DOES A MODIFIED GAMMA-GLUTAMYL CYCLE EXIST IN HUMAN ERYTHROCYTES

  • 1 January 1976
    • journal article
    • research article
    • Vol. 357  (11) , 1459-1463
Abstract
The 1st step in the biosynthesis of glutathione is the formation of .gamma.-glutamyl-cysteine by the enzyme glutamyl-cysteine synthetase [EC 6.3.2.2]. Since this enzyme is not specific for cysteine, different .gamma.-glutamylamino acids may be formed in vivo which represent potential substrates for the enzyme .gamma.-glutamylcyclotransferase [EC 2.3.2.4]; in this way 5-oxo-L-proline and free amino acid are formed. The competition between the biosynthesis of glutathione or ophthalmic acid and the degradation of .gamma.-glutamyl peptides was investigated in membrane free hemolysate by measuring the formation of 5-oxoproline. The endogenous rate of 5-oxoproline production was 0.13 .mu.M/min. This increased to 2 .mu.M/min after addition of 2-aminobutyrate, and to 10 .mu.M/min after addition of glutamate and 2-aminobutyrate to hemolysate. Addition of cysteine resulted in an increased oxoproline production only under conditions where glutamyl-cysteine accumulated. For glutamyl-2-aminobutyrate, the degradation to 5-oxoproline is faster than the utilization for the tripeptide synthesis. This was not the case for glutamyl-cysteine. Since membrane-free hemolysate (which lacks .gamma.-glutamyltransferase) is able to produce 5-oxoproline starting from glutamate, it is concluded that this 5-oxoproline synthesis does not necessarily reflect the function of a glutathione-dependent amino acid transport via a modified .gamma.-glutamyl cycle.