Further Studies on the Possible Compartmentation of the Precursor. Pool of Cholesterol for the Biosynthesis of Cholic Acid in the Rat

In vivo and in vitro experiments with rats were carried out to investigate the cholesterol pool in subcellular organelles of the liver that acts as the immediate precursor for the biosynthesis of cholic acid. When rats with a bile-fistula were given a mixture of [2-14C]mevalonate and [1, 2-8H]cholesterol intravenously, the 14C: 2H ratio in cholic acid in both whole homogenate and cytosol prepared from their livers was higher than that in free cholesterol in any subcellular fraction of the livers. When [2-14C]mevalonate was administered intravenously to bile-fistula rats, the specific radioactivity of free cholesterol in the hepatic microsomal fraction exceeded that in any other fraction, and the specific radioactivity of biliary cholic acid was remarkably high, exceeding that of microsomal free cholesterol. In similar experiments with [4–14C]cholesterol, the specific radioactivity of free cholesterol in the hepatic microsomal fraction exceeded that in any other subcellular fraction and the specific radioactivity of biliary cholic acid was lower than that of free cholesterol in any hepatic subcellular fraction. Tissue suspensions of rat livers in Krebs-Ringer bicarbonate (pH 7.4)-5.5 mM glucose were incubated with [2-14C]mevalonate in O2-CO2 (95 : 5, v/v) at 37°. The specific radioactivity of free cholesterol in the microsomal fraction prepared from the incubated tissue exceeded the specific radioactivities of free cholesterol in the other subcellular fractions. The estimated specific radioactivity of taurocholate formed during the incubation was far higher than that of microsomal free cholesterol. These data indicate that hepatic microsomal free cholesterol which was newly synthesized in situ was preferentially incorporated into cholic acid.