Monoglyceride hydrolase activities of rat plasma and platelets. Their properties and roles in the activity of lipoprotein lipase.

Rat plasma contains monoglyceride hydrolase activities against both 1(3)- and 2-monoglycerides. These activities are present as lipoprotein complexes recovered by either density flotation or by agarose gel chromatography with plasma high density lipoprotein. However neither activity is complexed with the major apoproteins (apo-A-I, apo-E) of this lipoprotein class. 2-Monoglyceride hydrolase (but not 1(3)-monoglyceride hydrolase) activity associates with the triglyceride-rich lipoprotein class. The two activities are also noncompetitive with respect to substrate, and differ in pH- and cofactor-dependence and sensitivity to inhibition by diethyl p-nitrophenyl phosphate. Rat platelets also contain both 1(3)- and 2-monoglyceride hydrolase activities. These differ in reactivity with antiesterase and after solubilization and electrophoretic migration with each other and with the corresponding plasma activities. Studies with the isolated perfused rat heart suggest that a major role in the catabolism of 2-monoglyceride generated from lipoprotein lipase activity at the coronary bed is played by the plasma 2-monoglyceride hydrolase activity.