Rose bengal-sensitized photooxidation of rat kidney gamma-glutamyltransferase.

Rose bengal-sensitized photooxidation of rat kidney gamma- glutamyltransferase (EC 2.3.2.2) resulted in irreversible inactivation of the enzyme. Inactivation revealed two distinct phases, the faster phase followed by the slower phase. During photooxidation, the enzyme lost its transpeptidation activity much faster than its hydrolysis activity. The rate of inactivation was markedly reduced by S- benzylglutathione , a substrate for the enzyme. Inactivation was also prevented by hippuric acid or by 3- benzylpropionate , acceptor site- directed ligands of the transferase. In contrast, D-glutamine, a donor site-specific substrate, had no effect on the inactivation process. Photooxidation of the enzyme affected neither the affinity for D-glutamine (apparent Km of 5.3-5.6 mmol/l) nor the reactivity with 6-diazo-5-oxo-D-(6-14C)norleucine, a donor site- specific affinity labeling reagent of the enzyme, suggesting preferential impairment of catalytic function of the acceptor site rather than that of the donor site. Amino acid analysis revealed that the photooxidative modulation of the catalytic activity represents a complex process accompanied by loss of multiple amino acid residues.