Effect of metal ions on the stable adduct formation of 16α- hydroxyestrone with a primary amine via the Heyns rearrangement

16α-Hydroxyestrone (16α-OHE₁), one of the major estrogen metabolites in humans that may plays a role in cell transformation, has been found to form stable adducts with nuclear proteins. The mechanism for the formation of a stable covalent adduct of 16α- OHE₁ with protein has been postulated via the Heyns rearrangement after Schiff base formation. The Heyns rearrangement on the steroidal D-ring α-hydroxyimine was investigated using 17-(2- methoxyethylimino)estra-1,3,5(10)-triene-3,16α-diol as a model intermediate. Rates of the Heyns rearrangement and hydrolysis of the steroidal α- hydroxyimine were determined by a high-performance liquid chromatography (HPLC) simultaneously. The Heyns rearrangement was demonstrated to be optimum at pH 6.2 and the reaction rate at physiological pH, 7.3-7.5, was more than 90% of that at the optimum pH. On the other hand, modulator(s) to the reactions were also examined. According to our previous finding of the proton-mediated mechanism of the Heyns rearrangement, the effects of cationic metal ions on the reactions were examined with 29 metal chlorides. Five metal ions, Pt⁴⁺, Cu²⁺, Ni²⁺, Co²⁺, and Mn²⁺, suppressed the formation of Heyns product significantly while Fe²⁺, Y³⁺, Gd³⁺, and Er³⁺ slightly increased it. The suppression rate was synergistically enhanced by the combination of Pt⁴⁺ with Co²⁺, Cu²⁺, or Ni²⁺. These results suggest the five metal ions, Pt⁴⁺, Cu²⁺, Ni²⁺, Co²⁺, and Mn²⁺, reduce the formation of the Heyns product in vivo and, therefore, would be useful tools to clarify the implication of the stable adduct formation of 16α-OHE₁ with protein.