ENHANCEMENT OF PEROXIDASE-DEPENDENT OXIDATION OF SINAPYL ALCOHOL BY AN APOPLASTIC COMPONENT, 4-COUMARIC ACID ESTER ISOLATED FROM EPICOTYLS OF VIGNA-ANGULARIS L

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 es ter was oxidized by a basic POX isozyme (Km, about 20 mu M) and by an acidic POX isozyme (K-m, about 40 mu M) that had been partially purifi ed from the apoplastic fraction of epicotyls of V. angularis. These PO X isozymes oxidized sinapyl alcohol at only a very low rate, but a 15- fold enhancement was observed upon addition of the ester. The concentr ations of the ester required for the half-maximal enhancement were sim ilar to the K-m values of the ester for its oxidation by the respectiv e isozymes. The apoplastic concentration of the ester was higher than 130 mu M, suggesting that this ester might act as a donor of electrons to the apoplastic POX isozymes in situ. Coniferyl alcohol also enhanc ed the POX-catalyzed oxidation of sinapyl alcohol. The concentrations of coniferyl alcohol required for half-maximal enhancement of the oxid ation of sinapyl alcohol were about 23 and 250 mu M when reactions wer e catalyzed by the basic and acidic POXs, respectively. These values w ere similar to the K-m values of coniferyl alcohol for its oxidation b y the respective isozymes. These results suggest that 4-coumaric acid ester and coniferyl alcohol, if it is present in the apoplast, can enh ance the POX-dependent oxidation of sinapyl alcohol in the apoplast of epicotyls of V. angularis.