Subcellular location and topography of rat hepatic monoacylglycerol acyltransferase activity.

Monoacylglycerol acyltransferase activity from suckling rat liver was localized to the microsomal subcellular fraction by differential centrifugation and comparison with the partitioning of selected marker enzymes. Chymotrypsin, pronase, and proteinase K inactivated the monoacylglycerol acyltransferase activity in detergent-disrupted microsomes, but not in intact microsomes, falsely suggesting a lumenal location for the enzyme. The impermeant inhibitors mercury-dextran and 4,4'-diisothiocyano,-2,2'-disulfonic acid stilbene inhibited monoacylglycerol acyltransferase in intact microsomes. These data, as well as the lack of latency and the inability of the substrate palmitoyl-CoA to readily permeate hepatic microsomes from suckling rats, strongly suggest that the enzyme's active site faces the cytosolic surface of the endoplasmic reticulum.