Soluble forms of gamma-glutamyltransferase in human adult liver, fetal liver, and primary hepatoma compared.

We isolated the soluble forms of gamma-glutamyltransferase (EC 2.3.2.2; gamma-GT) from adult and fetal human liver and primary hepatoma and compared their properties. The Km value for L-gamma-glutamyl-p-nitroanilide and glycylglycine, the Ki for anthglutin, and the pH optimum were identical for the enzyme from all three sources. Nor were significant differences observed among the three in their heat stability, inhibition by serine and borate, or ability to transfer the gamma-glutamyl moiety to various amino acids and dipeptides. Unlike membrane-bound gamma-GT, the soluble form from all three sources entered polyacrylamide gel and showed identical electrophoretic mobilities. Treatment with neuraminidase decreased the electrophoretic mobilities to a similar extent. The relative molecular mass of the enzyme from each of the three sources is about 84 000. Immunoinhibition and immunoprecipitation of gamma-GT from the three sources by antibody to fetal liver gamma-GT followed an identical pattern. Gamma-GT from fetal liver and hepatoma differed significantly from that of adult liver in affinity for wheat-germ agglutinin and Ricinus communis agglutinin (RCA-120). In many of the properties studied, soluble gamma-GT resembles the papain-digested form of membrane-bound gamma-GT.