OXIDATION OF HYDROQUINONE BY BOTH CELLULAR AND EXTRACELLULAR GRAPEVINE PEROXIDASE FRACTIONS

The oxidation of hydroquinone by two peroxidase (EC 1.11.1.7) fraction s obtained from the cells and spent medium of cell cultures of grapevi ne (Vitis vinifera cv Monastrell) has been studied, and their comparat ive efficacy (K(cat)/K(M) ratio) studied in both the H2O2-consuming an d hydroquinone-consuming reactions. While the efficacy in the H2O2-con suming reaction is practically identical for both enzyme fractions, th e cellular peroxidase has five-fold more efficacy in the hydroquinone- consuming reaction than the peroxidase located in the spent medium. Sc reening of cellular peroxidases capable of oxidizing hydroquinone on p olyacrylamide gels, by means of a staining reaction based on the nucle ophilic attack of 4-aminoantipyrine on p-benzoquinone in acidic media, reveals that all the cellular peroxidase isoenzymes are capable of ox idizing hydroquinone, probably yielding a quinone-diimine as a product of the staining reaction. Since isoperoxidases found in cellular frac tions are also present in the spent medium, the values found for the d ifferent efficacies in the hydroquinone-consuming reaction must be con sidered as the results of the different proportions in which each pero xidase isoenzyme was found in the two fractions. The localization of a benzoquinone-generating system of high efficacy inside the plant cell , and probably located in vacuoles, is discussed with respect to the h armful role which the quinone/semiquinone pair might play in cell deat h, as part of the hypersensitive response expressed within the mechani sm of plant disease resistance.