Citrate activates tyrosinase from B-16 murine melanoma and human skin

Citrate stimulates cresolase activity of tyrosinase from B-16 murine melanoma and human skin. Maximal stimulation by citrate was obtained at 2 mM, and stimulation was decreased at higher concentrations. Citrate stimulates tyrosinase not only from mammalian sources but also from mushroom. The stimulation was not due to reversal of inhibition of enzyme activity by excess tyrosine. On rapid decrease in pH of the enzyme solution from 6.8 to 5.0-5.2, the enzyme is no longer inhibited by excess tyrosine even when its activity was assayed at pH 6.8. Citrate also stimulates this form of enzyme. However, the stimulation is more at acidic pH than at pH 6.8. At higher concentrations of citrate the stimulatory effect decreases at both pH 5.0 and pH 6.8. Inhibition of this enzyme occurs at higher concentrations (22 mM) at pH 6.8. The physiological role of stimulation of cresolase activity of tyrosinase by citrate is yet to be unravelled.