Effect of milk constituents on hepatic cholesterol-genesis.

Two preparations active in reducing hepatic cholesterol biosynthesis were isolated from bovine skim milk. One of the inhibitors was in the dialysate and was identified as orotic acid (OA). The other inhibitor, present in the retentate, was not identified. Orotic acid appears to act by inhibiting cholesterol biosynthesis before the formation of mevalonate, whereas the retentate inhibitor exerts its effect beyond the formation of mevalonate in the biosynthetic pathway. Human milk also inhibited the incorporation of both labeled acetate and mevalonate into cholesterol by rat liver. Orotic acid was not detectable in human milk samples employed in this study. Administration of [6-14C]orotate to rats revealed its conversion to uracil in the liver. Subsequent work demonstrated that uracil had inhibitory activity on hepatic cholesterol biosynthesis similar to that of orotate when incubated with rat liver slices.