Homeostatic restoration of microsomal lipids and enzyme changes in HMG-CoA reductase and acyl-CoA: cholesterol acyltransferase in chick liver.

We have studied the correlation between changes in the lipid composition in chick liver microsomes and the activities of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) and acyl-CoA: cholesterol acyltransferase (ACAT) by in vivo and in vitro experiments with 21-day-old chicks. A 5% cholesterol diet for 3 hr produced an increase in the microsomal and plasmatic cholesterol content, a decrease in HMG-CoA reductase activity and a concomitant increase in ACAT activity. The effect produced by the short-term treatment virtually disappeared 27 hr after ending the cholesterol diet. In vitro experiments were carried out by using vesicles constituted by phosphatidylcholine/cholesterol and phosphatidylcholine.