Unexpected biosynthetic precursors of amarogentin - A retrobiosynthetic C-13 NMR study

A root culture of Swertia chirata (Gentianaceae) produced the natural bitte r compound amarogentin in 0.1% (dry weight) yield. The biosynthesis of amar ogentin was studied by in vivo experiments, using [1-C-13]glucose, [U-C-13( 6)]glucose, [1-C-13]acetate, [2-C-13]acetate, [7-C-13]benzoic acid, [ring-C -13(6)]cinnamic acid. or [7-C-13]shikimic acid as precursors. Comparison be tween labeling patterns of amarogentin and amino acids showed that dimethyl allyl pyrophosphate (DMAPP) and isopentenyl pyrophosphate (IPP), serving as building blocks of the sweroside moiety, are biosynthesized predominantly (> 95%) by the recently discovered deoxyxylulose phosphate pathway. The bip henylcarboxylic acid moiety is biosynthesized by a polyketide-type pathway, with three units of acetyl-CoA and one unit of 3-hydroxybenzoyl-CoA, this being formed from an early shikimate pathway intermediate and not via cinna mic or benzoic acid.