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Freshly cut slices darken as a result of oxidation of endogenous phenols in reactions catalyzed by polyphenol oxidase. Ascorbic acid, thiols, and other reducing substances prevent enzymic darkening (5, 17). Earlier studies showed that ATP substantially decreased browning in plant tissues, apparently via the reduction of quinones (14). ATP itself is not a reducing agent. The reducing compound is probably not ascorbic acid or GSH (12). In the presence of ATP, quinones are reduced under conditions favorable to oxidative phosphorylation (13). The mechanism by which ATP prevents browning is not known. -:This paper presents evidence in support of a hypothesis that DPNH serves as the reducing agent and that ATP is instrumental in maintaining a supply of DPNH for the reduction of quinones. Reduction of DPN in the presence of ATP had been demonstrated in minced pigeon breast muscle (8), phosphorylating animal mitochondria (3, 6), and subnmitochondrial particles (10, 16). Under our experinental conditions, DPNH may form from naturally present or added DPN, possibly via complex reactions found in animal systems. To our knowledge, the reduction of DPN induced by ATP has not yet been demonstrated in higher plants. The experimental work deals with the effect of pyridine and adenine nucleotides on enzymic browning of potato slices. In a phosphorylating system, the selected nucleotides could serve as possible precursors of ATP or of DPNH. The effects of the metabolic inhibitors, Amytal and antimycin A, were also investigated.

Effect of Nucleotides on Enzymic Browning in Potato Slices