Glutathionbiosynthese, VII. Die Biosynthese von Glutathion in Humanerythrozyten

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Abstract
Glutamate [300 .mu.M], glycine [375 .mu.M] and cysteine [10 .mu.M] were found by automated amino acid analysis. The concentration of 2-aminobutyrate, the precursor of ophthalmic acid, was 15 .mu.M. The influence of the activities of endogenous or added glutamyl-cysteine synthetase [EC 6.3.2.2] and glutathione synthetase [EC 6.3.2.3] on the rate of glutathione biosynthesis was measured in membrane-free hemolysates under physiological conditions. The rate of the overall biosynthesis mainly depends on the formation of the dipeptide glutamyl-cysteine. The effect of glutathione precursor concentrations on the synthesis of the tripeptide was investigated at constant (endogenous) activities of the synthesizing enzymes. The rate was not enhanced by addition of glutamate and/or glycine unless cysteine or glutamyl-cysteine was also added. The concentration of cysteine apparently limits the actual rate of the glutamyl-cysteine-synthetase reaction in vivo. No cystine or bis(glutamyl)cystine was detected in human hemolysate; these disulfides were converted to glutathione. Erythrocytes do have an appropriate system for their reduction, since the disulfides themselves are not substrates for the glutathione-synthesizing enzymes. Studies with intact human red cells indicate that the uptake of cysteine is the rate-determining step in the biosynthesis of glutathione.

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