'W'- 



127 

 When kojic acid was added, the apparent Michael is constants for 

 4-methyl catechol and chlorogenic acid were changed to 4.28 and 10.3 mM, 

 respectively. The inhibitor constant of kojic acid for 4-methyl catechol 

 and chlorogenic acid oxidation was 0.06 and 0.13 mM, respectively. The 

 former was similar to the K. of 2,3-naphthalenediol (Mayer et al., 1964). t 

 However, both K. values were lower than that of cinnamic acid (1.4 and 0.14 

 mM, respectively) when it was used as an inhibitor in a solubilized PPG 

 system (Walker and Wilson, 1975). Using 4-methyl catechol, chlorogenic 

 acid, and catechin as substrates, Walker and Wilson (1975) observed that 

 cinnamic acid, ferulic acid, and coumaric acid derivatives behaved as 

 competitive inhibitors of apple PPG. The Michael is constants for the 

 oxidation of L-DOPA and catechol by white shrimp PPG were determined to be 



.' 3.48 and 4.27 mM, respectively. The former (L-DGPA) K^ value was slightly 

 higher than that reported for DL-DGPA (2.8 mM) by Simpson et al . (1988a). 

 Regarding lobster PPG, the K^ values for oxidation of DL-DOPA and catechol ' 



• were 3.27 and 4.98 mM, respectively. These values were lower than those 



" reported by Chen et al . (1991a). Lobster PPG used in this study was 

 further purified by 7.5% acrylamide gel and thus possessed a higher 

 specific activity. For grass prawn PPG, the K^ values for DL-DOPA and 

 catechol were 3.64 and 5.29 mM, respectively. The former value was close 



' to that reported by Rolle et al . (1991). In comparison to pink shrimp 

 (Simpson et al., 1988a), white shrimp, grass prawn and lobster PPG showed 

 comparatively higher K^ values when either L-DGPA or DL-DGPA was used as 

 substrate. 



When kojic acid was added as an inhibitor, a mixed-type inhibition 

 was observed for the oxidation of both substrates by these three 



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