Isolation and characterization of relaxin from the sand tiger shark (Odontaspis taurus).

A peptide with relaxin activity in guinea pigs but not in mice has been extracted from the ovaries of pregnant sand tiger sharks (Odontaspis taurus). The structural similarity of this peptide to porcine relaxin includes molecular size approximately 6000 daltons), number of chains, and possibly, disulfide cross-links. The relaxin-type peptide isolated from shark ovaries contains the amino acid residues tyrosine, proline, and histidine, which are absent in the porcine hormone. The amino acid composition of shark relaxin, therefore, resembles that of porcine insulin to a greater extent than does the amino acid composition of porcine relaxin. This finding supports the idea that shark relaxin may be a primitive relaxin that has undergone fewer mutations than porcine relaxin since the putative duplication of the insulin gene. The data presented here suggest that the putative duplication of the insulin gene, which might have given rise to relaxin, has occurred much earlier than the separation of sharks from the general branch of animals that eventually gave rise to mammals.