Cholic Acid Biosynthesis: THE ENZYMATIC DEFECT IN CEREBROTENDINOUS XANTHOMATOSIS

Cholic acid biosynthesis is defective in individuals with cerebrotendinous xanthomatosis (CTX) and is associated with the excretion of 5β-cholestane-3α,7α, 12α,25-tetrol, an intermediate in the 25-hydroxylation pathway of cholic acid in CTX. To define the enzymatic defect in CTX, two suspected precursors of cholic acid, namely 5β-[7β-³H]cholestane-3α,7α, 12α-triol and 5β-[24-¹⁴C]cholestane-3α,7α, 12α,24S,25-pentol were examined by both in vivo and in vitro experiments. A third precursor, 5β-[7β-³H]-cholestane-3α,7α, 12α,25-tetrol, was compared with them in vitro.

In the in vivo experiments, each one of the labeled precursors was administered intravenously to two CTX and two control subjects. In the controls, 5β-[7β-³H]cholestane-3α,7α, 12α-triol as well as 5β-[24-¹⁴C]-cholestane-3α,7α, 12α,24S,25-pentol were rapidly converted to labeled cholic acid. Maximum specific activity values were reached within 1 d after pulse labeling, followed by exponential decay of the cholic acid specific activity curves. In contrast, these two precursors differed widely when administered to two CTX patients. While 5β-[24-¹⁴C]cholestane-3α,7α, 12α,24S,25-pentol was rapidly converted to [24-¹⁴C]cholic acid and yielded identical decay curves with those obtained in the control subjects, maximum specific activity values in [7β-³H]cholic acid were much lower and peaked only on the second day after the injection of 5β-[7β-³H]cholestane-3α,7α, 12α-triol. Furthermore, an appreciable amount of ³H label was present in the 5β-cholestane-3α,7α, 12α,25-tetrol isolated from the bile of the subjects with CTX.

In the in vitro experiments, three enzymes on the 25-hydroxylation pathway of cholic acid were examined in both control and CTX subjects. The rate of the 25-hydroxylation of 5β-cholestane-3α,7α, 12α-triol in CTX patients was comparable to that in the controls. Similarly, the transformation of 5β-cholestane-3α,7α, 12α,24S,25-pentol to cholic acid, catalyzed by soluble enzymes, proceeded at approximately equal rates in CTX and in control individuals. On the other hand, the rate of 5β-cholestane-3α,7α, 12α,24S,25-pentol formation was about four times greater in the control subjects than in the CTX patients.

The results of the in vivo as well as the in vitro experiments suggest that the site of the enzymatic defect in CTX is at the 24S-hydroxylation of 5β-cholestane-3α,7α, 12α,25-tetrol. The relative deficiency of this hydroxylase in CTX patients, accompanied by the accumulation of its substrate in bile and feces, probably accounts for the subnormal production of bile acids in CTX patients.

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