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Receptor binding and cell mediated metabolism of iodine 125 labeled monoiodoglucagon by isolated canine hepatocytes

, : Receptor binding and cell mediated metabolism of iodine 125 labeled monoiodoglucagon by isolated canine hepatocytes. Journal of Biological Chemistry 259(14): 8986-8993

A reverse-phase HPLC [high performance liquid chromatography] method to purify 125I-labeled products resulting from the chloramine-T-based iodination of glucagon was developed and the products [(125I)iodoTyr13]glucagon, [(125I)iodoTyr10,13]glucagon, and [(125I)iodoTyr10]glucagon used to study the receptor binding of glucagon and the cell-mediated metabolism of the hormone by isolated canine hepatocytes. The extent of binding of the 3 labeled glucagons to cell receptors differed at steady state (8.5, 11.9 and 12.6% of the 3 peptides, respectively, becoming cell-associated), but each of the labeled glucagons approached steady state binding at the same rate. Unlabeled glucagon competed for the binding of each of the labeled peptides in parallel under steady state conditions, and each of the peptides showed potent activity in inhibiting [14C]fructose incorporation into glycogen. Gel filtration of the acetic acid-extracted, cell-associated products of radiolabeled glucagon binding revealed 10-20% of the material as a shoulder on the descending limb of the peak of hormone for each of the 3 labeled peptides. Trypsin digestion of the lower MW peptide derived from [(125I)iodoTyr13]glucagon resulted in a fragment containing residues 13 to 17 as the only detectable radiolabeled product. On the other hand, trypsin digestion of the analogous peptide derived from [(125I)iodoTyr10]glucagon revealed, in addition to the radiolabeled fragment containing residues 1 to 12, a major fragment identified by radiosequence analysis to contain residues 4 to 12 and a minor fragment identified to contain residues 7 to 12. Notwithstanding apparent differences in affinities exhibited by [(125I)iodoTyr13]glucagon, [(125I)iodoTyr10,13]glucagon, and [(125I)iodoTyr10]glucagon for binding to canine hepatocytes, the interactions of all 3 peptides with the glucagon receptor are functionally equivalent. The cell mediated metabolism of receptor-bound glucagon involves the formation of hormone-derived peptides in which the biologically important NH2-terminal region of the hormone has been modified by limited proteolytic cleavage.


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