Role of sphingomyelin in the regulation of cholesterol esterification in the plasma lipoproteins. Inhibition of lecithin-cholesterol acyltransferase reaction.

In order to determine whether sphingomyelin (SPH) affects the rate of cholesterol esterification by plasma lecithin-cholesterol acyltransferase (LCAT), we studied the effects of its incorporation in to defined proteoliposome substrates containing phosphatidyl choline (PC), unesterified cholesterol, and apoprotein A-I, on the activity of purified LCAT. Cholesterol esterification was inhibited by up to 90% in the presence of SPH, and this inhibition was reversed by treatment with bacterial sphingomyelinase. The inhibition could be overcome by increasing the concentration of PC, but not unesterified cholesterol or apoprotein A-I, in the substrate. The effect of SPH was not related to the alterations in the size of the substrate particle and was not dependent on the type of acyl donor or apoprotein activator employed. The lysolecithin acyltransferase and phospholipase reactions carried out by LCAT were also inhibited by SPH. Kinetic studies suggested that: 1) LCAT binds better to substrate vesicles which contain SPH; 2) SPH competes with PC in binding to the active site of the enzyme; and 3) SPH is a more powerful competitive inhibitor than a diether analog of PC. The ability of various lipoproteins to act as substrates for purified LCAT varied inversely with the SPH/PC ratio. Treatment of the lipoproteins with sphingomyelinase activated the LCAT reaction, the percent activation being directly proportional to the SPH concentration in the native lipoprotein. Enrichment of high density lipoproteins with SPH inhibited cholesterol esterification in them by 50%, and this inhibition could be reversed by the degradation of SPH. These results show that SPH is a physiological inhibitor of cholesterol esterification in the plasma, by virtue of its competition with PC, the acyl donor for the reaction.