Glucagon structure and function. I. Purification and properties of bovine glucagon and monodesmidoglucagon.

Crystalline bovine or porcine glucagon was fractionated by chromatography on columns of DEAE-cellulose in the presence of buffers containing ‘7 M urea. About 89% of the recovered protein comprises highly purified glucagon; 10% is monodesamidoglucagon and another 0.8 % resembles glucagon in amino acid composition and in electrophoretic mobility. The monodesamidoglucagon was identified and characterized largely by means of amino acid analysis, electrophoretic mobility, circular dichroic spectra, and enzymatic degradations. Deamidation appears to occur randomly on the three glutamines in positions 3, 20, and 24, giving a mixture of three isomers. The penultimate asparagine remained intact. Monodesamidoglucagon had less biological and immunological activity than glucagon and exhibited an altered circular dichroic spectrum; either covalent or conformational changes, or both, may be responsible for the decrease in activity.