Radiometric analysis of oxidative reactions in aromatization by placental microsomes. Presence of differential isotope effects.

In order to study the initial as well as the final steps in the aromatization of androgens to estrogens, high-specific activity [19-C3H3]androstenedione and testosterone were synthesized. Incubations of [19-C3H3]androstenedione with human placental microsomes resulted in the generation of [3H]water, as a result of the dual hydroxylation at C-19, and [3H]formic acid reflecting final aromatization. After an initial lag in the production of [3H]formic acid, the two radiolabeled products were formed linearly with time at a ratio of 2 to 1 under subsaturating conditions and 2.2 to 1 when saturating levels of substrate were present. Incubation of a mixture of [19-C3H3]- and [4-14C]androstenedione with human placental microsomes yielded 19-hydroxy- and 19-oxoandrostenedione, respectively, products of one and two hydroxylations at C-19. The isotope ratios of these derivatives revealed the presence of a tritium isotope effect in the first but not in the second hydroxylation at that site. When [19-C3H2]- and [4-14C]19-hydroxyandrostenedione were used as the substrate, the isotope ratio of the isolated 19-oxoandrostenedione showed no evidence of any isotope effect in its formation. Thus, the second hydroxylation at C-19 exhibits no isotope effect irrespective of whether androstenedione or 19-hydroxyandrostenedione are the substrates, and therefore, a concerted process and catalytic commitment are not responsible for the difference in isotope effects between the first and second C-19 hydroxylation by the placental aromatase complex. Radiometric kinetic analysis employing [19-C3H3]- and [1 beta,2 beta-3H]androstenedione as the comparative substrates provided evidence that the isotope effect is exerted solely through the Vmax component of the reaction. The distinction between the successive hydroxylations at C-19 in the aromatization sequence suggests, but does not prove, that different mechanisms, and hence different catalytic sites, may be involved in these steps.