Relaxin and its structural relationship to insulin

THE sequence of the ovarian peptide, relaxin, has recently been reported1,2 and the observation1 that it can be accommodated into the insulin fold has been discussed3,4. Only 11 residues, including the cystines, are common to insulin and relaxin (Fig. 1), but the probable identity of the cystine pairings and the preservation of the hydrophobic character of buried residues suggests some structural homology between the two hormones. The extent of sequence changes and the remote relationship between the corpus luteum and the pancreas, the respective sources of relaxin and insulin, makes homology of the two hormones most interesting. We have therefore rigorously examined the relaxin conformation by a computer graphics system and found it possible to accommodate the relaxin sequence within the insulin main-chain geometry.