16alpha-Hydroxyestrone (16alpha-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 16alpha-OHE1 with protein has been postulated via the Heyns rearrangement after
Schiff base formation. The Heyns rearrangement on the steroidal D-ring alpha-hydroxyimine
was investigated using 17-(2-methoxyethylimino)estra-1,3,5(10)-triene-3,16alpha-dio
l as a model intermediate. Rates of the Heyns rearrangement and hydrolysis of the
steroidal a-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 16alpha-OHE1 with protein.