101 

 0.1 M sodium phosphate buffer (pH 6.0), respectively. Enzyme activity for 

 each plant PPG was monitored as previously described. 



L-DOPA and catechol at 1.5 - 7.0 mM and DL-DOPA and catechol at 1.67 

 - 9.92 mM in 0.05 M sodium phosphate buffer (pH 6.5) were respectively 

 used as substrate for white shrimp PPG (5,400 units/mg of protein) and 

 grass prawn (900 units/mg of protein) and lobster PPG (7,000 units/mg of 

 protein). The assay for white shrimp PPG was conducted for 5 min at 40°C 

 and at 25°C for grass prawn and lobster PPG. Enzyme activity on L-DGPA or 

 DL-DGPA and catechol was monitored at 475 and 395 nm, respectively. 



The inhibitory mechanism of kojic acid on enzyme activities was 

 also investigated. Except for the substitution of 0.5 mL of kojic acid 

 solutions for buffer, substrate concentrations used in the previous study 

 were employed. Kojic acid solutions at 0.28, 0.56, and 1.06 mM were used 

 in the mushroom, white shrimp, grass prawn, and lobster PPG assay systems, 

 while those at 0.56, 1.06, and 1.41 mM were used in the potato and apple 

 PPG systems. Prior to the addition of the substrate, an enzyme- inhibitor 

 mixture was pre-incubated at 37°C for 15 min. The assays for mushroom, 

 potato, grass prawn, and lobster PPG were carried out as previously 

 described. Kinetic parameters of apparent K^ (K^app) *"^ "^i ^°^ enzyme 

 activities were also determined according to the equations of Lineweaver- 

 Burk (1934) and Dixon (1953). 



The kinetic constants (K^, K^^^pp, and K.) of potato, apple, and white 

 shrimp PPG with DL-DGPA as the substrate were also determined. Sixty 

 microliters of PPG was added to 940 nl of DL-DGPA in 0.05 M sodium 

 phosphate buffer (pH 6.5). The final concentration of DL-DGPA varied from 

 1.4 to 8.9 mM. The reaction was monitored at 475 nm and 25°C for 10 min. 



