Radical inactivation of a biological sulphydryl molecule

Abstract

Reactive species produced from the free radical-induced chain oxidation of low molecular weight sulphydryl-containing molecules in aerated solutions deactivate the sulphydryl-containing enzyme papain, forming both reparable mixed disulphides and non-reparable products. This inactivation is highly efficient for penicillamine and glutathione, but almost negligible with cysteine, which is a protector of papain for [cysteine]/[papain] 5 under all conditions used. In the case of glutathione, superoxide dismutase caused only a small reduction in the inactivation and peroxide yields were small, implying that the deactivating species are not ·O^–₂ but RSOO· radicals or products form them. For penicillamine, however, dismutase was highly effective and the peroxide yields were relatively large, demonstrating that ·O^–₂ or a radical with similar capabilities for forming H₂O₂ and being deactivated by dismutase was involved. Although in the presence of dismutase penicillamine is a better protector of non-reparable papain inactivation than glutathione, it suffers from a deficiency in that the papain-penicillamine mixed disulphide, which is always formed, cannot be repaired by spontaneous reaction with RSH molecules.