[en] Studies were carried out to identify mammalian tissues capable of specifically binding mammalian pancreatic polypeptide (PP). Bovine PP (bPP) radiolabeled with ¹²⁵I was purified by HPLC to yield [¹²⁵I]iodo-(Tyr-27) bPP. The label was injected into three pairs of fasted littermate dogs and allowed to circulate for 5 min. One of the dogs was a control which received an excess of unlabeled porcine PP to provide competition for receptor binding. Unbound bPP was removed by perfusion with Krebs-Ringer bicarbonate and the tissue fixed in situ with Karnovsky's fixative. Tissue samples from various organs were removed, weighed, and counted. The entire gastrointestinal tract demonstrated high levels of ¹²⁵I after injection of the labeled peptide. The duodenum, jejunum, ileum, and colon were the only tissues to exhibit specific binding of bPP. These tissues (mucosal and muscle layers) from experimental animals exhibited 31-76% higher binding than the corresponding tissues from the control animals. Sections of the gastrointestinal tract were scraped to separate the mucosal layer from the underlying muscle layer. The mucosal layer of the duodenum, jejunum, and ileum exhibited 145-162% increases in binding compared to the control animals. The muscle layer of these tissues demonstrated no significant increase. These findings demonstrate that mucosal layer of the small intestine is a target tissue for mammalian PP

[en] [Leu-B25]insulin is a low affinity insulin analog which does not increase the rate of dissociation of ¹²⁵I-insulin from insulin receptors (i.e. does not display negative cooperativity). We have studied the characteristics of binding of this analog to IM-9 cultured lymphocytes, in order to determine the contribution of negative cooperativity to the curvilinear nature of Scatchard plots typical of insulin binding data. The affinity of [LeuB25]insulin for receptors was approximately 1% that of insulin, as determined by its ability to inhibit ¹²⁵I-insulin binding. Monoiodinated preparations of insulin and of [LeuB25]insulin were produced, labeled in the tyrosine at position 14 of the A chain. These ¹²⁵I-TyrA14-labeled species were used in all studies. Both native insulin and a serum containing antiinsulin receptor antibodies were equally potent at inhibiting binding of ¹²⁵I-[LeuB25]insulin and ¹²⁵I-native insulin, suggesting that they bind to the same population of receptors. Native insulin (100 ng/ml) increased the rate of dissociation of both ¹²⁵I-insulin and ¹²⁵I-[LeuB25]insulin. However, [LeuB25]insulin (2.5 micrograms/ml) did not increase the rates of dissociation of either ¹²⁵I-insulin or ¹²⁵I-[LeuB25]insulin (i.e. it did not display negative cooperativity). Competition curves and Scatchard plots were constructed using ¹²⁵I-[LeuB25]insulin and unlabeled analog. Half-maximal inhibition of ¹²⁵I-[LeuB25]insulin binding was seen at a [LeuB25]insulin concentration of approximately 500 ng/ml. More importantly, the Scatchard plot of these binding data was markedly curvilinear, as is typical of insulin binding data

[en] Rats were injected with [¹²⁵I]iodoinsulin labeled at either the A14 or B26 tyrosine, and the animals were killed and livers subcellularly fractionated to yield light (early or neutral) endosomes and heavy (late or acidic) endosomes. ¹²⁵I-Labeled material was extracted from endosomes and analyzed by Sephadex G-50 filtration and high performance liquid chromatography (HPLC). Radiolabeled material in both types of endosomes is comprised of high molecular weight, insulin-sized, and low molecular weight components, with B chain-labeled small molecular weight material in two peaks, one corresponding to iodotyrosine and one to small peptides. As compared with A chain label, however, less of the B chain material appears in the degradation components (both high and low molecular weight fractions) suggesting that a fragment of B chain containing the B26 residue is lost from the endosomes. Analysis on HPLC shows that significant amounts of the insulin-sized and high molecular weight material have proteolytic cleavage(s) in the B chain with an intact A chain. The B chain-derived labeled peptides elute from HPLC identically with products generated by insulin protease. These results therefore show substantial insulin degradation occurring in light endosomes prior to endosomal acidification and to receptor dissociation, suggesting receptor-bound insulin in a substrate for insulin protease

[en] The kidney is a major site for insulin removal and degradation, but the subcellular processes and enzymes involved have not been established. We have examined this process by analyzing insulin degradation products by HPLC. Monoiodoinsulin specifically labeled on either the A14 or B26 tyrosine residue was incubated with a cultured kidney epithelial cell line, and both intracellular and extracellular products were examined on HPLC. The products were then compared with products of known structure generated by hepatocytes and the enzyme insulin protease. Intracellular and extracellular products were different, suggesting two different degradative pathways, as previously shown in liver. The extracellular degradation products eluted from HPLC both before and after sulfitolysis similarly with hepatocyte products and products generated by insulin protease. The intracellular products also eluted identically with hepatocyte products. Based on comparisons with identified products, the kidney cell generates two fragments from the A chain of intact insulin, one with a cleavage at A13-A14 and the other at A14-A15. The B chain of intact insulin is cleaved in a number of different sites, resulting in peptides that elute identically with B chain peptides cleaved at B9-B10, B13-B14, B16-B17, B24-B25, and B25-B26. These similarities with hepatocytes and insulin protease suggest that liver and kidney have similar mechanisms for insulin degradation and that insulin protease or a very similar enzyme is involved in both tissues

