The action of tyrosinase on ortho-substituted monophenols (thymol, carvacrol, guaiacol, butylated hydroxyanisole, eugenol, and isoeugenol) was studied. These monophenols inhibit melanogenesis because they act as alternative substrates to L-tyrosine and L-Dopa in the monophenolase and diphenolase activities, respectively, despite the steric hindrance on the part of the substituent in ortho position with respect to the hydroxyl group. We kinetically characterize the action of tyrosinase on these substrates and assess its possible effect on browning and melanognesis. In general, these compounds are poor substrates of the enzyme, with high Michaelis constant values, K(m), and low catalytic constant values, k(cat), so that the catalytic efficiency k(cat)/K(m) is low: thymol, 161 ± 4 M(-1) s(-1); carvacrol, 95 ± 7 M(-1) s(-1); guaiacol, 1160 ± 101 M(-1) s(-1).

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Many phenolic compounds have been described in the scientific literature as inhibitors of tyrosinase. In this work a test is proposed that allows us to distinguish whether a molecule is an enzyme inhibitor or substrate. The test has several stages. First, the degree of inhibition of the studied molecule is determined on the monophenolase activity (i(M)) and on the diphenolase activity (i(D)). If i(M) = i(D), it is an inhibitor. If i(M) ≠ i(D), the molecule could be substrate or inhibitor. Several additional stages are proposed to solve this ambiguity. The study described herein was carried out using the following molecules: benzoic acid, cinnamic acid, guaiacol, isoeugenol, carvacrol, 4-tert-butylphenol, eugenol, and arbutin.

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