BIOSYNTHESIS OF FLAVAN-3-OLS BY LEAF EXTRACTS OF ONOBRYCHIS-VICIIFOLIA

A subcellular enzyme fraction (M(r) > 20 X 10(6)) from immature sainfo in leaves catalysed the two-step NADPH-dependent reduction of (+)-dihy dromyricetin to (+)-gallocatechin. No enzyme-mediated (-)-epigallocate chin was formed under the conditions employed, and only traces of 2R,3 S-trans-3S,4S-cis-leucodelphinidin were observed. The two-step reducta se activity mirrored proanthocyanidin content, rising to a maximum bef ore leaflet unfolding, then declining during leaf expansion. When [C-1 4](+)-dihydromyricetin and [C-4-H-3]2R,3S-trans-3S,4S-cis-leucodelphin idin were supplied to the same fraction, preferential utilization of ( +)-dihydromyricetin was observed. Enzymic formation of proanthocyanidi n dimers could not be detected using these preparations under a variet y of conditions. However, a small portion of radioactivity was bound t o the subcellular fraction after addition of labelled (+)-catechin. In addition, in vivo incorporation of(+)-dihydromyricetin into proanthoc yanidins was observed. Copyright (C) 1997 Elsevier Science Ltd