Characterization of the circulating form of porcine relaxin: biological activity and terminal amino acids.

Relaxin is structurally related to insulin, and it induces pregnancy-related changes in the reproductive tract of several mammalian species. Relaxin isolated from the ovaries of pregnant sows has been used for primary structure determination, for much of the biological characterization of the hormone, and for the development of RIAs. Immunoreactive (IR) relaxin is found in peripheral blood during pregnancy in pigs and other species, but it has not been established that the substance identified by RIA is structurally or biologically equivalent to the native ovarian hormone. IR relaxin was, therefore, isolated from peripheral plasma of late pregnant gilts (days 112-114) for bioassay and determination of terminal amino acid residues. IR relaxin was monitored by a specific homologous RIA during fractionation of plasma by gel filtration, cation exchange, and hydrophobic binding to octadecysilica. IR relaxin circulates unbound and is equipotent with ovarian relaxin in the mouse pubic ligament bioassay. Amino acids released from IR relaxin by pyroglutamic aminopeptidase and carboxypeptidase-Y were converted to their 4-dimethylamino-azo-4'-sulfonyl derivatives for identification by HPLC. The B-chain of IR relaxin had an amino-terminal pyroglutamic acid. Amino acids sequentially released from the carboxy-terminal indicated a chain length of 28-30 amino acids, suggesting a heterogeneity reminiscent of that of ovarian relaxin isolated by other methods. Arginine was released from the free amino-terminal by dimethylaminozaobenzene-isothiocyanate degradation, indicating an intact A-chain of 22 amino acids. Blood immunoreactive relaxin in pigs is, therefore, a secreted biologically active form of relaxin with an amino acid composition similar to that of the form stored in the corpus luteum.