A specific acetylhydrolase for 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (a hypotensive and platelet-activating lipid)

Abstract

1-Alkyl-2-acetyl-sn-glycero-3-phosphocholine, a phospholipid with platelet activating and hypotensive properties, has an extremely labile acetate grouping. The acetate group is obviously important in the expression of the biological properties of this unique derivative of plasmanic acid since once it is hydrolyzed from the parent compound to form the lyso product, all biological activity is lost. Our studies show that the enzyme responsible for the hydrolysis of the acetate moiety, 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine: acetylhydrolase, occurs in the cytosolic fraction of a variety of tissues and has a pH optimum of 7.5 to 8.5. Effects of calcium, magnesium, EDTA, dithiothreitol, deoxycholate, and diisopropylfluorophosphate on the enzyme activity and the fact that egg phosphatidylcholine was not inhibitory indicate that acetylhydrolase activity has different properties from those normally associated with the phospholipase A2 that utilizes phospholipids with two long chain acyl groups. The highest specific activity of the acetylhydrolase occurred in kidney; lung and brain were also good sources of the enzyme. The soluble fraction from the kidney cortex had an apparent Km and Vmax of 3.1 microM and 17.8 nmol/min/mg of protein, respectively. Our results indicate that acetylhydrolase plays a significant role in the catabolism of 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine.