Both creatine and creatine phosphate are found in muscle, brain and blood. Creatinine is formed in muscle from creatine phosphate by a nonenzymatic dehydration and loss of phosphate. The amount of creatinine produced is related to muscle mass and remains remarkably constant from day to day. The role of GSH as a reductant is extremely important particularly in the highly oxidizing environment of the erythrocyte. The sulfhydryl of GSH can be used to reduce peroxides formed during oxygen transport.
Several mechanisms exist for the transport of amino acids across cell membranes.
Many are symport or antiport mechanisms that couple amino acid transport to sodium transport. The g -glutamyl cycle is an example of a group transfer mechanism of amino acid transport. Although this mechanism requires more energy input, it is rapid and has a high capacity. The cycle functions primarily in the kidney, particularly renal epithelial cells. The enzyme g -glutamyl transpeptidase is located in the cell membrane and shuttles GSH to the cell surface to interact with an amino acid.