Vitellogenin of the northern leopard frog (Rana pipiens): Development of an ELISA assay and evaluation of induction after immersion in xenobiotic estrogens

- A competitive ELISA was developed for Rana pipiens vitellogenin.
- Male frogs immersed in DES showed time- and dose-dependent vitellogenin induction.
- Frogs treated (20 d) with 3 known xenobiotic estrogens had low or no vitellogenin.
- Frogs may be less sensitive than fish with regard to estrogen vitellogenin induction.
- R. pipiens vitellogenin may be useful for studying endocrine disruption in the field.

An immunoassay for leopard frog (Rana pipiens) vitellogenin was developed for studying endocrine disruption. Male frogs were injected with estradiol-17β to stimulate vitellogenin for purification. SDS-PAGE revealed high amounts of a 170-180 kDa protein, which was confirmed to be vitellogenin by Western blotting. Vitellogenin was purified by DEAE chromatography and used to generate a polyclonal antibody. A competitive ELISA was developed for leopard frog vitellogenin with a detection limit of 6.0 ng mL−1 and a working range of 20-1000 ng mL−1. The intra-assay coefficient of variation averaged 5.47% for control sera and 9.71% for estrogen-treated sera. The inter-assay coefficient of variation averaged 8.21% for control sera and 9.93% for estrogen-treated sera. Recovery of purified vitellogenin averaged 95.2%. Vitellogenin was measured in male frogs immersed in the estrogenic compound diethylstilbestrol (DES) for various times and doses. Serum vitellogenin was detected within five days after immersion in 1.0 mg L−1 DES and levels continued to increase through 20 d. In a 20-day dose-response experiment, serum vitellogenin was detected in frogs immersed in 0.01 mg L−1 DES and vitellogenin concentration increased with dose. Immersion of frogs in one of several xenobiotic estrogens (nonylphenol, octylphenol, bisphenol-A) for 20 d did not increase vitellogenin for any treatment, suggesting that this frog may be less sensitive than fish to endocrine disruptors. Vitellogenin induction in R. pipiens may be a useful amphibian model system for field studies of endocrine disruption, due to its broad geographic range.