To examine the defect in side-chain 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 5 beta-cholestane-3 alpha, 7 alpha-diol was present in all specimens and was 30-fold more active than the corresponding microsomal 25-hydroxylation. However, mean mitochondrial 26-hydroxylation of 5 beta-cholestane-3 alpha,7 alpha-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 beta-cholestane-3 alpha,7 alpha,12 alpha-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-hydroxylation of the same substrate (165 +/- 25 pmol/mg
protein per min). Moreover in
cerebrotendinous xanthomatosis, mitochondrial 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol-26-hydroxylase activity was one-seventh as great as in controls. Hepatic microsomal 12 alpha-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 12 alpha-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
5beta-cholestane-3alpha,7alpha,12alpha-triol was substantially lower in control and
cerebrotendinous xanthomatosis liver. Thus, separate mechanisms may exist for the cleavage of the
cholesterol side chain in
cholic acid and
chenodeoxycholic acid biosynthesis.