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

Characterization of the three 125I-iodination isomers of human insulin-like growth factor I (IGF1)

Human insulin-like growth factor I (IGF1) was labeled with 125I and the resulting mixture of iodination isomers was separated by reverse-phase HPLC. Three major radioactive peaks were isolated and identified by sequencing as the expected three monoiodinated species. The ranking of the affinities of the three isomers for the human IGF1 receptor was found to be Tyr24(125I) > Tyr31(125I) >> Tyr60(125I). The Tyr31(125I) isomer was shown to have an affinity similar to that of unlabeled IGF1 and is thus the tracer of choice for IGF1. The tracers were stable upon storage at -20 degrees C for at least 3 months.

Semisynthetic des-(B27-B30)-insulins with modified B26-tyrosine

Semisynthetic des-(B27-B30)-insulins containing modified B26-tyrosine residues were prepared to refine the understanding of the importance of position B26 with regard to biological and structural properties of the hormone. The following shortened insulin analogues were synthesized by trypsin-catalysed peptide-bond formation between the C-terminal amino acid ArgB22 of des-(B23-B30)-insulin and synthetic tetrapeptides as amino components: des-(B27-B30)-insulin, des-(B27-B30)-insulin-B26-methyl ester, -B26-carboxamide with varying C-terminal hydrophobicity of the B-chain, and [Tyr(NH2)B26]-, [Tyr(NO2)B26]-, [Tyr(I2)B26]-, [D-TyrB26]des-(B27-B30)-insulin-B26-carboxamide containing non-proteinogenic amino acids in position B26. Starting from insulin and an excess of synthetic Gly-Phe-Phe-Tyr-OMe as nucleophile, des-(B27-B30)-insulin-B26-methyl ester--the formal transpeptidation product at ArgB22--was formed in one step. Biological in vitro properties (binding to cultured human IM-9 lymphocytes, relative lipogenic potency in isolated rat adipocytes) of all semisynthetic analogues are reported, ranging from slightly decreased to two-fold receptor affinity and nearly three-fold biopotency relative to insulin. If the C-terminal tetrapeptide B27-B30 is removed, full relative insulin activity is still preserved, while the shortening results in the loss of ability to associate in solution. Only after carboxamidation or methyl esterification of TyrB26 the self-association typical of native insulin can be observed, and the CD-spectral effects in the near UV spectrum related to association and hexamerization of the native hormone are qualitatively reestablished. The results of this investigation underline the importance of position B26 to the modulation of hormonal properties and solution structure of the shortened insulins.

Synthesis and properties of [A19-(p-fluorophenylalanine)] insulin

The synthesis of [Phe(F)A19]insulin (porcine) is described. First the protected [Phe(F)19]A-chain was assembled by segment condensation of [1-12] and [13-21] using the dicyclohexyldiimide/1-hydroxybenzotriazole procedure. [Phe(F)19]A-chain was purified by ion exchange chromatography after removal of all the protecting groups (Boc, But, OBut and S-Trt) and its conversion into the tetra-S-sulfonated derivative. [Phe(F)A19]insulin was prepared by combination with porcine B-chain and purified by gel filtration and ion-exchange chromatography. The in vitro biological activity of this analogue was 60%. CD spectra in the near and far UV are qualitatively very similar to those of insulin.

Receptor-mediated degradation of insulin in isolated rat adipocytes. Formation of a degradation product slightly smaller than insulin

More than 90% of the radioactivity associated with isolated rat adipocytes incubated with [TyrA14-125I]monoiodoinsulin represented at steady state iodoinsulin possessing full binding affinity. In contrast, about half of the radioactivity dissociating from the cells was [125I]monoiodotyrosine. The other half was of a molecular size similar to that of iodoinsulin as judged from gel-filtration chromatography. However, the descending limb of the 'insulin' peak (i.e., the smaller molecules) possessed a reduced binding activity compared with native iodoinsulin, material from the ascending limb, or a similar fraction isolated from dissociation medium from IM-9 lymphocytes, a cell type devoid of receptor-mediated insulin degradation. The cells, thus, release an intermediary degradation product.

