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 3.3 ~M to 322/~M in rat liver homogenates inhibited the incorporation of [1-Carbon 14]acetate but not [1-Carbon 14]acetyl-coenzyme A, 3-hydroxy-3-methyl-[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 Lineweaver-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 x 10 -4 M for acetate with the yeast enzyme while a K i was 6.6 X 10 -s M for orotic acid.The approximate point of 50% inhibition with rat liver enzyme was 7 x 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 concentrationabout 12 mM.Neither raw milk nor pasteurized, homogenized milk inhibited 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase that had been partially purified from rat liver.