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

Ovarian microsomal aromatase (P450(arom)) activity was studied in relation to season and incubation of follicles with catecholestrogens [(2-hydroxyestradiol-17 beta (2-OHE2) and 2-methoxyestradiol-17 beta (2-methoxyE(2))] using a product (estradiol-17 beta) assay. Peak P450(arom) 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 K-m and V-max of the enzyme varied significantly and the values were high in the preparatory (vitellogenic) phase (K-m 74.62 +/- 1.73 nM, V-max 0.81 +/- 0.01 pmol/mg protein/min) and low in the spawning (post-vitellogenic) phase (K-m 62.01 +/- 1.68 nM, V-max 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 mu M 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 mu M or 100 mu M) did not elicit any significant change compared to the control groups. A comparison of the aromatase inhibition index (AI(50), indicates 50% inhibition of aromatase activity) of fadrozole, a known aromatase inhibitor and 2-OHE2 shows that the AI(50) 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-methoxyE(2) increased enzyme activity only at the higher concentrations (1-100 mu M). The results show seasonality in the potential of the ovary to synthesize E-2 and the potent enzyme inhibiting activity of 2-OHE2, which is reported for the first time. (c) 2008 Elsevier Inc. All rights reserved.