Four
peptides present in completely different
biological sources have been shown to exhibit a large degree of structural similarity. The
peptides include: (i) a 60
amino acid residue
breast cancer associated pS2
peptide isolated from human gastric juice and the
culture media of the human
breast cancer cell line MCF-7; (ii) a 106
amino acid residue
pancreatic spasmolytic polypeptide (PSP) isolated from porcine pancreas and pancreatic juice; and (iii) a 49 and 50
amino acid residue
peptide predicted from a
cDNA isolated from the skin of the frog, Xenopus laevis. These
peptides are characterized by having one (pS2 and the frog
peptides) or two (PSP) domains of a highly conserved 38-39
amino acid residue consensus sequence not found in any other known
peptides or
proteins. The domain sequences contain 6
cysteine residues in nearly the same positions and it is suggested that these 6 residues are linked by 3 disulphide bonds to form a characteristic 'trefoil' disulphide loop structure common in all four
peptides. From the sources of which the
peptides have been isolated and from experiments showing that PSP has a
growth factor stimulatory effect on MCF-7 cells, it is further suggested that these
peptides may represent members of a new family of
growth factors.