On the origin of the cholestenoic acids in human circulation

3β-Hydroxy-5-cholestenoic acid, 3β,7α -dihydroxy-5-cholestenoic acid, and 7α-hydroxy-3-oxo-4-cholestenoic acid are metabolites of cholesterol present at significant concentrations (40-80ng/ml) in human circulation. The 7α-hydroxylated acids may be formed from cholesterol via two major pathways initiated by oxidations at either the 7α- or 27-positions. In an attempt to clarify the origin and possible precursor-product relationships between these cholestenoic acids, we measured their deuterium enrichment in a unique experiment, after infusion of 10g of [²H₆]-cholesterol to a healthy volunteer. The observed extent and time-course of deuterium enrichment of circulating 3β-hydroxy-5-cholestenoic and 3β,7α-dihydroxy-5-cholestenoic acid were almost identical, while different from that of cholesterol and 7α-hydroxycholesterol. Notably, the deuterium enrichment of 7α-hydroxy-3-oxo-4-cholestenoic acid was similar to that of 7α-hydroxycholesterol (and its metabolite 7α -hydroxy-4-cholesten-3-one), though distinct from the other cholestenoic acids. Finally, the enrichment of unesterified 27-hydroxycholesterol followed a similar, though less pronounced, time curve to the Δ ⁵-cholestenoic acids. In conclusion, these results suggest that plasma 3β-hydroxy-5-cholestenoic acid is formed from a pool of cholesterol distinct from that used for the formation of the bulk of 27-hydroxycholesterol. The results are also in accordance with a formation of 3β,7α -dihydroxy-5-cholestenoic acid directly from 3β-hydroxy-5-cholestenoic acid, and a formation of most of the circulating 7α -hydroxy-4-cholesten-3-one from 7α-hydroxycholesterol. These results are consistent with a flux of 7α-hydroxycholesterol from the liver into the circulation, and an extrahepatic metabolism of this steroid into 7α-hydroxy-3-oxo-4-cholestenoic acid.