Purification and Properties of Bovine Liver Gamma-glutamyltransferase

gamma-Glutamyltransferase [EC 2.3.2.2] (gamma-GTP) was purified to a homogeneous state from bovine liver. It had an apparent molecular weight of about 110,000, as judged by polyacrylamide gel electrophoresis, and consisted of two non-identical glycopeptides with molecular weights of 68,000 and 27,000. The amino acid composition of the bovine liver enzyme was similar to those of other gamma-GTPs. The enzyme contained large amounts of neutral sugars, amino sugars and sialic acid, although the sialic acid content varied in different preparations. The Michaelis constant of the enzyme was estimated to be 0.8 mM for gamma-L-glutamyl-p-nitroanilide in the presence of glycylglycine and 1.25 mM in the absence of glycylglycine. Glutathione competitively inhibited the release of p-nitroaniline from gamma-L-glutamyl-p-nitroanilide with a Ki value of 0.3 mM. The specific activities of the enzyme for gamma-L-glutamyl-p-nitroanilide in the presence of glycylglycine (pH 8.6) and for glutathione, a natural substrate (pH 7.4), were comparable to those reported for gamma-GTPs from other mammalian sources. The bovine liver enzyme showed the same gamma-glutamyl group acceptor specificity as other gamma-GTPs from normal mammalian tissues. The phosphate-independent glutaminase activity of the enzyme was much lower than that of the rat kidney enzyme both in the presence and absence of maleate.