Studies on the Structure of γ-Glutamyltranspeptidase1

Abstract

The amino acid compositions of the subunits of γ-glutamyltranspeptidase solubilized with Triton X-100 (T-γ-GTP) and papain (P-γ-GTP) were compared. The results showed that the light subunits of the two forms are identical in amino acid composition, but that the heavy subunit of T-γ-GTP contains about 52 amino acid residues more than that of P-γ-GTP. The susceptibilities of the two forms to endopeptidase and the endo-groups of the two forms were studied to determine whether the membrane binding segment was located near the amino-terminus or the carboxyl-terminus. Leucine amino peptidase released amino acids from T-γ-GTP, converting it to a hydrophilic form like P-γ-GTP without loss of activity, and the composition of the amino acids released was similar to that of the difference in the amino acid compositions of the heavy subunits of T-γ-GTP and P-γ-GTP. No amino acids were released from P-γ-GTP by treatment with this enzyme. The amino-terminal residues of the heavy and light subunits of T-γ-GTP were methionine and threonine, respectively, while those of P-γ-GTP were glycine and threonine, respectively. Neither T-γ-GTP nor P-γ-GTP was affected by treatment with carboxypeptidase Y. Amino acids were released from both forms by treatment with carboxypeptidase Y in the presence of sodium dodecyl sulfate, and the patterns of these released amino acids were very similar. These results indicate that the amino terminal portion of the heavy subunit of γ-glutamyl-transpeptidase contains the portion that anchors the enzyme to the membrane.