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crustacean PPO. The inhibitor constant for the oxidation of DL-DOPA by 

 grass prawn PPO was 0.05 mM and 0.07 mM for catechol. Similarly, the K. 

 values were determined to be 0.07 mM for DL-DOPA and 0.10 mM for catechol 

 when lobster PPO was used. For white shrimp PPO, the K. of L-DOPA and 

 catechol was determined to be 0.15 and 0.18 mM, respectively. Antony and 

 Nair (1975) studied the inhibitory effect of several chemicals on prawn 

 phenolase and found L-tyrosine (K. = 0.38 mM) was a competitive inhibitor 

 to the oxidation of DOPA. In contrast, L-cysteine and sodium diethyl 

 dithiocarbamate were found to behave as mixed-type inhibitors. Madero 

 (1982) reported that bisulfite was a competitive inhibitor to brown shrimp 

 PPO. The observation of lower K. values for grass prawn and lobster PPO 

 than white shrimp PPO further demonstrate that kojic acid exhibited a 

 greater inhibitory effect on those two enzymes than the latter one 

 (Figures 26, 27, and 28). Also, when the same amount of kojic acid was 

 applied to the assay mixtures, greater inhibition was observed for DL-DOPA 

 than for catechol (Figures 27 and 28). 



It was noted that the K^ value of white shrimp PPO for DL-DOPA (3.20 

 mM) was close to that for L-DOPA (3.48 mM) . For potato and apple PPO, 

 both enzymes showed an extremely high affinity for DL-DOPA than for 

 chlorogenic acid, 4-methyl catechol , and catechol (Table 4). Also, these 

 two enzymes showed a higher affinity for DL-DOPA than other PPOs. Kojic 

 acid also exhibited the same inhibitory mechanism on the oxidation of DL- 

 DOPA as well as on other diphenolic substrates by white shrimp, potato, 

 and apple PPO (Table 4). Kojic acid exerts more effective inhibition 

 (Table 4; K.) on DL-DOPA oxidation by potato and apple PPO than on other 

 diphenolic substrates. 



