Bile acid synthesis in man. In vivo activity of the 25-hydroxylation pathway

During biosynthesis of the 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 three-carbon 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 [¹⁴C]acetone from [¹⁴C]26-cholesterol. In the present study, we adapted this method to human subjects. 4 d after oral administration of 100 μCi of [¹⁴C]26-cholesterol and 1 d after beginning a constant infusion of 16.6 μmol/min unlabeled acetone, three men and two women underwent breath collections. Expired acetone was trapped and purified as the 2,4-dinitrophenylhydrazine derivative. ¹⁴CO₂ was trapped quantitatively using phenethylamine. Specific activity of breath acetone was multiplied by the acetone infusion rate to calculate production of [¹⁴C]acetone. [¹⁴C]Acetone production averaged 4.9% of total release of ¹⁴C from [¹⁴C]26-cholesterol, estimated by ¹⁴CO₂ output. The method was validated by showing that [¹⁴C]acetone production from [¹⁴C]isopropanol averaged 86.9% of the [¹⁴C]isopropanol infusion rate. We conclude that in man, as in the rat, the 25-hydroxylation pathway accounts for < 5% of bile acid synthesis.