Demonstration of γ-glutamyl transpeptidase activity in human jejunal mucosa

Abstract

A quantitative study of γ-glutamyl transpeptidase (GGT) activity of human jejunal, gastric, and colonie mucosa is reported. This enzyme, which probably plays an important role in the transport of amino acids across cell membranes and possibly also in peptide-nitrogen storage in the intact cell, in protein synthesis, and in the regulation of tissue glutathione levels, was found to be present in relatively high concentrations in the jejunal epithelium as compared to low levels in the stomach and colon. Failure of most previous investigators to find such high jejunal mucosal concentrations was probably due to differences in the techniques employed. Determination of enzyme activity was based on the colorimetric measurement of β-naphthylamine released enzymatically from an artificial substrate, namely N-(γ- l-glutamyl)-β-naphthylamide. Methods used for the preparation of tissue specimens and for the biochemical determination of enzyme activity have been described in detail. Leucine aminopeptidase (LAP), an enzyme known to be present in intestinal epithelial cells and localized primarily in the brush border, was determined simultaneously in jejunal specimens as a reference enzyme. Results of the LAP studies were in agreement with those previously reported for normal human jejunal epithelium. Histochemical studies showed GGT activity to be localized in both the nucleus and the apical portion of the surface epithelial cells covering the jejunal villi. No activity was found in the lining cells of the crypts. Differential centrifugation of homogenized jejunal epithelial cells demonstrated that enzyme activity was about equally distributed between the crude sediment of nuclei and the soluble supernatant subfractions. The physiologic significance of the relatively high levels of GGT activity in jejunal mucosa is as yet conjectural.