The interaction of immobilized transition-metal ions with some gastrointestinal polypeptides.

Amino acid residues in proteins that possess an ionizable proton can display ligand properties. Nitrogen donors include the imidazole group of histidine, the &-amino group of lysine and the guanadinium group of arginine. Thecarboxyl groups of aspartate and glutamate and the hydroxyl group of tyrosine can function as oxygen donors, and the thiol group of cysteine can act as a sulphur donor. The nitrogen atom of the peptide linkage and the N-terminal amino group can also co-ordinate to metal ions (Vallee & Wacker, 1970). The affinity of proteins for heavy metal ions has been used by Porath et al. (1975) in the development of a chromatographic technique in which columns of immobilized zinc and copper ions were used to fractionate serum proteins. In the present study, the interaction of immobilized transition-metal ions with the lzSI-labelled polypeptide hormones, gastrin and glucagon and with glucagon-like-immunoreactivity polypeptide material, which competes with glucagon, for binding sites on antibodies raised against glucagon, was investigated. The glucagon-like-immunoreactivity material (mol.wt. approx. 12000) was purified from extracts of pig colon by immunoaffnity chromatography (Murphy et al., 1973). The metal-binding conjugate (I) was synthesized by the method of Porath et al. (1975) by coupling iminodiacetic acid to Sepharose-4B activated with 1,4bis-(2,3epoxypropoxy)butane.