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Physical Properties of Nitroglucagons and Aminoglucagons

From the ¹ From the Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, Indiana 46206

Mononitroglucagon (NO₂ group on tyrosine residue 13) exhibits physical properties very similar to those of unmodified glucagon. The far ultraviolet CD spectra of nitroglucagon and glucagon are extremely similar over a wide range of pH (2 to 10). The CD spectrum of monoaminoglucagon, the reduced form of mononitroglucagon, is also very similar to that of glucagon. The low ultraviolet CD spectra of nitroglucagon and glucagon show the same concentration dependency (pH 10.2). The two proteins also exhibit very similar self-association properties. We observed no evidence of any conformational change in mononitroglucagon as compared to glucagon.

The pK_a values of the NO₂-tyrosine and tyrosine residues in nitroglucagon are 6.7 and 10.2, respectively. In addition, the pK_a of the tyrosine group (residue 10) is concentration dependent. Based on these titration results, along with ultraviolet difference spectroscopy studies, we propose that the tyrosine group in glucagon whose ionization is concentration dependent is tyrosine 10 and not tyrosine 13 as proposed by Gratzer and Beaven (Gratzer, W. B., and Beaven, G. H. (1969) J. Biol. Chem. 244, 6675).

Nitroglucagon exhibits near ultraviolet and visible region CD bands which are attributable to the NO₂-tyrosine group. Attempts to relate these bands to a particular orientation of the side chain group have been unsuccessful. Only the sequence of residues 7 through 13 is needed to retain the visible region CD spectrum observed with the intact nitroglucagon molecule.

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