5 



primary products of its action on o-diphenols, the o-quinones, react with 

 tryptophan to form indole acetic acid via indolepyruvic acid (Gordon and 

 Paleg, 1961). Thus, PPO together with auxin degrading enzyme 

 (peroxidase), might play a role in plant growth regulation (Vamos-Vigyazo, 

 1981). The quinones formed upon PPO action may also participate in 

 reactions similar to those leading to nonenzymatic browning and 

 humification and thus contribute to producing organic matter of soil 

 (Synge, 1975). 



PPO from insects and crustaceans plays an important role in the 

 sclerotization during molting (Andersen, 1971; Brunet, 1980; Cobb, 1977; 

 Ferrer et al . , 1989a; Summers, 1967; Vinayakam and Nellaiappan, 1987). In 

 this process, PPO oxidizes diphenols to quinones, which interact with 

 certain side groups on adjacent proteins, thus linking them together 

 (Stevenson, 1985). N-Acetyldopamine (Andersen, 1979) and 3,4- 

 dihydroxybenzoic acid (Pryor et al . , 1962) have been identified as the 

 cross-linking agents in crustacean and cockroach, respectively. Another 

 function closely related to plant systems was PPO's involvement in the 

 process of wound repair and calcification of the cuticle (Stevenson, 

 1985). ' ■ V r -, 



Most PPO's are copper-containing enzymes which catalyze two entirely 

 different reactions: (a) the hydroxylation of monophenols to the 

 corresponding o-di hydroxy compounds, which is called cresolase activity, 

 and (b) the oxidation of o-dihydroxy phenols to o-quinones, which is 

 termed catechol activity. The activity of cresolase involves three steps 

 which can be represented by the following equation (Mason, 1956): , . 



Protein-Cu^*-02 + monophenol ■* Protein-Cu^* + o-quinone + H2O 



