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.
The binding affinity to insulin receptors in isolated rat adipocytes at 37 degrees C of the four isomers of [125I]monoiodoinsulin was ranked as B26 greater than B16 = A14 greater than A19. It was demonstrated that the difference in affinity was mainly due to a change in the association rate constant, rather than in the dissociation rate constant. 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 greater than 90% and independent of the position of the iodine. It was also shown that the formation of [125I]-monoiodotyrosine as a consequence of receptor-mediated degradation was proportional to the respective binding affinities of the four isomers. The two isomers with binding affinities different from that of [A14-Tyr-125I]monoiodoinsulin (i.e. the B26 and the A19 isomers, respectively) were shown to have 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 degrees C was B26 greater than B16 greater than A14 greater than A19, and in cultured human colon adenocarcinoma cells of the HT-29 line the binding affinities were ranked in the order B26 greater than B16 greater than A14 greater than or equal to A19 indicating that the functional properties of the insulin receptor vary within cell types and/or species.
D000273:Adipose Tissue / Q000378:metabolism; D000818:Animals; D006863:Hydrogen-Ion Concentration; D007328:Insulin / Q000031:analogs & derivatives* / Q000378:metabolism; D007457:Iodine Radioisotopes; D007536:Isomerism; D007700:Kinetics; D011485:Protein Binding; D051381:Rats; D011972:Receptor, Insulin / Q000378:metabolism; D013997:Time Factors; D014443:Tyrosine / Q000378:metabolism*
D007328:Insulin; D007457:Iodine Radioisotopes; C008147:insulin, iodo-; D014443:Tyrosine; D011972:Receptor, Insulin