Biosynthesis of flavan-3-ols by leaf extracts of Onobrychis viciifolia

A subcellular enzyme fraction (M(r) > 20 X 10(6)) from immature sainfoin leaves catalysed the two-step NADPH-dependent reduction of (+)-dihydromyricetin to (+)-gallocatechin. No enzyme-mediated (-)-epigallocatechin was formed under the conditions employed, and only traces of 2R,3S-trans-3S,4S-cis-leucodelphinidin were observed. The two-step reductase activity mirrored proanthocyanidin content, rising to a maximum before leaflet unfolding, then declining during leaf expansion. When [C-14](+)-dihydromyricetin and [C-4-H-3]2R,3S-trans-3S,4S-cis-leucodelphinidin were supplied to the same fraction, preferential utilization of (+)-dihydromyricetin was observed. Enzymic formation of proanthocyanidin dimers could not be detected using these preparations under a variety of conditions. However, a small portion of radioactivity was bound to the subcellular fraction after addition of labelled (+)-catechin. In addition, in vivo incorporation of(+)-dihydromyricetin into proanthocyanidins was observed. Copyright (C) 1997 Elsevier Science Ltd