THEORETICAL-STUDIES ON THE MECHANISM OF CONVERSION OF ANDROGENS TO ESTROGENS BY AROMATASE

Semiempirical molecular orbital calculations (AM1) were used to model several possible reaction mechanisms for the third oxidation of the aromatase-catalyzed conversion of androgens to estrogens. The reaction mechanisms considered are based on the assumption that the third oxidation is initiated by 1-beta-hydrogen atom abstraction. Homolytic cleavage of the C-10-C19 bond was modeled for both the 3-keto and 2-en-3-ol forms of the androgen 1-radicals. The addition of a protein nucleophile to the 19-oxo intermediate was also considered, and -OCH3, -SCH3, and -NHCH3 were used to represent the Ser, Cys, and Lys adducts. The transition states were estimated and optimized from the reaction coordinates obtained by constraining and increasing the C-10-C19 bond lengths. The enthalpies of activation range from 14 to 21 kcal and are approximately 2 kcal lower for cleavage of the enol form. Given the tendency for AM1 to overestimate activation energies, all reactions may be energetically accessible. Other reactions modeled include a homolytic cleavage reaction from a thioether radical cation and the direct additions of oxygen radical compounds to the carbonyl of the 1-radical-2-en-3-ol-19-oxo androgen. A mechanism is proposed in which the 19-oxo intermediate is subject to initial nucleophilic attack by the protein. Since rotation of the 19-carbonyl can bring the oxygen within 2.1 angstrom of the 2-beta-hydrogen, the formation of a tetrahedral intermediate can occur with concomitant removal of the 2-beta-proton. Enolization activates the C1-position for hydrogen atom abstraction, since the resulting radical is resonance stabilized. Homolytic cleavage of this radical is followed by recombination of the C19-radical with the hydroxy radical equivalent. The geometry of the carbonyl oxygen in the androgens is between that for the 1-radicals and estrogen, suggesting that some transition-state stabilization for the homolytic cleavage reaction may occur.