Purification and characterization of gonadotropin I and II from pituitary glands of tuna (Thunnus obesus)

The duality of teleost pituitary gonadotropins was established in an advanced marine fish, the tuna (Thunnus obesus). Two different molecular forms of gonadotropins, designated tGTH I and tGTH II, were isolated from an alcoholic extract of pituitary glands following ion-exchange chromatography and reversed-phase HPLC. Both tGTH I and tGTH II stimulated estradiol-17β and testosterone production in tuna ovarian follicles in vitro, although responses to tGTH II were significantly greater than those to tGTH I. Each gonadotropin consisted of α- and β-subunits. tGTH I was stable in acidic conditions, whereas tGTH II dissociated into two subunits after acid treatment. Alpha subunits of tGTH I and tGTH II had identical amino acid sequences of 94 amino acid residues. The tGTH Iβ and tGTH IIβ consisted of 102 and 115 amino acid residues, respectively, and showed 35% sequence identity. tGTH Iβ is structurally more similar to salmon GTH Iβ than to salmon GTH IIβ, whereas tGTH IIβ is more similar to salmon GTH IIβ. Thus it is evident that the tuna pituitary gland produces two chemically distinct gonadotropins.