6 



The protein-copper-oxygen complex is formed by combining one molecule of 

 oxygen with the protein to which two adjacent cuprous atoms are attached. 

 Catecholase activity involves the oxidation of 2 molecules of o-diphenols 

 to 2 molecules of o-quinones, resulting in the reduction of one molecule 

 of oxygen to two molecules of water. The sequence for the PPO-catalyzed 

 reaction proposed by Mason (1957) is shown in Figure 1. The enzyme-oxygen 

 complex serves as the hydroxylating or dehydroxylating intermediate, and 

 {Cu)„ represents the actual charge designation of the copper at the active 

 site. The overall reaction involves the use of one molecule of oxygen; 

 one atom of which goes into the formation of diphenol, and the other is 

 reduced to water. Substrate specificity varies considerably for PPO from 

 various sources (Aurand and Woods, 1973). 



Enzymatic Browning (Melanosis) 



Food scientists' primary interest in PPO is in the enzymatic 

 browning phenomenon and its effect on food quality. o-Quinones, the 

 primary products of PPO oxidative reaction can (a) interact with each 

 other to form high molecular weight polymers, (b) form macromolecular 

 complexes with amino acids or proteins, and (c) oxidize compounds of lower 

 oxidation-reduction potentials (Mathew and Parpia, 1971). Enzymatic 

 browning is a desirable process for the manufacture of black tea, sultana 

 grape, and ground coffee bean (De Amorim and Silva, 1968; Grncarevic and 

 Hawker, 1971; Takeo, 1966). 



Unfavorable darkening on the surface of many fruits, vegetables, and 

 seafood products is primarily due to the indirect consequence of PPO 

 oxidizing phenols to orthoquinones, which in turn rapidly polymerize to 



