Estrogen receptor in malignant melanoma: fact or artefact?

A model system is presented to explain how tyrosinase, an enzyme unique to pigmented cells such as normal and malignant melanocytes can oxidize [3H]-estradiol to radiolabeled products which closely resemble the tight binding of [3H]-estradiol to estrogen receptor. In the model system studied, tyrosinase oxidized [2,4,6,7-3H]-estradiol to [3H]-water and [3H]-estradiol metabolites, the latter of which formed ring-substituted conjugates with nucleophiles like monothioglycerol and BSA. Radiolabeled estradiol without tritium in the C-2 position (i.e. [6,7-3H]-estradiol) failed to liberate [3H]-water when exposed to tyrosinase but, nevertheless, did form ring-substituted [3H]-estradiol adducts with nucleophiles. The [3H]-water and the ring-substituted radiolabeled products possessed several characteristics of a genuine estrogen receptor protein in that they were resistant to dextran-coated charcoal (DCC) adsorption and their enzymatic formation was inhibited with non-steroidal estrogens like diethylstilbestrol. Other natural (estradiol) and synthetic (hydroxytamoxifen) estrogens which contain the phenol grouping also inhibited the enzymatic oxidation of [3H]-estradiol. Although it was difficult to differentiate estrogen receptor from tyrosinase using the conventional DCC assay system, several differences in these two proteins permitted a distinction to be made between them. First, tyrosinase oxidation of [3H]-estradiol was markedly inhibited by sulfhydryl reducing agents (monothioglycerol) that stabilize [3H]-estradiol binding to estrogen receptor. Second, estrogen receptor adsorbed by hydroxylapatite whereas tyrosinase did not, thus permitting the separation of these two proteins prior to incubation with [3H]-estradiol. We conclude that the [3H]-estradiol binding components in melanoma previously reported to be estrogen receptor probably represent instead the radiolabeled products of the tyrosinase-catalyzed oxidation of [3H]-estradiol.