Terpenoid indole alkaloid biosynthesis and enzyme activities in two cell lines of Tabernaemontana divaricata

Abstract

The possible limitation of the rate of biosynthesis of terpenoid indole alkaloids by low enzyme levels was investigated in two cell lines of Tabernaemontana divaricata with different terpenoid indole alkaloid biosynthetic capacities. The activities of tryptophan decarboxylase (TDC), strictosidine synthase (SSS), strictosidine glucosidase (SG), isopentenyl pyrophosphate isomerase (IPP isomerase) and geraniol 10-hydroxylase (G10H) of both cell lines were compared. The activities of TDC, SSS and IPP isomerase did not show a direct relationship with the biosynthetic capacity but SG and G10H might be limiting. In order to test whether the availability of the terpenoid precursor limits the biosynthesis of the terpenoid indole alkaloids, loganin was fed to the cultures. Loganin-feeding did not influence any of the measured enzyme activities but increased the terpenoid indole alkaloid accumulation of both cell lines to similar levels. A five-fold increase was observed for the accumulating line and a more than 100-fold increase for the low-accumulating one. Strictosidine accumulated mainly in the low-accumulating cell line which has high TDC and low SG activity; the amounts and types of the other terpenoid indole alkaloids which accumulated were similar in both lines. From this it can be concluded that the biosynthesis of terpenoid indole alkaloids in both cultures is limited by the availability of terpenoid precursors; this pathway is not saturated with substrates under normal culture conditions.