A novel pathway for biosynthesis of cholestanol with 7 alpha-hydroxylated C27-steroids as intermediates, and its importance for the accumulation of cholestanol in cerebrotendinous xanthomatosis.

Abstract

A mixture of 7a-3H-and 4-'4C-labeled cholesterol was administered intravenously to rats. Cholestanol with 20-30% lower ratio between 3H and '4C than in cholesterol could be isolated from different organs. In a healthy human control, cholestanol isolated from feces had a 3H/'4C ratio which was 28% lower than in administered cholesterol. Cholesterol and coprostanol reisolated in these experiments had the same ratio between 3H and 14C as in the precursor. A previously unknown pathway for formation of cholestanol, involving 7a-hydroxylated intermediates, may explain these results. Under normal conditions, this pathway is responsible for at most 30% of the cholestanol synthesized from cholesterol.Intravenous administration of the 7a-3H-and 4-'4C-labeled cholesterol to a patient with cerebrotendinous xanthomatosis (CTX) resulted in formation of cholestanol which had 70-75% lower 3H/'4C ratio. It is concluded that the novel pathway involving 7a-hydroxylated intermediates is accelerated in patients with CTX. This acceleration may contribute essentially to the accumulation of cholestanol, which is a predominant feature of this disease.7a-Hydroxycholesterol and 7a-hydroxy4-cholesten-3-one might be intermediates in the novel pathway to cholestanol. After intravenous administration of 7_-3H-labeled 7a-hydroxycholesterol in a patient with CTX, significant amounts of 3H were incorporated into plasma and fecal cholestanol. Only small amounts of 7a-hydroxycholesterol and 7a-hydroxy-4-cholesten-3-one are excreted into the intestine, and we therefore conclude that the 7a-dehydroxylation step mainly occurs in the liver. In CTFX, the synthesis of cholestanol may be accelerated because the concentrations of 7a-hydroxylated bile acid intermediates in the liver are increased. A possible mechanism for the conversion of a minor fraction of 7a-hydroxycholesterol into cholestanol is suggested.

On the mechanism of accumulation of cholestanol in the brain of mice with a disruption of sterol 27-hydroxylase

“…There is a clear link between the increased cholesterol 7 ␣ -hydroxylase activities in patients with CTX and the accumulation of cholestanol. By injecting 7 ␣ -tritium-labeled cholesterol and measuring incorporation of the label in cholestanol, we could demonstrate that the major part of the cholestanol formed in CTX involves 7 ␣ -hydroxylated intermediates ( 10 ). We have also shown that the bile acid precursor 7 ␣ -hydroxy-4-cholesten-3-one is metabolized into cholestanol under in vitro conditions, with cholesta-4,6-dien-3-one and 4-cholesten-3-one as intermediates ( 10 ).…”

“…In the alternative pathway (pathway II in the fi gure), 4-cholesten-3-one is not formed directly from cholesterol but from 7 ␣ -hydroxy-4-cholesten-3-one by dehydration followed by saturation of the ⌬ 6 double bond ( 2,10 ). The enzymes involved in the latter two reactions have been characterized with respect to reaction mechanisms ( 25 ) and are analogous to reactions involved in the bacterial 7 ␣ -dehydroxylation of cholic acid into deoxycholic acid ( 26 ).…”

Identification of bile acid precursors as endogenous ligands for the nuclear xenobiotic pregnane X receptor

“…Cholestanol (5) levels were only modestly increased in these hepatic microsome preparations. The concentration of the putative intermediate 4-cholesten-3-one (4) was not determined (28,29). We tested whether these compounds could activate PXR, CAR, FXR, or VDR in cell-based reporter assays (Fig.…”

A blood test for cerebrotendinous xanthomatosis with potential for disease detection in newborns

“…We previously utilized keto-moiety derivatization with Girard's P reagent (1-(carboxymethyl) pyridinium chloride hydrazide) ( 23,24 ) to enable sensitive isotope dilution LC-ESI-MS/MS quantifi cation of plasma ketosterol BA precursors that accumulate in CTX (25)(26)(27). Although the method lower limit of quantifi cation (LLOQ) that we reported (50-100 ng/ml) was close to the maximum circulating concentration of 7 ␣ -hydroxy-4-cholesten-3-one (7 ␣ C4) in healthy individuals (28)(29)(30)(31), we demonstrated 7 ␣ C4 possessed improved diagnostic utility over 5 ␣ -cholestanol as a marker of CTX ( 21 ).…”