

125 

 inhibitor for both potato and apple PPO, but a mixed-type inhibitor for 

 white shrimp, grass prawn and Florida spiny lobster PPO. For mushroom 

 PPO, it was a competitive inhibitor for L-tyrosine, while a mixed-type 

 inhibitor for DL-DOPA. 



The Michael is constant for the oxidation of DL-DOPA and L-tyrosine 

 by mushroom tyrosinase was 0.29 mM and 0.69 mM, respectively. The K^ value 

 for DL-DOPA was close to that for the oxidation of catechol (0.22 mM) and 

 chlorogenic acid (0.22 mM) reported by Sisler and Evans (1958) but was 

 lower than the values reported by Smith and Kruger (1962) on catechol (2.5 

 - 4.0 mM). Similarly, the Michael is constant for L-tyrosine was lower 

 than that for the oxidation of p-cresol (1.5 - 10 mM) (Sisler and Evans, 

 1958). Mayer et al . (1966) have attributed this discrepancy to different 

 enzyme preparations and assay methods used. Based on the K^^ values for 

 these two substrates, it was noted that mushroom tyrosinase had a higher 

 affinity for DL-DOPA than for L-tyrosine. Boughilloux et al . (1963) 

 isolated 4 different forms of tyrosinase from mushroom, all of which were 

 capable of oxidizing DOPA more actively than tyrosine. A similar 

 observation was also reported by Harrison et al . (1967) using a 

 fluorescence spectrophotometric technique. 



Apparent K^^^ values for the oxidation of L-tyrosine and DL-DOPA by 

 mushroom tyrosinase in the presence of kojic acid were determined to be 

 2.02 and 0.66 mM, respectively. The inhibitor constant (K.) was determined 

 to be 0.03 mM for the former (competitive inhibition) and 0.02 mM for the 

 latter (mixed-type inhibition). Since kojic acid was a competitive 

 inhibitor, it would compete with L-tyrosine for the active site 

 (Segel ,1976) . The results also showed that the inhibitory properties of 



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