■ 10 



1962), the activity of this enzyme can be inhibited by metal -chelating 

 agents. The effects of cyanide, carbon monoxide, sodium-diethyl-dithio- 

 carbamate (DIECA), mercaptobenzothiazole, dimercaptopropanol , potassium 

 methyl xanthate, fluoride, azide, borate, benzoic acid, small peptides, 

 polyvinylpyrrolidone (PVP), 2,3-naphthalenediol , ascorbic acid, and 

 dichlorodifluoromethane on plant PPO activity have been extensively 

 studied; these chemical inhibitors act primarily on the enzyme (Bedrosian 

 at al., 1959; Harel et al . , 1967; Jones et al . , 1965; Mayer at al . , 1964; 

 Palmer and Roberts, 1967; Pierpoint, 1966; Pifferi et al . , 1974; Robb et 

 al., 1966; Taeufel and Voigt, 1964; Walker, 1964, 1975, 1976; Walker and 

 Wilson, 1975; Yasunobu and Norris, 1957). 



Other compounds acting as reducing agents which reduce o-quinones to 

 diphenols or react with plant PPO substrates to lessen the extent of 

 enzymatic browning are ascorbic acid, SO2, sodium sulfite, sodium 

 bisulfite, sodium metabisulfite, potassium metabisulfite, cysteine, 

 2-mercaptoethanol, glutathione, benzenesulphinic acid, DIECA, Na-ethyl 

 xanthate, and PVP (Cash et al . , 1976; Embs and Markakis, 1965; Feinberg et 

 al., 1976; Haisman, 1974; Hope, 1961; Markakis and Embs, 1966; Mapson, 

 1965; Mapson and Wager, 1961; Muneta, 1966; Muneta and Walradt, 1968; 

 Muneta and Wang, 1977; Ponting, 1960; Ponting and Jackson, 1972; 

 Sayavedra-Soto and Montgomery, 1986; Singh and Ahlawati, 1974; Taeufel and 

 Voigt, 1964; Vamos-Vigyazo, 1981; Walker, 1964). For the prevention of 

 melanosis in crustaceans, numerous agents including ascorbic acid and 

 sodium ascorbate, ascorbate and citrate, L-tyrosine, L-methionine, citric 

 acid, L-cysteine, sodium diethyl dithiocarbamate, sodium tripolyphosphate 

 (STP), sodium sulfite, and sodium bisulfite have been broadly studied 



