Translocation of intracellular glutathione to membrane-bound γ-glutamyl transpeptidase as a discrete step in the γ-glutamyl cycle: Glutathionuria after inhibition of transpeptidase
Abstract
Several inhibitors of γ-glutamyl transpeptidase in vitro [L-serine plus borate, 6-diazo-5-oxo-L-norleucine, and L- and D-γ-glutamyl-(o-carboxy)phenylhydrazide] are active in vivo, as indicated by their effect in decreasing the conversion of administered D-γ-glutamyl-L-α-amino[14C]butyrate to respiratory 14CO2 in mice. The hydrazides (both L and D isomers) are the most potent inhibitors in vitro and in vivo. Inhibition of γ-glutamyl transpeptidase in vivo by the hydrazides is accompanied by extensive glutahionuria. The evidence suggests that a substantial fraction of the urinary glutathione arises from the kidney. The findings support the view that renal intracellular glutathione is normally translocated to the membrane-bound γ-glutamyl transpeptidase as a separate step in the γ-glutamyl cycle. Studies on in vivo inhibition of glutathione synthesis and of γ-glutamyl transpeptidase provide direct evidence that glutathione is normally translocated from tissues to the blood plasma and that the turnover of plasma glutathione is relatively high. The data suggest that the low but significant steady-state level of glutathione in the plasma reflects synthesis of glutathione (predominantly in the liver) and its utilization by γ-glutamyl transpeptidase (predominantly in the kidney). Thus, glutathione synthesized in cells that have transpeptidase may be translocated to and used by the membrane-bound enzyme, whereas glutathione synthesized in cells that lack the transpeptidase may be transported via the plasma to transpeptidase located on the membranes of other cells.
