THE PLATELET-ACTIVATING-FACTOR ACETYLHYDROLASE OF MOUSE PLATELETS

platelet-activating factor (PAF) acetylhydrolases are a family of dist inct enzymes with the common property of hydrolyzing and inactivating PAF. It has been shown that the structure and the biochemical behavior of these enzymes depend on their cellular origin. We studied the PAF acetylhydrolase activity in mouse platelets in order to investigate th e unusual response of these platelets to PAF. We found that mouse plat elets contain a PAF acetylhydrolase with an apparent K-m value of 0.8 mu M, suggesting a very high affinity for PAF. Contrary to other norma l mammalian cells and tissues, mouse platelet PAF acetylhydrolase is a lmost equally distributed in the membranes and the cytosol and is char acterized by an extreme sensitivity to heating. The enzyme requires th e presence of dithioerythritol for maximal activity, it is affected by 5,5'-dithiobis(2-nitrobenzoic acid) and N-ethylmaleimide, and it is s trongly inhibited by phenylmethylsulfonylfluoride. We purified, to nea r homogeneity, the PAF acetylhydrolase from mouse platelet membranes a nd demonstrated that it is a protein relatively abundant in the membra nes with an apparent molecular weight of 270 kDa. Electrophoretic anal ysis, under reducing conditions, revealed four bands and one duplet wi th molecular weights of 66, 55, 52, 49 and 62 kDa, respectively. Thus, PAF hydrolysis in mouse platelets is mediated by a PAF acetylhydrolas e having biophysical and biochemical properties more intricate than th ose of the PAF acetylhydrolases found in other species.