Enzymatic synthesis of aromatic compounds in higher plants: Formation of Bis-noryangonin (4-Hydroxy-6[4-hydroxystyryl]2-pyrone) from p-coumaroyl-CoA and malonyl-CoA

Abstract

Cell-free extracts from light-induced cell suspension cultures of Petroselinum hortense catalyzed, in the presence of mercaptoethanol or dithioerythritol, the formation of bisnoryangonin from p-coumaroyl-CoA and malonyl-CoA. Radioactivity from the 3H- and 14C-labeled acyl moieties of p-coumaroyl-CoA and malonyl-CoA, respectively, was incorporated into the product at a molar ratio of 1:2. This result supports earlier conclusions from experiments in vivo favoring a mechanism of synthesis for the pyrone ring of bisnoryangonin according to the “acetate rule.” Bis-noryangonin could not be detected in cultured Petroselinum hortense cells in vivo. Our present results suggest that the styrylpyrone derivative formed in vitro is an artificial product of the first enzyme of the flavonoid pathway, flavanone synthetase. In the course of a 300-fold purification of this enzyme, the bis-noryangonin-synthesizing activity was always associated with the flavanone synthetase activity. The concentration of certain thiol reagents, such as mercaptoethanol or dithioerythritol, the ionic strength of the buffer, and the degree of purity of the enzyme preparation had a pronounced, differential effect on the amounts of flavanone and styrylpyrone formed by the flavanone synthetase. A possible explanation for the mechanism of formation of the artificial product, bis-noryangonin, is discussed.