Demonstration and characterization of partial glyceride specific lipases in pig thyroid plasma membranes

Purified pig thyroid plasma membranes were treated with bacterial phospholipase C. Phospholipids were hydrolyzed, producing diacylglycerols and monoacylglycerol. The products, however, decreased appreciably in the later period of incubation, suggesting the presence of lipase hydrolyzing partial glycerides in the membranes. The presence of the lipase(s) was proved by the use of exogenous substrates. In contrast to ordinary lipases, the plasma membrane enzyme did not hydrolyze triacylglycerol, but diacylglycerol. Apparent Km of the enzyme for 1,2-diacylglycerol was 1.25 mM and optimal pH, 7 – 7.5. The membranes also had the activity to hydrolyze 2-monoacylglycerol but not 1-monoacylglycerol. On the basis of these results, we proposed a scheme in which the enzyme(s) participate in arachidonic acid production from phosphatidylinositol (PI) under the influence of thyrotropin.