Preparative reversed-phase high-performance liquid chromatography of iodinated insulin retaining full biological activity

Insulin monoiodinated in Tyr A14, A19, B16 and B26 can be separated from insulin and diiodoinsulins using reversed-phase high-performance liquid chromatography on LiChrosorb RP-18 columns. Monoiodoinsulins with high and low specific activities were isolated from a number of buffer systems without any reduction in binding affinity and biological activity in isolated rat fat cells. The reason for the previously observed reduction in the binding affinity was probably column bleeding, i.e., chemical degradation of the column support.

Uptake and degradation of insulin and alpha 2-macroglobulin-trypsin complex in rat adipocytes. Evidence for different pathways

The cell association and degradation of insulin and alpha 2-macroglobulin-trypsin complex were measured in rat adipocytes with or without various inhibitors in the attempt to clarify whether the two ligands were taken up by the same or by different pathways. Several inhibitors, and particularly those of membrane traffic, lysosomal function and transglutaminase activity, affected the two ligands differently. Thus, chloroquine (100 microM) reduced both the uptake of alpha 2-macroglobulin X trypsin and its receptor-mediated degradation by about 70%. In contrast, the uptake of insulin was increased 2-3-times and the receptor-mediated degradation was only slightly reduced. Methylamine (10 mM) and ammonium chloride (10 mM) reduced degradation of alpha 2-macroglobulin X trypsin markedly without affecting that of insulin. Leupeptin (100 microM) increased uptake and reduced degradation of alpha 2-macroglobulin X trypsin without affecting insulin. Dansylcadaverine (500 microM) almost abolished uptake and degradation of alpha 2-macroglobulin X trypsin but had little effect on insulin. Moreover, uptake and degradation of alpha 2-macroglobulin X trypsin was much more sensitive than insulin to the action of metabolic inhibitors such as dinitrophenol and cyanide. The results show that the two ligands are taken up by functionally different systems. In addition, they support the hypothesis that lysosomes play a relatively minor role in the receptor-mediated degradation of insulin.

Receptor-mediated degradation and internalization of insulin in the adenocarcinoma cell line HT-29 from human colon

In the adenocarcinoma cell line HT-29 receptor-bound insulin is substrate for a proteolytic process leading to the release of about half of the cell-associated [125I]monoiodoinsulin in the form of [125I]iodide and [125I]monoiodotyrosine. Classical lysosomal inhibitors (NH+4, methylamine, leupeptin) did not inhibit this proteolysis. Inhibitors of membrane traffic (chloroquine and monensin) and of metabolism (CN-) inhibited the fractional receptor-mediated degradation. The former led to an increased cell-associated 125I activity whereas the latter reduced the uptake. Sulphydryl reagents inhibited the receptor-mediated degradation. The data are not compatible with a quantitatively major role of lysosomes in the receptor-mediated insulin degradation. However, since the process requires energy it is suggested that the receptor-mediated degradation takes place in vesicles other than secondary lysosomes. The responsible enzyme(s) may belong to the thiol group of proteases. Both insulin and the insulin receptor are internalized as a consequence of incubation of HT-29 cells with insulin.

High-performance liquid chromatography of iodine-labeled insulin and glucagon derivatives with on-line gamma-detection

Insulin and glucagon were labeled with iodine. The reaction products were analyzed by high-performance liquid chromatography. It is shown that the pH of the reaction medium has a large effect on the position and the degree of iodine substitution as well as on the oxidation of the Met-containing glucagon and, furthermore, that the molar ratio of iodine to polypeptide hormone used during the labeling procedure affects not only the amount of iodine incorporated but also the distribution of iodinated products. The results show that certain iodinated derivatives are separated from each other and from the respective unlabeled polypeptide and thus can be obtained in a pure state.

Separation, isolation and characterization of the four monoiodinated insulin tracers using reversed-phase high-performance liquid chromatography

Baseline separation between insulin and insulin monoiodinated in Tyr A14, A19, B16 and B26 can be obtained using isocratic elution from a C18 column with triethylammonium trifluoroacetate-acetonitrile and the iodinated insulin derivatives can be isolated by lyophilization. Compared with similar tracers purified and isolated by disc electrophoresis/ion-exchange chromatography, the reversed-phase high-performance liquid chromatographically purified tracers are more homogeneous but show reduced binding affinity to adipocytes.