Analysis of metabolic pathways via quantitative prediction of isotope labeling patterns:: a retrobiosynthetic 13C NMR study on the monoterpene loganin

The monoterpene loganin serves as a precursor in the biosynthetic pathways of numerous indole alkaloids. In contrast to earlier studies, we present evidence that the biosynthesis of loganin in Rauwolfia serpentina cells proceeds mainly via the deoxyxylulose pathway and not by the mevalonate pathway. This conclusion is based on experiments using a R. serpentina cell culture supplied with C-13-labeled samples of glucose, ribose/ribulose, pyruvate or glycerol. Loganin was isolated from biomass, and the hydrolysis of cellular protein afforded amino acids. The isolated metabolites were analyzed by NMR spectroscopy. The C-13-labeling patterns of isolated amino acids were then used to reconstruct the labeling patterns of phosphoenol pyruvate, pyruvate and acetyl CoA. These labeling patterns were subsequently used to predict labeling patterns for dimethylallyl pyrophosphate and isopentenyl pyrophosphate via the mevalonate and deoxyxylulose pathway, respectively. The observed labeling patterns of the terpenoid moieties in loganin were in excellent agreement with the deoxyxylulose prediction. The minor incorporation of mevalonate into loganin observed in earlier studies can be attributed to metabolite exchange between the two terpenoid pathways. The possibility of crosstalk between the two pathways in plants and plant cell cultures stresses the need for a quantitative analysis of general carbon metabolism in order to determine the partitioning between the mevalonate and deoxyxylulose pathway. The present study shows that a wide variety of general metabolic precursors can fulfill this task in conjunction with the retrobiosynthetic concept. (C) 1999 Elsevier Science Ltd. All rights reserved.