Ovarian P450 aromatase activity in the catfish Heteropneustes fossilis: Seasonal changes and effects of catecholestrogens

Ovarian microsomal aromatase (P450arom) activity was studied in relation to season and incubation of follicles with catecholestrogens [(2-hydroxyestradiol-17β (2-OHE2) and 2-methoxyestradiol-17β (2-methoxyE2)] using a product (estradiol-17β) assay. Peak P450arom activity was noticed in late preparatory phase (April) and it decreased significantly in pre-spawning, spawning and post-spawning phases to give the lowest value in resting phase. Apparent Km and Vmax of the enzyme varied significantly and the values were high in the preparatory (vitellogenic) phase (Km 74.62 ± 1.73 nM, Vmax 0.81 ± 0.01 pmol/mg protein/min) and low in the spawning (post-vitellogenic) phase (Km 62.01 ± 1.68 nM, Vmax 0.69 ± 0.002 pmol/mg protein/min). The incubation of the ovarian microsomes with 2-OHE2 elicited significant biphasic effects on enzyme activity. In the vitellogenic phase, concentrations of the steroid up to 1 μM inhibited enzyme activity significantly with the highest inhibition at 10 nM. However, in the post-vitellogenic ovary, the highest inhibition was registered at 100 nM. The higher concentrations (10 μM or 100 μM) did not elicit any significant change compared to the control groups. A comparison of the aromatase inhibition index (AI50, indicates 50% inhibition of aromatase activity) of fadrozole, a known aromatase inhibitor and 2-OHE2 shows that the AI50 was 4.4 nM for fadrozole and 0.864 nM (vitellogenic phase) and 1.31 nM (post-vitellogenic phase) for 2-OHE2 indicating higher potency of the latter. The incubation of the ovarian microsomes with 2-methoxyE2 increased enzyme activity only at the higher concentrations (1–100 μM). The results show seasonality in the potential of the ovary to synthesize E2 and the potent enzyme inhibiting activity of 2-OHE2, which is reported for the first time.