Tyrosinase catalyzes two distinct sequential reactions in
melanin biosynthesis: the hydroxylation of
tyrosine to
DOPA followed by the oxidation of
DOPA to
dopaquinone. The central roles of
melanin in living species have motivated researchers to maintain constant efforts to discover new agents that modulate
tyrosinase activity. In this study, we report on the inhibition of
tyrosinase by
ethionamide and its analogues.
Ethionamide, 2-ethylpyridine-4-carbothioamide, is a second-line antituberculosis
drug used for the treatment of
multidrug-resistant tuberculosis. The chemical similarity of
ethionamide to
phenylthiourea, a well-known
tyrosinase inhibitor, led us to investigate its inhibitory effects on mushroom
tyrosinase and the IC50 was calculated as 4 μM. Five analogues of
ethionamide, including another antituberculosis
drug,
prothionamide, were also inhibitory, with values for IC50 in the range of 3-43 μM. Fluorescence quenching experiments supported a mechanism of direct binding. In contrast,
isoniazid, a structural analogue and first-line antituberculosis
drug, was a poor inhibitor of
tyrosinase. We also tested the effects of
ethionamide and its analogues on
melanin content in B16F10 cells. At a concentration of 50 μM, the molecules,
pyridine-2-carbothioamide and
thiobenzamide substantially decreased the
melanin content by 44% and 37%, respectively. In addition to identifying other interactions, docking simulations showed that the carbothioamide groups of the molecules make essential contacts with the catalytic di-
copper atoms. Our results suggest that carbothioamide can be a central moiety for the development of new and potent
tyrosinase inhibitors.