Enhancement of Peroxidase-Dependent Oxidation of Sinapyl Alcohol by an Apoplastic Component, 4-Coumaric Acid Ester Isolated from Epicotyls of Vigna angularis L.

Abstract

A water-soluble component that enhanced the peroxidase-dependent (POX-dependent) oxidation of sinapyl alcohol was isolated from epicotyls of Vigna angularis. This compound was an ester of 4-coumaric acid and a hexose, and it was found in both the apoplast and the symplast. The ester was oxidized by a basic POX isozyme (Km, about 20 μM) and by an acidic POX isozyme (Km, about 40 μM) that had been partially purified from the apoplastic fraction of epicotyls of V. angularis. These POX isozymes oxidized sinapyl alcohol at only a very low rate, but a 15-fold enhancement was observed upon addition of the ester. The concentrations of the ester required for the half-maximal enhancement were similar to the Km, values of the ester for its oxidation by the respective isozymes. The apoplastic concentration of the ester was higher than 130 μM, suggesting that this ester might act as a donor of electrons to the apo-plastic POX isozymes in situ. Coniferyl alcohol also enhanced the POX-catalyzed oxidation of sinapyl alcohol. The concentrations of coniferyl alcohol required for half-maximal enhancement of the oxidation of sinapyl alcohol were about 23 and 250 μM when reactions were catalyzed by the basic and acidic POXs, respectively. These values were similar to the Km values of coniferyl alcohol for its oxidation by the respective isozymes. These results suggest that 4-coumaric acid ester and coniferyl alcohol, if it is present in the apoplast, can enhance the POX-dependent oxidation of sinapyl alcohol in the apoplast of epicotyls of V. angularis.