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Activity of polyphenol oxidase in bananas as expressed three ways, units of activity per mg protein, per g fresh weight of banana, and per g dry weight of banana, is given in Table 7 and Table 7 continued. The specific activity of the enzyme (units of activity/mg protein) averaged 6.4, 5.2, and 4.1 for bananas analyzed as purchased, after two days of additional ripening, and after four days of additional ripening, respectively. Specific activity of the polyphenol oxidase decreased with ripening in each banana, but it must be kept in mind that protein increased with ripening. The decrease in specific activity in those bananas analyzed two days apart averaged 15.6 percent and for those analyzed four days apart averaged 31.0 percent. The difference in the mean changes between bananas analyzed two days apart and those analyzed four days apart was significant at the 1 percent level (Table 2). The difference in specific activity due to hand was also significant at the I percent level (Table 2).
While it is customary to report the activity of an enzyme in terms of specific activity, the protein content of bananas increased with ripening. Therefore, activity of polyphenol oxidase was also calculated and reported as units of activity/g of both fresh and dry weight of banana. The activity of the enzyme averaged 46.8, 45.0, and 45.3 units of activity/g of banana on a fresh weight basis for bananas analyzed as purchased and after two and four days of additional ripening, respectively. On a dry weight basis, the corresponding figures are 189, 183, and 195 units of activity/g. On this basis, the decrease in activity of polyphenol oxidase with ripening was not significant, but the difference in activity due to hand was significant at the 5 percent level (Table 2). Nandi (1972) reported a decrease in enzyme activity with ripening of bananas. However, Palmer (1963) recommended that a final dopamine concentration of 5 x 10 M be used in the assay of polyphenol oxidase. This would require a stock solution of dopamine of 2. 84 mg/ml. Nandi used a solution with a concentration of 0. 2 mg/ml which may have caused the substrate to be a limiting factor in her assay.
Updated: Wednesday, June 20, 2007.
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