The accumulation of various 25-hydroxylated C(27)-bile alcohols in blood and their excretion in urine are characteristic features of cerebrotendinous xanthomatosis (CTX) a recessively inherited inborn error of bile acid synthesis caused by mutations in the mitochondrial sterol 27-hydroxylase (CYP27) gene. These bile alcohols may be intermediates in the alternative cholic acid side chain cleavage pathway. The present study was undertaken to identify enzymes and reactions responsible for the formation of these bile alcohols and to explain why Cyp27(-/-) mice do not show CTX-related abnormalities. Microsomal activities of 5beta-cholestane-3alpha,7alpha,12alpha-triol 25- and 26-hydroxylases, 5beta-cholestane-3alpha,7alpha,12alpha,25-tetrol 23R-, 24S-, and 27-hydroxylases and testosterone 6beta-hydroxylase, a marker enzyme for CYP3A, in Cyp27(-/-) mice livers were markedly up-regulated (5.5-, 3.5-, 6.5-, 7.5-, 2.9-, and 5.4-fold, respectively). In contrast, these enzyme activities were not increased in CTX. The activities of 5beta-cholestane-3alpha,7alpha,12alpha-triol 25- and 26-hydroxylases and 5beta-cholestane-3alpha,7alpha,12alpha,25-tetrol 23R-, 24R-, 24S-, and 27-hydroxylases were strongly correlated with the activities of testosterone 6beta-hydroxylase in control human liver microsomes from eight unrelated donors. Troleandomycin, a specific inhibitor of CYP3A, markedly suppressed these microsomal side chain hydroxylations in both mouse and human livers in a dose-dependent manner. In addition, experiments using recombinant overexpressed human CYP3A4 confirmed that these microsomal side chain hydroxylations were catalyzed by a single enzyme, CYP3A4. The results demonstrate that microsomal 25- and 26-hydroxylations of 5beta-cholestane-3alpha,7alpha,12alpha-triol and microsomal 23R-, 24R-, 24S-, and 27-hydroxylations of 5beta-cholestane-3alpha,7alpha,12alpha,25-tetrol are mainly catalyzed by CYP3A in both mice and humans. Unlike Cyp27(-/-) mice, CYP3A activity was not up-regulated despite marked accumulation of 5beta-cholestane-3alpha,7alpha,12alpha-triol in CTX.