The synthesis of [7α-3H]lanosterol is described. It is shown that in the conversion of [7α-3H,26,27-14C2]lanosterol into cholesterol by a rat liver system, it is the 7β-hydrogen atom that is predominantly removed. On the other hand, the conversion of doubly labelled lanosterol into ergosterol by whole yeast cells results in the loss of the 7α-hydrogen atom. These results therefore suggest that the C-7 hydrogen atoms with opposite stereochemistry are labilized by the rat liver and the yeast Δ8–Δ7 steroid isomerases.

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