Donor substrate specificity of bovine kidney gamma-glutamyltransferase

The enzyme γ-glutamyltransferase (GGT) catalyzes the hydrolysis of the γ-glutamyl isopeptide bond of glutathione conjugates (donor substrates), releasing glutamic acid, or the transfer of the donor's γ-glutamyl group to an acceptor substrate, such as a dipeptide. The specificity of GGT for xenobiotic donor substrates has not been fully characterized. The transpeptidation activity of bovine kidney GGT was measured with glycylglycine as the acceptor substrate and several glutathione conjugate donor substrates, representative of detoxication products of polycyclic aromatic xenobiotics. HPLC separation with UV detection was used for quantitation. The commonly-used chromogenic substrate γ-glutamyl-p-nitroanilide was also tested. Michaelis constants (K(m)) were obtained for 4-nitrobenzylglutathione (0.075 mM), 2,4-dinitrophenylglutathione (0.30 mM), 4-methylbiphenylylglutathione (0.12 mM), 1-menaphthylglutathione (0.23 mM), and 9-methylanthracenylglutathione (0.22 mM), indicating a trend towards higher values for bulkier substrates. These results provide insight into the capacity of GGT to act in the biotransformation of aromatic compounds, many of which are of toxicological importance.