The amino acid sequence of cystatin, the protein from chicken egg-white that is a tight-binding inhibitor of many cysteine proteinases, is reported. Cystatin is composed of 116 amino acid residues, and the Mr is calculated to be 13143. No striking similarity to any other known sequence has been detected. The results of computer analysis of the sequence and c.d. spectrometry indicate that the secondary structure includes relatively little a-helix (about 20%) and that the remainder is mainly f-structure.
Relaxin, a peptide hormone responsible for the widening of the birth canal in mammals, has been purified from the ovaries of pregnant hogs. The amino acid sequences of its constituent A and B chains were determined, and the positions of the disulfide cross-links were established. Relaxin was shown to be identical to insulin with respect to its disulfide bond distribution, but significant homology was lacking in other positions. These findings suggest that relaxin and insulin were derived from a common ancestral gene. Since the intrauterine mode of propagation is synonymous with the development of mammals, the genetic distance between insulin and relaxin should therefore permit an estimate of the earliest possible time of commitment of one evolutionary branch to the development of mammals. This event was estimated to have occurred about 5 X 10(8) years ago.
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