The Biosynthesis of the Triterpene, Eburicoic Acid

The C_(31) triterpene, eburicoic acid, has been biosynthesized by allowing P. sulfureus to grow on a medium containing carboxyl-labeled acetate. Labeled eburicoic acid has been obtained and by stepwise degradation of the molecule the following facts have been established: (a) the acetate is utilized as a two carbon unit, (b) C_4 of ring A and C_(11) and C_(12) of ring C are labeled, and (c) the gem-dimethyl group, C_(30) and C_(31), of ring A, the carboxyl group, C_(21), and the extra methylene carbon atom, C_(28), in the side-chain are not labeled. The specific activities of the labeled carbon atoms are those expected for an eburicoic acid molecule containing 12 carbon atoms derived from the carboxyl of acetate. The location of the labeled atoms and their specific activities are those predicted on the basis of the squalene hypothesis as the mechanism of biosynthesis for steroids and triterpenes. Of particular interest is the demonstration that both C_(11) and C_(12) are derived from the carboxyl carbon of acetate since such is a necessary requirement if squalene is to be utilized as an intact molecule