Purification of glucagon by subunit exchange chromatography.
Glucagon was immobilized onto Sepharose matrices activated with CNBr or tresyl chloride, as a function of several parameters including pH of coupling, concentration of added polypeptide, and presence or absence of urea. The hormone was linked to the matrix through a single point per molecule, namely, the epsilon -amino group of Lys(12) when the coupling was carried out at alkaline pH, or the imidazole group of His(1) when the coupling was carried out at acidic pH. Glucagon immobilized at alkaline pH interacted specifically with soluble glucogon. The extent of self-association was similar to that of free glucagon, which exists in solution in a monomer-trimer equilibrium whose association constant is highly dependent on the characteristics of the buffer (pH, ionic strength, and nature of anions). The immobilized hormone proved to be suitable for the purification of the free one from a pancreatic extract. After a preliminary treatment with charcoal-dextran, the extract was percolated on a glucagon-Sepharose column under associating conditions (high concentrations of salting out anions and alkaline pH) and then, following a washing to remove extraneous compounds, the specifically bound hormone was eluted under dissociating conditions (low ionic strength). The subunit exchange chromatography of the extract gave a ca. 90% pure product. The overall recovery of the process was ca. 66%. The leakage of immobilized hormone was 40% in the case of CNBr activation of Sepharose and 15% in the case of tresyl chloride activation, after an eight-day treatment under working conditions.
Istituto di Chimica degli Ormoni, C.N.R., Via Mario Bianco 9, 20131 Milan, Italy.
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Antonini