Amphibian lutropin from the bullfrog Rana catesbeiana. Complete amino acid sequence of the beta subunit.

The amino acid sequence of lutropin (LH) β subunit of an amphibian, the bullfrog Rana catesbeiana, has been determined. The primary structure was determined by sequencing the intact protein (residues 1–44) and peptides originated by cyanogen bromide cleavage and lysyl endopeptidase digestion. 12 cysteine residues are conserved in the bullfrog and mammalian LH β subunit. One sugar-chain-binding site at Asn-8 is also conserved in the bullfrog and in all mammals except humans. This glycoprotein is composed of 112 amino acid residues with a molecular mass of 12675 Da, considering the six cystine bridges and excepting the sugar chain. The bullfrog β subunit has approximately 50% sequence identity with that of mammals and with the fish gonadotropin β subunit, and about 40% with bullfrog follicle-stimulating hormone β subunit.