Effects of acidic phospholipids, nucleotides, and heparin on the activity of lipase from rat liver lysosomes

Abstract

Purification and characterization of endogenous lipid factors that stimulate rat liver lysosomal lipase has led to the identification of cardiolipin, phosphatidylserine, and phosphatidic acid as stimulators of this activity. Bovine heart cardiolipin (half-maximal stimulation at 1.5 x 10(-4) m) and bovine brain phosphatidylserine (half-maximal stimulation at 9.5 x 10(-4) m) were the most potent of the phospholipids from other sources tested. The major rate-enhancing effect of phosphatidylserine is expressed as a 35-fold increase in the apparent V(max) of the enzyme. The effect is produced by acid phospholipids specifically, since in no case was there greater than a twofold stimulation by synthetic detergents, zwitterionic phospholipids, taurocholic acid, or gum acacia. The observed degree of stimulation depends upon the detergent used to disperse tripalmitin substrate and the relative concentrations of factor and detergent in reaction mixtures. The concentration of phosphatidylserine to produce half-maximal stimulation is directly dependent upon the Triton X-100 concentration, but the effects of this detergent on cardiolipin stimulation are more complex. Enzyme activity is inhibited 50% by 1 mm nucleoside triphosphate and 2.5 mm ADP, 80% by 1 mm PP(i), 100% by 20 U/ml heparin and 0.25 mg/ml chondroitin sulfate, and 80% by 10 mm sulfate ion. Inhibition is partially prevented by phosphatidylserine.