Solubilization and properties of the liver plasma membrane transglutaminase

We recently reported that rat liver contains a transglutaminase activity which is specifically associated with the lateral plasma membrane domain [D. J. Tyrrell, W. S. Sale, and C. W. Slife (1986) J. Biol. Chem. 261, 14833-14836]. In this manuscript, conditions for maintaining the activity of this plasma membrane-associated enzyme are described and an unusual method for solubilizing the enzyme is detailed. When rat liver plasma membranes were stored at 4 degrees C, the transglutaminase activity was rapidly lost unless dithiothreitol was present. If calcium or EDTA were included with the reducing agent, a time-dependent enhancement of enzyme activity occurred. These reagents probably prevented and perhaps reversed the oxidation of critical thiol residues in the transglutaminase. When the membranes were incubated at 37 degrees C, increased enzyme activity was found only if 50% glycerol was added to the dithiothreitol and calcium-containing buffer. Under these latter conditions, a selective release of the enzyme from the membrane also occurred, with the enzyme remaining soluble after the glycerol was removed. These data, and our inability to solubilize the enzyme with detergents, indicate that the plasma membrane transglutaminase is a peripheral membrane protein which associates only with a specific plasma membrane domain.