Citrus Fruit Enzymes. I. Ascorbic Acid Oxidase in Oranges

Ascorbic acid oxidase was first described by Szent-Gyorgyi (14) in 1931 from cabbage. Frankenthal postulated its presence in citrus peel on the basis of indirect evidence in 1939 (6). In a survey of different tissues, Huelin and Stephen (8) reported the presence of ascorbic acid oxidase in citrus peel but found negligible amounts in orange juice. These reports, indicating the presence of this enzyme, were based on the ability of the enzyme preparation to oxidize ascorbic acid; however, many compounds are capable of oxidizing ascorbic acid in the absence of ascorbic acid oxidase. This criterion alone is not sufficient evidence for determining the presence of ascorbic acid oxidase (1). The purpose of this paper is to establish the fact that ascorbic acid oxidase is present in citrus. The citrus enzyme characteristics are compared with ascorbic acid oxidase from other sources (2, 3, 6, 8, 14) on the basis of spectrophotometric and manometric studies, inhibition by potassium cyanide and diethyldithiocarbamate, optimum pH, copper content, thermolability, dependence on oxygen, and ammonium sulfate fractionation studies.