Reversible dimerization of avian pancreatic polypeptide

The dimerization of avian pancreatic polypeptide (PP) was studied by large-zone gel chromatography using integral boundary analysis. The association constant has been determined as a function of temperature and pH. The dimerization is endothermic and entropically driven, which suggests hydrophobic interactions, and is enhanced with increasing pH. Analysis of the pH dependence indicates the involvement of ionizable groups. with pKa values of 4.5-5.5. In the avian PP molecule, there are six groups which are potentially titratable in this pH range. A comparison of the amino acid sequence of avian PP with that of the bovine and canine homologues, which also exhibit pH-dependent dimerization Noelken, M.E., Chang, P.J. & Kimmel, J.R. (1980) Biochemistry (preceding paper in this issue), shows that they have three carboxylate and two guanidinium groups in common. It is suggested that salt linkages involving these groups participate in dimerization. In the avian peptide, histidine-34 may also be involved.