Abstract
A number of amino acids and peptides are substrates for gamma-glutamyl transferase (γ-GT) and their presence increases the activity of this membrane-bound enzyme. The enzyme has been postulated to play a role in intestinal transport of amino acids and peptides. In this study the degree of activation of both human and rat intestinal brush border γ-GT activity by various amino acids and peptides was assessed. Of the 22 l-amino acids tested, rat γ-GT was activated by 13, inhibited by isoleucine, and not significantly affected by 8. Human γ-GT was activated by 7 amino acids, inhibited by isoleucine, and unaffected by 14. Meaningful correlation of γ-GT activation with known characteristics of human intestinal amino acid tansport was not possible. Results obtained when γ-GT was tested for activation by peptides revealed that significant errors may occur due to hydrolysis of test peptides by contaminating peptide hydrolases. Thus, peptide hydrolases present in intact brush borders result in significant underestimation of γ-GT activation by dipeptides and overestimation of activation by tripeptides. The magnitude of the error varied greatly with the individual peptide used. Maximal activation of both rat and human peptide hydrolase-free γ-GT by dipeptides was more than 2-fold greater than that observed for amino acids white the four tripeptides tested were generally less activating than amino acids. The pattern of activation of both enzymes by peptides is compatible with the currently known specificity of the peptide transport system.
Published Version
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