. J- 



m.^t' 



117 

 chloroplasts. Walker (1964) demonstrated that diethylthiocarbamate or 

 dimercaptopropanol at 1 mM was able to inhibit 90% of the oxidation of 

 chlorogenic acid. 



Effect of Ko.iic Acid nn Crustacean PPQ Activity 



Kojic acid inhibition on white shrimp (Figure 26) and grass prawn 

 PPO (Figure 27) was less effective than on spiny lobster (Figure 28). The 

 former two enzymes were only inhibited by about 20%. while the oxidation 

 of DL-DOPA and catechol by lobster PPO was inhibited to 80 and 70%, 

 respectively, when kojic acid at 20 /xg/mL was used. Kojic acid appeared 

 to be more effective in inhibiting lobster PPO than white shrimp and grass 

 prawn PPO and plant PPO. L-Tyrosine, L-cysteine. and sodium diethyl 

 dithiocarbamate effectively inhibited the oxidation of DOPA (Antony and 

 Nair, 1975). p-Aminobenzoic acid (PABA) and NaNg was more effective than 

 EDTA and cysteine in inhibiting pink and white shrimp PPO (Simpson et al . , 

 1988a). On the basis of the molar ratio between crustacean PPO and 

 inhibitor used, kojic acid was found to be less effective in inhibiting 

 white shrimp, grass prawn, and lobster PPO than PABA and NaN3 on pink and 

 white shrimp PPO. 



Enzyme Kinptir<: ' 



Kinetic parameters (K^, K^^^^, and K.) for the various PPOs utilizing 

 different substrates in the absence or presence of kojic acid,and the type 

 of inhibition are listed in Table 4. Kojic acid was a competitive 



w. 



c: 







