Indole alkaloid biosynthesis: An investigation into leucine as a possible precursor of the monoterpene portion of vindoline

Abstract

Because of the nonspecific incorporation of acetate into the monoterpenoid portion of the indole alkaloids, the amino acid leucine has been investigated as an alternative precursor. Incorporations of leucine into the alkaloids vindoline and catharanthine in Vinca rosea are found to be 0.07 and 0.02%, respectively, levels comparable with those of acetate. An equation is developed to calculate the quantity of [ 13C]leucine needed to be fed in order to obtain alkaloids with sufficient 13C enrichment to be analysed by 13C nmr spectroscopy. This equation assumes that percentage incorporations do not decrease with increasing quantities of leucine fed, an assumption found to be true. [2- 13C]Leucine was synthesized from [2- 13C]acetic acid in an eight-step procedure in 11.5% overall yield, and fed to Vinca rosea plants. Incorporations into vindoline obtained using both 14C and 13C were in reasonable agreement, and the activity was found to be spread over seven carbon atoms, none of which corresponded to the two atoms at which activity was expected. It is concluded that the leucine → mevalonate → monoterpene route does not operate in indole alkaloid biosynthesis, leaving open the question of the origin of the monoterpene portion of the alkaloids. An attempt to confirm these results by degradation of 14C-labelled alkaloids was unsuccessful, vindoline proving to be unexpectedly resistant to oxidation.