The Mechanism of Introduction of Alkyl Groups at C-24 of Sterols

Abstract 1. In germinating peas the methyl group of methionine was incorporated into 24-ethylcholesterol, the major component of pea sterol, but not into the pentacyclic triterpenoid, β-amyrin, under conditions which lead to the incorporation of label from 2-14C-mevalonate into both substances. Since only C-28 and C-29 are not common to β-amyrin and the sterol, the methionine label must reside in these two carbon atoms. 2. The minor component of pea sterol was shown to be 24-methylcholesterol. 3. From the relative incorporations of label into 24-methyl- and 24-ethylcholesterol from mevalonate and from methionine it was possible to show that both C-28 and C-29 are labeled in the 24-C2 compound. 4. When labeled ethionine was used, the amount of label which appeared in sterol was nearly zero under conditions which strongly label 24-ethylcholesterol from methionine. 5. When a species of brown algae was cultured in the presence of methyl-14C-methionine, radioactive fucosterol was produced. 6. The results reported here are in agreement with the mechanism we proposed earlier (1) in which the C2 group at C-24 of naturally occurring sterols and tetracyclic triterpenoids is derived by two C1 transfers.