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

26-Hydroxylation of 5β-cholestane-3α,7α,12α-triol and other C₂₇-steroids was demonstrated in cultured skin fibroblasts from healthy individuals. Activities in skin fibroblasts were ~5-10% of those previously found in human liver homogenates, and were inhibited by CO. The apparent K(m) was lowest for 5β-cholestane-3α,7α,12α-triol (1.3 μmol/liter) and highest for 5-cholestene-3β,7α-diol (12 μmol/liter). The rate of 26-hydroxylation was highest with 7α-hydroxy-4-cholesten-3-one. These characteristics are similar to those of hepatic mitochondrial C₂₇-steroid 26-hydroxylase. In skin fibroblasts from three patients with cerebrotendinous xanthomatosis (CTX), 26-hydroxylation of C₂₇-steroids proceeded at a rate of only 0.2-2.5% of healthy controls. No accumulation of endogenous 5β-cholestane-3α,7α,12α-triol could be demonstrated in these cells, and the lowered formation of radioactive, 26-hydroxylated products could not be explained by dilution of the labeled exogenous substrate. The present results add strong evidence to the concept that the primary metabolic defect in CTX is a deficiency of C₂₇-steroid 26-hydroxylase.