Molecular Pharmacology, Vol 15, 375-385, Copyright © 1979 by the American Society for Pharmacology and Experimental Therapeutics

Responses of Microsomal UDP-Glucuronyltransferase to Trypsin

¹ Drug Metabolism Research Unit, Department of Pharmaceutical Chemistry, Strathclyde University, Glasgow G1 1XW, U. K.

The UDP-glucuronyltransferases of rat and guinea pig liver microsomal fractions prepared in 154 mM KCl responded very differently to proteolysis of the membrane surface by trypsin. Guinea pig transferase was inactivated biphasically: a rapid inactivation stage was followed by a much slower one. The slow inactivation was accelerated by chymotrypsin and the combined effects of the two proteases caused almost total loss of activity. A large part of the activity lost on trypsin treatment was recovered on adding phosphatidylcholine. Rat transferase, on the other hand, was much more resistant to proteolysis. Trypsin elicited a triphasic response which occurred on a long time scale. Firstly, a small, rapid inactivation was observed, followed by a slow activation, then finally, inactivation. However, when areas of the microsomal membranes shielded from proteolysis in intact preparations were made accessible to trypsin by adding Triton X-100, the UDP-glucuronyltransferases of both animals were rapidly inactivated in similar fashion. These results may be explained if UDP-glucuronyltransferase molecules are located differently in KCl-prepared microsomal membranes of the two species. In guinea pigs the enzyme molecules probably are situated at or near the membrane surface, while in rats the majority appear to occupy sites deep within the membrane.

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ACKNOWLEDGMENTS We thank Mrs. Carolyn Hibberd, Mrs. Marion Kinnear and Mrs. S. Stafford for their technical assistance.