Monoiodoinsulin Labelled in Tyrosine Residue 16 or 26 of the B-Chain or 19 of the A-Chain. II. Characterization of the Kinetic Binding Constants and Determination of the Biological Potency

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Abstract
The binding affinity to insulin receptors in isolated rat adipocytes at 37.degree. C of the 4 isomers of [125I]monoiodoinsulin was ranked as B26 > B16 = A14 > A19. The difference in affinity was mainly due to a change in the Ka, rather than in the Kd. At steady state in the binding process the fraction of cell-associated 125I-activity eluting from a Sephadex G-50 Fine column at a position identical to that of iodoinsulin was > 90% and independent of the position of the iodine. The formation of [125I]monoiodotyrosine as a consequence of receptor-mediated degradation was proportional to the respective binding affinities of the 4 isomers. The 2 isomers with binding affinities different from that of [A14-Tyr-125I]monoiodoinsulin (i.e. the B26 and the A19 isomers, respectively) had biological potencies which corresponded within .+-. 8% to the observed changed binding affinities. In cultured human lymphocytes of the IM-9 line the hierarchy of binding affinities at 37.degree. C was B26 > B16 > A14 > A19, and in cultured human colon adenocarcinoma cells of the HT-29 line the binding affinities were ranked in the order B26 > B16 > A14 .gtoreq. A10 indicating that the functional properties of the insulin receptor vary within cell types and/or species.