Bile Acid Synthesis

During biosynthesis of bile acid, carbons 25-26-27 are removed from the cholesterol side-chain. Side-chain oxidation begins either with hydroxylation at the 26-position, in which case the three-carbon fragment is released as propionic acid, or with hydroxylation at the 25-position, in which case the threecarbon fragment is released as acetone. We have previously shown in the rat that the contribution of the 25-hydroxylation pathway can be quantitated in vivo by measuring production of 1'4Cjacetone from I'4C126-cholesterol. In the present study, we adapted this method to human subjects. 4 d after oral administration of 100 gCi of ['4Cj26-cholesterol and 1 d after beginning a constant infusion of 16.6 Mmol/min unlabeled acetone, three men and two women underwent breath collections. Expired acetone was trapped and purified as the 2,4 dinitrophenylhydrazine derivative. '4C02 was trapped quantitatively using phenethylamine. Specific activity of breath acetone was multiplied by the acetone infusion rate to calculate production of 1'4Ciacetone. ['4CiAcetone production averaged 4.9% of total release of '4C from 1'4(126-cholesterol, estimated by "CO2 output. The method was validated by showing that ['4qacetone production from I'4Ciisopropanol averaged 86.9% of the i'4Ciisopropanol infusion rate. We conclude that in man, as in the rat, the 25-hydroxylation pathway accounts for < 5% of bile acid synthesis.