Homogeneous [mono-125I-Tyr10]- and [mono-125I-Tyr13] glucagon.

The two monoiodinated forms of glucagon were prepared by lactoperoxidase-catalysed iodination followed by separation by reversed-phase high-performance liquid chromatography. The intramolecular distribution of 125I was analysed by tryptic and chymotryptic cleavage of the isolated isomers. The results show that [mono-125I-Tyr10]- and [mono-125I-Tyr13]glucagon can be separated from each other and from the respective unlabelled polypeptide and thus can be obtained in a pure state with the highest possible specific activity. We have studied the receptor binding ability of both tracer isomers to isolated intact rat hepatocytes. The resulting Kd values were 2.0 +/- 0.2 nM for the tyrosine-13-labelled glucagon and 4.2 +/- 0.3 nM for the tyrosine-10-labelled glucagon.