The role of plasma membranes in the transfer of non-esterified cholesterol to the acyl-CoA:cholesterol acyltransferase substrate pool in liver microsomal fraction.

The incubation at 37 degrees C of rat-liver microsomal fraction followed by re-isolation of the treated microsomal vesicles results in a time-dependent increase in the activity of acyl-CoA:cholesterol acyltransferase. The rate of this increase was higher in the microsomal fraction from rats fed cholesterol-supplemented diet or starved overnight as compared with that in the microsomal fraction from rats fed standard diet. The presence of a plasma membrane preparation in the incubation mixture also resulted in a time-dependent increase in acyl-CoA:cholesterol acyltransferase activity at a rate that was dependent on the concentration of plasma membranes. During the incubation of the microsomal fraction in the presence of phosphatidylcholine liposomes, cholesterol is transferred from the microsomal to liposomal vesicles. This transfer followed first-order kinetics with respect to cholesterol concentration in the donor with a rate that increased with the concentration of liposomes in the incubation mixture. The presence of phospholipid was also associated with a decrease in the activity of the acyltransferase that was related to the concentration of phospholipid in the incubation mixture. The incubation of the microsomal fraction in the presence of phosphatidylcholine-cholesterol liposomes resulted in a time-dependent and concentration-dependent transfer of liposomal cholesterol to the microsomal fraction and the acyltransferase substrate pool. The measurement of the rate of transfer of liposomal cholesterol to the microsomal vesicles and to the acyltransferase substrate pool at various temperatures showed that activation energies for the two processes are similar. Similar to these various was also the activation energy for the increase in acyl-CoA:cholesterol acyltransferase activity due to preincubation in the absence of artificial membrane vesicles. The present results suggest that there is, under the present conditions, a time-dependent and temperature-dependent flow of cholesterol from plasma membranes to the acyltransferase substrate pool and that this flow is either diverted in the presence of phospholipid liposomes or increased in the presence of cholesterol-phospholipid liposomes.