A Mechanism for the Biosynthesis of a - Carotene

from [2-14C]mevalonic acid is labelled only in the methyl carbon shows that the reduction of the A24-double bond of desmosterol to form cholesterol, as well as the oxidation of the terminal carbon atom of the side chain of cholesterol, is stereospecific. If this were not the case, the C-26 and C-27 would become equivalent, and the [14C]propionic acid removed enzymically from the side chain would be labelled equally in the methyl and carboxyl carbon atoms. The results from the two experiments, in both of which the pattern of labelling in the cholesterol side chain was retained in the [14C]propionate, are difficult to explain if the [14C]propionate was formed by recombination of C1 and C2 fragments derived from the side chain, but would be expected if the carbon skeleton of the isopropyl unit was removed intact as a C3 fragment. This could be propionic acid itself or, as suggested by Suld et al. (1962), the CoA ester of propionic acid.