Abstract
The properties of γ-glutamyltransferase in homogenates of human kidney were investigated by measuring its susceptibility to inactivation by heating at 56°C. In freshly prepared homogenates, or after short storage at −20°C, the enzyme was observed to be relatively heat-stable. However, it could be transformed into a heat-labile form by treatment with thiol compounds or the proteolytic enzymes trypsin and papain. This process was not necessarily accompanied by solubilisation of the enzyme from the particulate fraction of the homogenate. The addition of 1 mmol/l glutathione during the heating step completely protected the enzyme against heat inactivation in either the relatively heat-stable form or the heat-labile form produced as a result of prior treatment with enzymes or thiol reagent. The observation that glutathione concentrations lower than the apparent Km for glutathione as a substrate fully protect the enzyme against heat inactivation suggests that the enzyme contains a second binding site for glutathione. This second site would be of higher affinity for glutathione than the substrate-binding site but would not itself participate in the enzyme reaction.