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Ascorbic Acid |
Ascorbic acid is a naturally-occurring inhibitor of the oxidation of dopamine. Harris and Poland (1939) found that ascorbic acid increased from an average of 5.3 mg/100 g pulp in green bananas to 11.0 mg/100 g pulp during ripening, but decreased to an average of 3.2 mg/100 g pulp as the fruit became overripe. The range of values of ascorbic ac id was 1.0 to 14.3 mg/100 g pulp. The average value was 10 mg/100 g pulp.
Palmer (1964) studied the effects of various levels of ascorbic acid on delay of the enzymatic oxidation of dopamine and on the activity of banana polyphenol oxidase. He found a delay of 0.2 minute and 14 percent inhibition of polyphenol oxidase activity with 1.4 x 10-5 M ascorbic acid. The delay increased to 12.7 minutes, and inhibition of polyphenol oxidase activity increased to 78 percent with 1.7 x 10-4 M ascorbic acid.
Many questions remain unanswered about the importance in the metabolism of plant tissue of the oxidation of dopamine by polyphenol oxidase. Formation of melanin from dopamine is associated with root wounding and infection, and may play a significant role in resistance to infection. Sondheimer (1962) reported that plants susceptible to infection accumulate ascorbic acid, whereas resistant species accumulate polyphenols with an accompanying decrease in ascorbic acid. he says that oxidation products of ortho-dihydroxy compounds may be the antiviral substances. DeSwardt et al. (1967) suggested that low molecular weight tannins may control the activity of enzymes in preclimacteric fruit, thus influencing ripening. As tannins polymerize, they may be bound by hydrogen bonds to these proteins or to cell walls. Palmer (1963) suggested that further study of this unique relationship could help answer some of the problems concerning the synthesis, localization, and role of phenol oxidases in plant tissue.
Updated: Wednesday, June 20, 2007.
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