Effect of ferrous ions on the monophenolase activity of tyrosinase

The effect of ferrous ions on the monophenolase activity of tyrosinase has been studied. Although a shortening of the lag period which characterizes this hydroxylation reaction was observed, no direct effect on the enzyme was found. The reaction between ferrous ions and molecular oxygen in the presence of chelating agents, such as phosphate or EDTA, produces hydroxyl radicals. These radicals can hydroxylate tyrosine to generate L-3,4-dihydroxyphenylalanine (dopa). Catalase and scavengers of hydroxyl radicals inhibited both the shortening of the lag period and dopa formation. On the basis of these results, it is proposed that the influence of ferrous ions on tyrosinase is due to the formation of dopa in the chemical hydroxylation of tyrosine. Dopa transforms the Emet form of the enzyme (Cu2+Cu2+) into the Edeoxy form (Cu1+Cu1+) and, thus, shortens the lag period.