Sea bass (Dicentrarchus labrax) sex hormone binding globulin: molecular and biochemical properties and phylogenetic comparison of its orthologues in multiple fish species

Sex hormone binding globulin (SHBG) binds and transports androgens and estrogens in the blood of vertebrate species including fish. We have used oligonucleotide primers corresponding to highly conserved regions of the SHBG coding sequences within the zebrafish and fugufish genomes to obtain a 1528 bp cDNA encoding SHBG from tissue RNA extracts from the European sea bass. Amino-terminal sequence analysis of recombinant sea bass SHBG indicated that its deduced precursor polypeptide includes a 35-residue secretion signal polypeptide, and the 361-residue mature sea bass SHBG sequence exhibits 45-67% sequence identity with SHBGs from other fish species that have been determined directly (for zebrafish) or deduced (for rainbow trout, medaka and fugufish) from sequences within public databases. The sea bass SHBG (39,894 Da) comprises a tandem repeat of laminin G-like domains typical of SHBG sequences; contains three N-glycosylation sites, and exists as a 118,300 ± 11,500 Da homodimer. Sea bass SHBG exhibits a high affinity (Kd = 8.8 nM for 17β-estradiol) and specificity for gonadal steroids and their precursors (e.g., 17β-estradiol > testosterone > dehydroepiandrosterone > 5α-dihydrotestosterone > androstenedione > 11-ketotesterone). Interestingly, the affinity of sea bass SHBG for the synthetic estrogen, 17α-ethynylestradiol was found to be essentially identical to that for 17β-estradiol. The availability of SHBG sequences in sea bass and other fish set the stage for detailed studies of SHBG function in fish reproductive physiology, as well as its potential role as a target of endocrine disruptors.