Purification, characterization, and molecular cloning of gonadotropin subunits of silver carp (Hypophthalmichthys molitrix).

Abstract The α and β subunit of silver carp gonadotropin (scGTH-α and scGTH-β) were isolated by high-performance liquid chromatography. Heterogeneity of N-terminal amino acid sequence was observed in scGTH-α but not in scGTH-β. For determining the complete primary structures of scGTH-α and scGTH-β, their cDNAs were cloned. Combining the data of N- and C-terminal sequences determined from proteins and the amino acid sequences deduced from cDNAs, we infer that scGTH-α consists of 95 and/or 93 residues and scGTH-β consists of 115 residues. Both scGTH-α and scGTH-β are glycoprotein. Their carbohydrate content is about 20 g per 100 g protein. The molecular weights of scGTH-α and scGTH-β were calculated to be 12,700 and 15,700 Da, respectively. The amino acid sequences of scGTH-α and scGTH-β are very similar to those of the corresponding subunit of carp GTH, different in only 2 and 4 residues, respectively. In addition, a high extent of homology (70%) was also observed between the α subunits of fish and mammalian GTHs. In the case of β subunit, homology among various species of fish (75 to 98%) is much higher than that between fish and mammal (40%). These data suggest that the α subunit is conserved while the β subunit is diversified during the molecular evolution of vertebrate GTH.