Phospholipase A2-treated human high-density lipoprotein and cholesterol movements: exchange processes and lecithin: cholesterol acyltransferae reactivity

Human HDL₃ (d 1.125?1.21 g/ml) were treated by an exogenous phospholipase A₂ from Crotalus adamenteus in the presence of albumin. Phosphatidylcholine hydrolysis ranged between 30 and 90% and the reisolated particle was essentially devoid of lipolysis products.

• 1.(1) An exchange of free cholesterol was recorded between radiolabelled erythrocytes at 5–10% haematocrit and HDL₃ (0.6 mM total cholesterol) from 0 to 12–15 h. Isotopic equilibration was reached. Kinetic analysis of the data indicated a constant rate of free cholesterol exchange of 13.0 μM/h with a half-time of equilibration around 3 h. Very similar values of cholesterol exchange, specific radioactivities and kinetic parameters were measured when phospholipase-treated HDL replaced control HDL.
• 2.(2) The lecithin: cholesterol acyltransferase reactivity of HDL₃, containing different amounts of phosphatidylcholine, as achieved by various degrees of phospholipase A₂ treatment, was measured using a crude preparation of lecithin: cholesterol acyltransferase (the d 1.21–1.25 g /ml plasma fraction). The rate of esterification was determined between 0 and 12 h. Following a 15–30% lipolysis, the lecithin:cholesterol acyltransferase reactivity of HDL₃ was reduced about 30–40%, and then continued to decrease, though more slowly, as the phospholipid content was further lowered in the particle.
• 3.(3) The addition of the lecithin:cholesterol acyltransferase preparation into an incubation medium made of labelled erythrocytes and HDL₃ promoted a movement of radioactive cholesterol out of cells, above the values of exchange, and an accumulation of cholesteryl esters in HDL. This reflected a mass consumption of free cholesterol, from both the cellular and the lipoprotein compartments upon the lecithin:cholesterol acyltransferase action. As a consequence of a decreased reactivity, phospholipase-treated HDL (with 2/3 of phosphatidylcholine hydrolyzed) proved much less effective in the lecithin:cholesterol acyltransferase-induced removal of cellular cholesterol.