2-3-oxidosqualene and lupeol

The biosynthesis of lupeol-3-(3'R-hydroxy)-stearate (procrim b, 1) was investigated in the Mexican medicinal plant Pentalinon andrieuxii by (13)CO2 pulse-chase experiments. NMR analyses revealed positional enrichments of (13)C2-isotopologues in both the triterpenoid and the hydroxystearate moieties of 1. Five of the six isoprene units reflected a pattern with [1,2-(13)C2]- and [3,5-(13)C2]-isotopologues from the respective C5-precursors, IPP and DMAPP, whereas one isoprene unit in the ring E of 1 showed only the [3,5-(13)C2]-connectivity of the original C5-precursor, due to rearrangement of the dammarenyl cation intermediate during the cyclization process. The presence of (13)C2-isotopologues was indicative of [(13)C2]acetyl-CoA being the precursor units in the formation of the fatty acid moiety and of the triterpene via the mevalonate route. The observed labeling pattern was in agreement with a chair-chair-chair-boat conformation of the (S)-2,3-oxidosqualene precursor during the cyclization process, suggesting that the lupeol synthase from P. andrieuxii is of the same type as that from Olea europea and Taraxacum officinale, but different from that of Arabidopsis thaliana. The study shows that (13)CO2 pulse-chase experiments are powerful in elucidating, under in vivo conditions and in a single experiment, the biosynthesis of complex plant products including higher terpenes.

Topics: Amino Acid Sequence; Carbon Isotopes; Cyclization; Intramolecular Transferases; Magnetic Resonance Spectroscopy; Olea; Pentacyclic Triterpenes; Squalene; Stearates; Taraxacum; Triterpenes

The Arabidopsis thaliana LUP1 gene encodes an enzyme that converts oxidosqualene to pentacyclic triterpenes. Lupeol and beta-amyrin were previously reported as LUP1 products. Further investigation described here uncovered the additional products germanicol, taraxasterol, psi-taraxasterol, and 3,20-dihydroxylupane. These results suggest that the 80 known C(30)H(50)O compounds that are structurally consistent with being oxidosqualene cyclase products may be derived from fewer than 80 enzymes and that some C(30)H(52)O(2) compounds may be direct cyclization products of oxidosqualene.

Topics: Arabidopsis; Chromatography, High Pressure Liquid; Cyclization; Intramolecular Transferases; Pentacyclic Triterpenes; Squalene; Triterpenes