Metabolism of Uronic Acids in Plant Tissues

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Abstract
A new enzyme, named uronic acid oxidase, was extracted and purified 67-fold by (NH4)2SO4 fractionation and CM-Sephadex column chromatography from ethylene-treated Shamouti orange (Citrus sinensis L. Osbeck) leaves. The enzyme catalyzes the oxidation of d-galacturonic acid and d-glucuronic acid to the corresponding hexaric acids in the presence of molecular oxygen with the production of H2O2. The pH optimum for the oxidation of d-galacturonic acid and d-glucuronic acid is between 7 and 8. The enzyme is highly specific for d-galacturonic acid and d-glucuronic acid. It also oxidizes polygalacturonic acid. The apparent Michaelis constant values of the enzyme for d-galacturonic acid and d-glucuronic acid are 0.13 and 0.5 mm, respectively. The molecular weight of the enzyme, as determined by gel filtration, is about 98,000. The enzyme is inhibited by sodium hydrosulfite and other sulfites, indicating that it contains a flavin prosthetic group.