Gag (Mycteroperca microlepis) vitellogenin: purification, characterization and use for enzyme-linked immunosorbent assay (ELISA) of female maturity in three species of grouper

Circulating levels of the egg yolk precursor protein, vitellogenin (VTG), can be used as a biochemical indicator of maturation in female fish. Here we report on purification and partial characterization of VTG from a temperate marine serranid, the gag(Mycteroperca microlepis). Development of a competitive, enzyme-linked immunosorbent assay (ELISA) for gag VTG (gVTG) is also described. The gVTG was purified by DEAE-agarose anion exchange chromatography from a pooled plasma sample collected from several juvenile gag after they were injected with 17βestradiol. The protein appeared as a major band of Mr≅183 000 after SDS-PAGE ± Western blotting using either a specific rabbit antiserum to gVTG or a universal monoclonal antibody for vertebrate VTGs. Amino acid composition analysis and N-terminal peptide sequencing verified that gVTG is similar in primary structure to VTG from several other teleost species. The purified gVTG and its specific antiserum were used to develop a sensitive, competitive, antibody-capture ELISA for quantifying the protein in blood plasma from maturing females. VTG levels in maturing female gag were highly correlated with oocyte growth and circulating testosterone and 17β-estradiol levels, whereas VTG was non-detectable in juveniles, immature females or males. Two size-based maturity schedules for female gag were constructed, one utilizing detection of VTG in their circulation as a marker of maturity and the other relying on histological evidence that their ovaries were in vitellogenic or later stages of maturation. The two schedules were virtually identical. The gVTG ELISA was also used to detect VTG in blood plasma from mature Nassau grouper (Epinephelus striatus) and red hind (E. guttatus). As with gag, the assay was completely reliable for discriminating between reproductively mature females versus males from these two grouper species.