EFFECT OF KOJIC ACID ON THE OXIDATION OF TRIHYDROXYPHENOLS BY MUSHROOM TYROSINASE

Kojic acid [5-hydroxy-2- (hydroxymethyl)-4-pyrone] inhibited effective ly the rare of pigment formation during the oxidation of pyrogallol, 2 ,3,4-THAP (2,3,4-trihydroxyacetophenone) and 2,4,5-THBP (2,4,5-trihydr oxybutyrophenone) by tyrosinase. On the other hand, kojic acid had a s ynergistic effect on the rate of methyl gallate and n-propyl gallate o xidation to pigmented product(s) (lambda(max) = 360 nm and lambda(max) = 380 nm, respectively). However, kojic acid inhibited effectively th e rate of oxygen uptake when each of the above trihydroxyphenols was o xidized by tyrosinase. These results suggest that kojic acid inhibits tyrosinase per se (probably due to its ability to bind copper at the a ctive site of the enzyme) and that it exerts only an apparent stimulat ory effect during the formation of pigmented product(s) from methyl ga llate and n-propyl gallate. Proof for the latter was obtained by a tim e-course experiment of kojic acid addition and examination of the spec tra of pigmented product(s) formed in the absence versus presence of k ojic acid, which suggested that the o-quinone of n-propyl gallate and the o-quinone of methyl gallate can each convert kojic acid to a yello w product(s) absorbing at the 360-380 nm region.