Substrate specificity of the placental microsomal aromatase

Using an accurate and sensitive assay for the human placental aromatase we have found apparent Km values for androstenedione (4-androstene-3,17-dione) and testosterone to be 14 ± 4.0 nM and 41 ± 12 nM respectively. These values were significantly different (p < 0.001). Analyses at substrate concentrations 5–10 fold above and below the Km values did not indicate any anomalous kinetic behavior. Mixed substrate experiments were consistent with a single enzyme metabolizing both steroids: each competitively inhibited the aromatization of the other, and the “Ki” values were the same as their apparent Km values. Sodium chloride (1.2M) significantly increased the rate of testosterone aromatization by decreasing its Km value and had no significant effect on the aromatization of androstenedione. However, in the presence of this salt testosterone still inhibited the aromatization of androstenedione competitively with a “Ki” equal to its apparent Km. Our data is therefore consistent with the proposal that human placental microsomes contain a single “high affinity” site for the aromatization of androstenedione and testosterone.