Inhibition of Cholesterol Biosynthesis and Acetyl-Coenzyme A Synthetase by Bovine Milk and Orotic Acid

Pasteurized, homogenized bovine milk or orotic acid in solution at final concentrations varying from 3.3μM to 322μM in rat liver homogenates inhibited the incorporation of [1-Carbon 14] acetate but not [1-Carbon 14] acetyl-coenzyme A, 3-hydroxy-3-methyj-[Carbon 14] glutaryl-coenzyme A or [5-hydrogen 3] mevalonic acid into cholesterol. Milk inhibited cholesterol biosynthesis up to 72±10% before acetyl-coenzyme A formation, thus indicating that acetyl-coenzyme A synthetase is the affected enzyme. Kinetics of the inhibition were studied with acetyl-coenzyme A synthetase purified from yeast. From a Line-weaver-Burk plot of the inhibition of yeast acetyl-coenzyme A synthetase, orotic acid is a noncompetitive inhibitor of acetyl-coenzyme A synthetase. A Michaelis constant was 6.0×10−4 M for acetate with the yeast enzyme while a Ki was 6.6×10−5 M for orotic acid. The approximate point of 50% inhibition with rat liver enzyme was 7×10−6 M orotic acid indicating the mammalian enzyme may be more sensitive to the orotic acid. Nicotinic acid also inhibited the yeast enzyme. Fifty percent inhibition required a relatively high final concentration–about 12mM. Neither raw milk nor pasteurized, homogenized milk inhibited 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase that had been partially purified from rat liver.