Kinetic properties of gamma-glutamyltransferase from human liver

We studied the kinetic properties of purified "heavy" form of human liver gamma-glutamyltransferase (EC 2.3.2.2) in the presence and absence of the acceptor substrate glycylglycine under Vmax conditions and as a function of pH. gamma-Glutamyl carboxynitroanilide was used as the donor substrate. Our data suggest that hydrolysis of donor substrate is the major pathway in the absence of acceptor. Autotransfer, if it occurs, is not important. Hydrolysis and transfer reactions have different pH profiles both for Vmax and Km. The pH dependency of Vmax and Km for both the hydrolase and the transferase reactions most probably reflects a change in the rate-limiting step: deacylation of the enzyme at acidic pH and acylation at alkaline pH. Negative cooperativity, observed for both donor and acceptor substrates near neutral pH, is interpreted in terms of more than one active site per dimer for each substrate.