Inactivation of 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine by a plasma acetylhydrolase: higher activities in hypertensive rats.

We have partially characterized the properties of a specific acetylhydrolase in plasma from spontaneous hypertensive rats. This enzyme inactivates 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (a lipid involved in platelet aggregating, hypotensive, and allergic responses) by removal of the acetate group. The extent of acetate hydrolysis was linear with both time and protein concentration, and the enzyme had an apparent Km of 2.5 microM and a Vmax of 2.6 nmol/min/mg protein. As with an intracellular acetylhydrolase previously characterized by us, the plasma activity was not affected by addition of phosphatidylcholine, EDTA, or Ca2+. However, in contrast to the acetylhydrolase activity in the rat kidney soluble fraction, the plasma activity was associated with a higher molecular weight protein resolved on a Sepharose 6B column and the plasma acetylhydrolase was not inhibited by treatment with trypsin, pronase, or subtilisin. We also compared the acetylhydrolase activity in plasma of age-matched spontaneous hypertensive rats and their normotensive controls, and found approximately 20% higher levels of activity in plasma from the hypertensive animals (P less than 0.01).