Activators and Inhibitors

The activation mechanism of PPO from different sources is not well understood, but it has been suggested that conformational changes of protein and perhaps protein association or dissociation are involved in the process. Anionic detergents, such as sodium dodecyl sulfate, were found to reactivate PPO in crude and partially purified preparations from different avocado cultivars (55). Short exposures to acid pH or urea also resulted in a severalfold reversible activation of grape and bean PPO.

Many compounds inhibit PPO activity but only a limited number of PPO inhibitors are considered acceptable on grounds of safety and expense for use to control enzymatic browning during food processing. Practical aspects of browning inhibition are discussed later. There are two principal types of PPO inhibitor: compounds that interact with the copper in the enzyme and compounds that affect the site for the phenolic substrate. The metal-chelating agents such as cyanide, diethyldithiocarbamate, carbon monoxide, dimercaptopropanol, sodium azid, phen-ylthiourea, and potassium methyl xanthate are all inhib itory. In general, there are five major types of PPO inhibitors based on their chemical characters (56,57): (1) reducing agents, such as ascorbic acid and sulfur dioxide; (2) copper-chelating agents, such as carbon monoxide, tropolone, and diethyl dithiocarbamate; (3) quinone couplers, such as cysteine and glutathione; (4) substrate analogues, such as cinnamic acid, p-coumaric acid, and benzoic acid and some substituted cinnamic acids; and (5) miscellaneous compounds, such as 4-hexylresorcinol and poly(vinylpyrrolidone). Some other natural inhibitors of PPO from various sources have been reported (57). The activity of laccase is not inhibited by carbon monoxide or phenylhydrazine, both of which inhibit PPO. This response permits an easy differentiation between laccase and PPO.

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