Relaxin: an insulin-related growth factor.

Relaxin is an ovarian peptide that is released just prior to parturition to effect changes in the tissues of the birth canal that aid in the delivery of the fetus. Structural studies established that the two constituent polypeptide chains, composed of 22 and 30 amino acids, are joined by two interchain disulfide bonds with an additional third intrachain bridge. As well as the identical pattern of disulfide bonds, relaxin shows an overall 25% identity with insulin. Furthermore, the sequence of relaxin can be incorporated into the known three-dimensional structure of insulin without significant distortion of the main polypeptide chain backbone. The discovery of the insulin-relatedness of relaxin brings to three the number of growth factors that share a common structural gene precursor with insulin. Nerve growth factor and insulin-like growth factor have already been so identified. Inclusion in this hormone family suggests that the mechanism of action may involve internalization as well as complexation with cell surface receptors of target cells.