to inhibit yeast glyoxalase I (EC 4.4.1.5) at 25 degrees C, including haemin, protoporphyrin IX, coproporphyrin III, haematoporphyrin, deuteroporphyrin as well as meso-(tetrasubstituted) porphines. Bilirubin and chlorophyllin were also inhibitory, but not cobalamin, adipic, pimelic or suberic acids. Whilst the Ki value for linear competitive inhibition by meso-tetra(4-methylpyridyl)porphine was pH-dependent, analogous Ki values for meso-tetra(4-carboxyphenyl)- and meso-tetra(4-sulphonatophenyl)porphines followed the Henderson-Hasselbalch equation with pKapp values of 7.10 and 6.50, respectively. Protoporphyrin showed similar behaviour (pKapp 7.06) with a deviation at lower pH. The haemin pH profile for Ki showed a maximum at approx. pH 6.5. The redox reaction between haemin and glutathione did not interfere in the inhibition studies. The Ki value for S-(p-bromobenzyl)glutathione was pH-independent. A detailed analysis of porphyrin binding modes was undertaken.