The Substrate Specificity and Inhibition Char- Acteristics of Two Copper-protein “oxidases””

Evidence has been presented in earlier publications that the rapid oxidation of ascorbic acid in plant press-juices, when exposed to air, is primarily due to the catalytic effect of copper combined with protein. The evidence was of three types : (a) sensitivity to a number of organic and inorganic copper inhibitors, (b) the presence of adequate quantities of copper in the press-juices and enzyme preparations, and (c) the similarity between the enzyme preparations and known copper-protein complexes (1, 2). Other plant and animal oxidative catalysts, such as (a) those that lead to quinone formation, (b) the cytochrome system, and (c) hemochromogens, can obviously serve as catalysts for the oxidation of ascorbic acid, but the dominant active agent in many plants appears to be copper. In view of the subsequent finding by Kubowitz that potato “polyphenol oxidase” was also a copper-protein type of catalyst (3), it was of interest to investigate the relative substrate specificities and inhibition characteristics of these two forms of copperprotein. Previous evidence has indicated that neither type of catalyst is highly specific in relation to substrates. We have pointed out the undesirability of naming the catalyst that serves for the oxidation of ascorbic acid in terms of a specific substrate, at least until further evidence of its substrate specificity is available (l), and Kubowitz