Significance of gamma-glutamyl transpeptidase and thiols in amino acid uptake by rat pancreatic islets: studies with glutamine and leucine.

The importance of gamma-glutamyl transpeptidase, the key enzyme of the gamma-glutamyl cycle and of thiols for the uptake of amino acids into rat pancreatic islets was investigated. Both serine-borate, an inhibitor of gamma-glutamy transpeptidase, and serine which does not inhibit this enzyme, but probably is a competitive inhibitor of amino acid uptake, inhibited the uptake of glutamine. The inhibitory effect of serine-borate was not greater than that of serine alone. The uptake of glutamine was not affected by either GSH (reduced glutathione) or diamide (a thiol oxidant). Neither substances affected the uptake of leucine. The results indicate that the uptake of glutamine by rat pancreatic islets is not dependent on the functioning of gamma-glutamyl transpeptidase and that thiols are not important for the uptake of the amino acids glutamine and leucine.