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3,3',5-L-triiodothyronine (T3) can inhibit tyrosinase activity in B16/C3 melanoma cells in culture and block the induction of that enzyme by imidazole (Endocrinol. 119, 2118, 1986). The current study examined the effect of thyroxine (T4) on tyrosinase activity. Proliferating cultures were exposed to either T3 (0-500 nM) or T4 (0-50,000 nM) in addition to imidazole (10 mM). Imidazole induced enzyme activity by approximately 3-fold and T3 could block that induction with a maximal inhibition occurring at 10 nM. Even at the highest concentrations used, T4 had no effect on tyrosinase activity. These studies disclose that T4, the tetraiodinated parent compound from which T3 is derived, is devoid of intrinsic biological activity in this T3 responsive system.
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