Solubilization by Heparin of the Phospholipase A1 from the Plasma Membranes of Rat Liver

Abstract The phospholipase A1 of rat liver plasma membranes was stimulated 2- to 3-fold by physiologic concentrations of heparin (1 to 2.4 µg per ml for humans). The stimulation did not change appreciably with increasing enzyme or substrate concentration, nor with the time of incubation. However, activation of hydrolysis by Ca2+ was altered by the addition of heparin. Treatment of the plasma membranes with heparin displaced the enzyme from the membrane which accounts for the stimulation of heparin. Heparin at high concentrations inhibited the enzyme. The amount of 35S-heparin bound to the membrane (about 10 µg per mg of plasma membrane protein) was the same as the amount required for maximal stimulation and solubilization of the membrane-bound enzyme. The enzyme freed from the membrane was stimulated only 2-fold by Ca2+. Gel filtration of the solubilized enzyme indicated that the enzyme was heterogenous in size. Linoleic acid, NaCl, albumin, deoxycholate, and protamine inhibited the activity of the enzyme. Protamine prevented binding of heparin to the membrane and probably blocked displacement of the enzyme from the membrane. We believe our results support the hypothesis that the liver plasma membrane is the origin of the postheparin phospholipase A1.