Characterization of oviductal aromatase in the northern leopard frog, Rana pipiens

Previous work has shown that the oviduct of the northern leopard frog, Rana pipiens, can convert testosterone to estradiol. The present paper examines the characteristics of the aromatase responsible for this reaction and compares it with human placental aromatase. Microsomes were isolated from the oviducts, and aromatase activity was assayed by a tritiated water release method. The Km and the Vmax for androstenedione were 188.1 +/- 30.2 nM and 1.42 +/- 0.11 fmol of estrogen produced/min/mg protein, respectively. Using the same method, human placental aromatase had a Km of 123.0 nM and a Vmax of 113.57 fmol of estrogen produced/min/mg protein. When tested at four temperatures between 15 and 45 degrees C, the frog enzyme showed maximum activity at 37 degrees C. The enzyme had a broad pH optimum between 7.4 and 10.4. The aromatase inhibitor 4-hydroxyandrostenedione inhibited activity by 20% at 0.3 microM and 40% at 0.5 microM. The present study provides additional evidence that an aromatase is present in the frog oviduct. Characterization of this enzyme revealed similarities to human placental aromatase, but the specific activity was much lower in the frog oviduct.