Biosynthesis of bile acids in cerebrotendinous xanthomatosis. Relationship of bile acid pool sizes and synthesis rates to hydroxylations at C-12, C-25, and C-26.

Abstract

To examine the defect in side-cbain oxidation during the formation of bile acids in cerebrotendinous xanthomatosis, we measured in vitro hepatic microsomal hydroxylations at C-12 and C-25 and mitochondrial hydroxylation at C-26 and related them to the pool size and synthesis rates of cholic acid and chenodeoxycholic acid as determined by the isotope dilution technique. Hepatic microsomes and mitochondria were prepared from seven subjects with cerebrotendinous xanthomatosis and five controls. Primary bile acid synthesis was markedly reduced in cerebrotendinous xanthomatosis as follows: cholic acid, 133±30 vs. 260±60 mg/d in controls; and chenodeoxycholic acid, 22±10 vs. 150±30 mg/d in controls. As postulated for chenodeoxycholic acid synthesis, mitochondrial 26-hydroxylation of 5jt-cholestane-3a,7a-diol was present in all specimens and was 30-fold more active than the corresponding microsomal 25-hydroxylation. However, mean mitochondrial 26-hydroxylation of 5j8-cholestane-3a,7a-diol was less active in cerebrotendinous xanthomatosis than in controls: 59±17 compared with 126±21 pmol/mg protein per min. As for cholic acid synthesis, microsomal 25-hydroxylation of 5-cholestane3a,7a,12a-triol was substantially higher in cerebrotendinous xanthomatosis and control preparations (620±103 and 515±64 pmol/mg protein per min, respectively) than the corresponding control mitochondrial 26-hyroxylation of the same substrate (165±25 pmol/mg protein per min). Moreover in cerebrotendinous xanthomatosis, mitochondrial 5a-cholestane-3a,7a,2a-triol-26-hydroxylase activity was one-seventh as great as in controls. Hepatic microsomal 12a-hydroxylation, which may be rate-controlling for the cholic acid pathway, was three times more active in cerebrotendinous xanthomatosis than in controls: 1,600 vs. 500 pmol/mg protein per min. These results demonstrate severely depressed primary bile acid synthesis in cerebrotendinous xanthomatosis with a reduction in chenodeoxycholic acid formation and pool size disproportionately greater than that for cholic acid. The deficiency of chenodeoxycholic acid can be accounted for by hyperactive microsomal 12a-hydroxylation that diverts precursors into the cholic acid pathway combined with decreased side-chain oxidation (mitochondrial 26-hydroxylation). However, side-chain oxidation in cholic acid biosynthesis may be initiated via microsomal 25-hydroxylation of 5ft-cholestane-3a,7a,12a-triol, since the corresponding mitochondrial 26-hydroxylation of 5#t-cholestane-

Demonstration of 26-hydroxylation of C27-steroids in human skin fibroblasts, and a deficiency of this activity in cerebrotendinous xanthomatosis.

“…This would be compatible with their proposal that the "25-hydroxylation pathway" may be, physiologically, the most important route for the formation of bile acids (10). Recently, Salen's group has also found evidence for depressed 26-hydroxylation in CTX (11), and they conclude that the primary enzymatic defect in this disease remains to be established (1 1).…”

“…were tested also with 5a-cholestane-3a,7a-diol and 7a-hydroxy-4-cholesten-3-one, and the 26-hydroxylase activity was <3% of the controls also with these substrates (Table II). The experiments shown in Table II were based upon the detection of labeled product by the described HPLC method (see Methods), which separated the products very efficiently from the substrates (e.g., for 5g3-cholestane-3a,7a,12a,26-tetrol and 5l#-cholestane-3a,7a, 12a-triol, the R. were 11 and 38 min, respectively). Fig.…”

Human CYP7A1 deficiency: progress and enigmas

“…Mice deficient in the canalicular bile salt export pump have mild cholestasis (24), whereas humans with this defect develop progressive familial cholestasis, a merciless pediatric disease causing early cirrhosis (25). Loss of sterol 27-hydroxylase in humans causes a striking deficiency of bile acids, cerebrotendinous xanthomatosis, and an accumulation of cholestanol in tissues (26)(27)(28). Mice with the same enzyme defect have a bile acid synthesis defect but do not develop cerebrotendinous xanthomatosis.…”

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

“…These metabolites can be either excreted from the body in the bile or urine or converted to CA. Notably, CYP3A activity does not appear to be increased in human CTX, suggesting a potential explanation for the dichotomy in CYP27A1 Ϫ/Ϫ phenotypes between mice and humans (7,10).…”