Citrus Fruit Enzymes. I. Ascorbic Acid Oxidase in Oranges

Abstract

Evidence for the occurrence of ascorbic acid oxidase in immature oranges (Citrus sinensis) is presented. Assay was conducted by measuring the rate of oxygen uptake manometrically and by measuring the rate of diminution of the substrate ascorbic acid spectrophotometrically at 265 m[mu]. The evidence is based on the findings that the enzyme was 1) specific for ascorbic acid and not for various phenolic compounds tested; 2) was stoichiometric with respect to substrate and oxygen uptake (1/2 O2/ mole ascorbic acid); 3) was inhibited by cyanide and diethyldithiocarbamate; 4) was oxygen-dependent; and 5) had an optimum pH of 5.6. Approximately 2/3 of the enzyme was soluble in 0.1 [image] potassium phosphate at pH 5.6 and the balance remained with the cell wall fraction. The soluble enzyme contained 26 enzyme units/mg N and 30 enzyme units/[mu]g Cu.