126 . 

 kojic acid on mushroom tyrosinase depends upon whether o-diphenol 

 (DL-DOPA) or monophenol (L-tyrosine) is used as a substrate. In this 

 study, a mixed-type inhibition implies that kojic acid affected the 

 affinity of the enzyme for DL-DOPA but did not bind at the active site 

 (Webb, 1963). 



The K^ value for the oxidation of chlorogenic acid by potato PRO was 

 5.20 mM, while it was 7.89 mM for the oxidation of catechol. These values 

 were lower than those reported by Abukharma and Wool house (1966), 

 Alberghina (1964), and Macrae and Duggleby (1968). Variations in enzyme 

 preparations and assay methods could have contributed to these differences 

 in K^ values (Macrae and Duggleby, 1968). 



In the presence of kojic acid, the apparent Michael is constant for 

 the oxidation of chlorogenic acid became 7.23 mM while it was 10.5 mM for 

 catechol. The inhibitor constant for the oxidation of chlorogenic acid 

 was 0.60 mM and 0.71 mM for catechol oxidation. Thus, kojic acid was more 

 competitive with chlorogenic acid than with catechol for the active site. 

 The K. values for kojic acid using chlorogenic acid and catechol as 

 substrate were lower than those reported for p-nitrophenol , ferulic acid, 

 p-coumaric acid, 2,3-dihydroxy-naphthalene, and cinnamic acid (Macrae and 

 Duggleby, 1968). 



The Michael is constants for the oxidation of 4-methyl catechol and 

 chlorogenic acid by apple PPG were determined to be 3.85 and 8.20 mM, 

 respectively. The K^ value for 4-methylcatechol was close to that reported 

 by Harel et al . (1965) and Mayer et al . (1964), whereas the K^^ for 

 chlorogenic acid was higher (Walker, 1964). 



