Stilbene synthases and stilbenecarboxylate synthases, I Enzymatic synthesis of 3,5,4-trihydroxystilbene from p-coumaroyl coenzyme A and malonyl coenzyme A.

1. Cell-free extracts prepared via acetone powder from rhizome of Rheum rhaponticum were found to be capable of converting p-coumaroyl-CoA and [2-14C]malonyl-CoA into a 3,5,4'-trihydroxystilbene, resveratrol. The product was characterized by repeated chromatography and by recrystallization to constant specific radioactivity. 2) The mode of condensation of the phenylpropanoid moiety with C2 units was determined by chemical degradation of the enzymatically formed product. 3) The enzyme system which catalyses the condensation of p-coumaroyl moiety with 3 malonyl units, the intramolecular aldol reaction and the subsequent decarboxylation was classified as stilbene synthase. It could be activated by dithiothreitol. 4) When the extraction of the rhizome was performed with aqueous grinding medium, a high percentage of this stilbene synthase activity was found to be membrane-bound.