[Rat liver plasma membrane phospholipase A].

The plasma membranes phospholipase A2 studied in situ shows very little sensitivity for pH variations ; the optimal concentration for Ca++ is 5 mM ; the enzymatic kinetics are of the Michaelis type. An inactive state of the phospholipase A2 exists : when rats are injected with heparin, their plasma membranes contain a very low phospholipase A2 activity. These membranes recover a high A2 activity if they are incubated in the presence of a rat platelets lysate. Treatment of membranes by NaCl 1 M displaces phospholipase A1 and phospholipase A2 but for the latter only when being in active state. Treatment of animals, conditions of preparation and of incubation of membranes influence the respective amounts of phospholipases A1 and A2 present in these membranes and are discussed in this paper. The localisation of these enzymatic proteins inside the membrane according to the membranous model proposed by Singer et al. [18] is also discussed.