The gamma-glutamyl cycle: a possible transport system for amino acids.

Evidence is presented that rat kidney contains enzymes that catalyze the synthesis and utilization of glutathione; these reactions, which involve the uptake and release of amino acids from gamma-glutamyl linkage, constitute a cyclical process which is termed "the gamma-glutamyl cycle." The gamma-glutamyl cycle has properties that fulfill the requirements of an amino acid transport system. Thus, gamma-glutamyl transpeptidase may function in translocation and gamma-glutamylcysteine synthetase and glutathione synthetase may catalyze energy-requiring "recovery" steps in transport. These and other considerations suggest that glutathione serves a carrier function in amino acid transport.