[en] Insulin degradation is an integral part of the cellular action of insulin. Recent evidence suggests that the enzyme insulin protease is involved in the degradation of insulin in mammalian tissues. Drosophila, which has insulin-like hormones and insulin receptor homologues, also expresses an insulin degrading enzyme with properties that are very similar to those of mammalian insulin protease. In the present study, the insulin cleavage products generated by the Drosophila insulin degrading enzyme were identified and compared with the products generated by the mammalian insulin protease. Both purified enzymes were incubated with porcine insulin specifically labeled with ¹²⁵I on either the A19 or B26 position, and the degradation products were analyzed by HPLC before and after sulfitolysis. Isolation and sequencing of the cleavage products indicated that both enzymes cleave the A chain of intact insulin at identical sites between residues A13 and A14 and A14 and A15. These results demonstrate that all the insulin cleavage sites generated by the Drosopohila insulin degrading enzyme are shared in common with the mammalian insulin protease. These data support the hypothesis that there is evolutionary conservation of the insulin degrading enzyme and further suggest that this enzyme plays an important role in cellular function

[en] Kidneys degrade small proteins such as cytochrome c (CYT c) by the classic lysosomal pathway. However, because alternate routes for the transport and degradation of protein hormones have been identified in other tissues, the authors set out to determine whether extralysosomal sites might participate in the renal degradation of insulin. First, they compared the effect of the lysosomal inhibitor NH₄Cl on insulin and CYT c degradation by isolated perfused rat kidneys. After kidneys were loaded with radiolabeled proteins to allow for absorption and transport to lysosomes, degradation was measured in the presence or absence of inhibitors. Next they followed the subcellular distribution of ¹²⁵I-labeled insulin in kidneys exposed to ¹²⁵I-labeled insulin in vivo or when isolated and perfused. Under both circumstances the distribution of insulin on a linear sucrose gradient differed from that of the lysosomal enzyme N-acetyl-β-glucosaminidase. In contrast, [¹⁴CH₃]CYT c, injected in vivo, distributed over a density similar to the lysosomal marker. Thus important differences exist between the renal metabolism of CYT c, which proceeds in lysosomes, and the renal metabolism of insulin. These include rate of degradation, sensitivity to NH₄Cl, and subcellular sites of localization. Accordingly, they suggest that insulin degradation may occur, at least in part, in a different compartment from the classic lysosomal site of protein degradation

[en] The proinsulin-insulin system provides a general model for the proteolytic processing of polypeptide hormones. Two proinsulin-specific endopeptidases have been defined, a type I activity that cleaves the B-chain/C-peptide junction (Arg³¹-Arg³²) and a type II activity that cleaves the C-peptide/A-chain junction (Lys⁶⁴-Arg⁶⁵). These endopeptidases are specific for their respective dibasic target sites; not all such dibasic sites are cleaved, however, and studies of mutant proinsulins have demonstrated that additional sequence or structural features are involved in determining substrate specificity. To define structural elements required for endopeptidase recognition, the authors have undertaken comparative ¹H NMR and photochemical dynamic nuclear polarization (photo-CIDNP) studies of human proinsulin, insulin, and split proinsulin analogues as models or prohormone processing intermediates. The overall conformation of proinsulin is observed to be similar to that of insulin, and the connecting peptide is largely unstructured. In the ¹H NMR spectrum of proinsulin significant variation is observed in the line widths of insulin-specific amide resonances, reflecting exchange among conformational substrates; similar exchange is observed in insulin and is not damped by the connecting peptide. The aromatic ¹H NMR resonances of proinsulin are assigned by analogy to the spectrum of insulin, and assignments are verified by chemical modification. These results suggest that a stable local structure is formed at the CA junction, which influences insulin-specific packing interactions. They propose that this structure (designated the CA knuckle) provides a recognition element for type II proinsulin endopeptidase

[en] Insulin provides an important model for the application of genetic engineering to rational protein design and has been well characterized in the crystal state. However, self-association of insulin in solution has precluded complementary 2D NMR study under physiological conditions. The authors demonstrate here that such limitations may be circumvented by the use of a monomeric analogue that contains three amino acid substitutions on the protein surface (HisB10 → Asp, ProB28 → Lys, and LysB29 → Pro); this analogue (designated DKP-insulin) retains native receptor-binding potency. Comparative ¹H NMR studies of native human insulin and a series of three related analogues-(i) the singly substituted analogue [HisB10→Asp], (ii) the doubly substituted analogue [ProB28→Lys; LysB29→Pro], and (iii) DKP-insulin-demonstrate progressive reduction in concentration-dependent line-broadening in accord with the results of analytical ultracentrifugation. Extensive nonlocal interactions are observed in the NOESY spectrum of DKP-insulin, indicating that this analogue adopts a compact and stably folded structure as a monomer in overall accord with crystal models. Site-specific ²H and ¹³C isotopic labels are introduced by semisynthesis as probes for the structure and dynamics of the receptor-binding surface. These studies confirm and extend under physiological conditions the results of a previous 2D NMR analysis of native insulin in 20% acetic acid. Implications for the role of protein flexibility in receptor recognition are discussed with application to the design of novel insulin analogues

[en] The aromatic ¹H NMR resonances of the insulin monomer are assigned at 500 MHz by comparative studies of chemically modified and genetically altered variants, including a mutant insulin (PheB25 → Leu) associated with diabetes mellitus. The two histidines, three phenylalanines, and four tyrosines are observed to be in distinct local environments; their assignment provides sensitive markers for studies of tertiary structure, protein dynamics, and protein folding. The environments of the tyrosine residues have also been investigated by photochemically induced dynamic nuclear polarization (photo-CIDNP) and analyzed in relation to packing constrains in the crystal structures of insulin. Dimerization involving specific B-chain interactions is observed with increasing protein concentration and is shown to depend on temperature, pH, and solvent composition. The differences between proinsulin and mini-proinsulin suggest a structural mechanism for the observation that the fully reduced B29-A1 analogue folds more efficiently than proinsulin to form the correct pattern of disulfide bonds. These results are discussed in relation to molecular mechanics calculations of insulin based on the available crystal structures