Polyphenol Oxidase Enzymes in the Sap and Skin of Mango Fruit

Severe sapburn occurs in mango fruit of the cultivar Kensington when sap contacts the fruit, resulting in browning and then blackening of the skin. Both the sap and skin of mango fruit contained considerable polyphenol oxidase (PPO) activity. The sap enzyme was not activated by SDS, was inhibited by hexadecyltrimethylammonium bromide, and was active with both para- and ortho-diphenol substrates. The skin enzyme was activated by SDS, was inhibited by salicylhydroxamic acid and polyvinylpyrrolidone, and was active only with ortho-diphenol substrates. These properties suggest that the sap PPO is a laccase-type enzyme (EC 1.10.3.2) whereas the skin contains the more common catechol oxidase-type PPO (EC 1.10.3.1). The skin enzyme had a temperature optimum at 30ºC but the sap enzyme had maximum PPO activity at 75ºC. Both enzymes were relatively thermostable, requiring more than 15 min at 80ºC for 50% loss of activity. It is concluded that browning of mango skin induced by the sap is predominantly catalysed by PPO in the skin and that this is unlikely to be prevented by heat treatment of the fruit.