Ageing of Neurospora crassa. II. Organic hydroperoxide toxicity and the protective role of antioxidant and the antioxygenic enzymes.

Cumene hydroperoxide and tert-butyl hydroperoxide at sublethal concentrations initially prevent growth of mycelia of wild-type Neurospora crassa, but after a time the cells grow at a subnormal steady-state rate. The antioxidant nordihydroguaiaretic acid protects unadapted cells from hydroperoxide inhibition, leading to a decrease in the time before growth begins, an increase in steady-state growth rate and an increase in biomass production. The results of growth transfer experiments and enzyme measurements indicated that the acquired resistance to the hydroperoxides is physiological and most likely involves the induction of the synthesis of the antioxygenic enzymes superoxide dismutase, glutathione peroxidase and glutathione reductase. Nordihydroguaiaretic acid normalizes the levels of activities of glutathione peroxidase and glutathione reductase during culture with hydroperoxide. Molecular-induced homolysis of the hydroperoxides, a process that is induced by unsaturated fatty acids of membrane lipids, leads to lipid autoxidation in a chain reaction which produces lipid hydroperoxides, which in turn decomposes to form more free radicals. Nordihydroguaiaretic acid, a well-known free radical scavenger, probably serves to minimize hydroperoxide decomposition, lipid autoxidation and molecular damage from free radicals, whereas the coupled enzyme system glutathione peroxidase and glutathione reductase minimizes these processes by decomposing the hydroperoxides to harmless alcohols. We suggest that either free radicals derived from these processes or some consequent non-radical products may serve as the inducers of this enzyme system, rather than the hydroperoxide substrates.