Isolation of maturational gonadotropin subunits from spotted seatrout ( Cynoscion nebulosus) and development of a β-subunit-directed radioimmunoassay for gonadotropin measurement in sciaenid fishes

Abstract

Maturational gonadotropin (GTH) subunits were isolated from pituitaries of the spotted seatrout ( Cynoscion nebulosus), a marine perciform teleost, by ethanolic extraction and ion-exchange, gel-filtration, and reverse-phase chromatography. Partial amino acid sequencing of the N-terminal regions of the α and β subunits indicated 60–80% identities with various carp and salmon GTH subunits. The spotted seatrout GTH β-subunit was used as radioligand in a radioimmunoassay (RIA) with Atlantic croaker ( Micropogonius undulatus) GTH antiserum. Pituitary extracts and plasmas from a variety of sciaenid fishes diluted parallel to the croaker GTH standard in the RIA. These data suggest that there is a high degree of immunological similarity among the GTH β subunits of sciaenid fishes. The RIA measured increased levels of GTH in the plasmas of three species of sciaenid fishes, spotted seatrout, orangemouth corvina ( Cynoscion xanthulus), and red drum ( Sciaenops ocellatus), following injections of a luteinizing hormone-releasing hormone analog. The β-subunit-directed GTH RIA increases considerably the number of species in which studies of GTH physiology can now be conducted.