• : - .. ' ' : 50 



potato, apple, grass prawn, pink shrimp, and white shrimp PPOs. Varied K^ 

 values were also reported for peach (K^ = 4.2 mM, Wong et al . , 1971), pear 

 (K^ = 20.9 mM, Rivas and Whitaker, 1973), and banana (K^ = 0.63 mM, Palmer, 

 1963) PPOs when different phenolic compounds were used as substrates 

 (Table 2). Data obtained from this study showed that mushroom, potato, 

 and apple PPOs had comparatively lower K^ than those of crustacean PPOs, 

 indicating that the former PPOs had a higher affinity for DL-DOPA than the 

 latter ones. It has been reported that the affinity of PPO towards a 

 given substrate may vary widely, even if isozymes of the same origin are 

 used. The differences might be due, at least partly, to steric factors 

 connected to differences in protein structure (Vamos-Vigyazo, 1981). 

 Substrate specificity varies considerably for PPO from various 

 sources (Aurand and Wood, 1977; deMan, 1985). Simpson et al . (1988a) 

 identified higher levels of free tyrosine and phenylalanine as natural 

 substrates of melanosis in the carapace of pink shrimp. Further studies 

 by Ali (1991) showed PPO isolated from Florida spiny lobster was capable 

 of hydroxylating free tyrosine to DOPA. Tyramine, produced by bacterial 

 activity on free L-tyrosine (Veciana-Nogues et al . , 1989; Santos-Buelga et 

 al . , 1986) was also identified as the substrate of crab PPO (Summers, 

 1967). For mushroom {Agaricus bisporum) PPO, DOPA and catechol were, 

 respectively, reported by Bouchilloux et al . (1962) and Nakamura et al . 

 (1966) as specific substrates. For potato and apple PPOs, chlorogenic 

 acid (Patil and Zucker, 1965) was considered a specific substrate for the 

 former, while 4-methyl catechol for the latter (Stelzig, 1972). Compared 

 to pear PPO with pyrocatechol (Rivas and Whitaker, 1973) and banana PPO 

 with dopamine (Palmer, 1963), D-catechin was demonstrated as the specific 



