Glutathione-deficient mutants () of the yeast , impaired in the first step of glutathione (GSH) biosynthesis were studied with respect to the regulation of enzymes involved in GSH catabolism and cysteine biosynthesis. Striking differences were observed in the content of the sulphur amino acids when mutants were compared to wild-type strains growing on the same minimal medium. Furthermore, all mutants examined showed a derepression of γ-glutamyltranspeptidase (γ-GT), the enzyme initiating GSH degradation. However, γ-cystathionase and cysteine synthase were unaffected by the GSH deficiency as long as the nutrient sulphate source was not exhausted. The results suggest that the mutants are probably not impaired in the sulphate assimilation pathway, but that the γ-glutamyl cycle could play a leading role in the regulation of the sulphur fluxes. Studies of enzyme regulation showed that the derepression of γ-GT observed in the strains was most probably due to an alteration of the thiol status. The effectors governing the biosynthesis of cysteine synthase and γ-cystathionase seemed different from those playing a role in γ-GT regulation and it was only under conditions of total sulphate deprivation that all these enzymes were derepressed. As a consequence the endogenous pool of GSH was used in the synthesis of cysteine. GSH might, therefore, fulfil the role of a storage compound.


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