A shortened insulin with full in vitro potency

Prandial Insulins: A Person-Centered Choice

PURPOSE OF REVIEW

Postprandial hyperglycemia, or elevated blood glucose after meals, is associated with the development and progression of various diabetes-related complications. Prandial insulins are designed to replicate the natural insulin release after meals and are highly effective in managing post-meal glucose spikes. Currently, different types of prandial insulins are available such as human regular insulin, rapid-acting analogs, ultra-rapid-acting analogs, and inhaled insulins. Knowledge about diverse landscape of prandial insulin will optimize glycemic management.

RECENT FINDINGS

Human regular insulin, identical to insulin produced by the human pancreas, has a slower onset and extended duration, potentially leading to post-meal hyperglycemia and later hypoglycemia. In contrast, rapid-acting analogs, such as lispro, aspart, and glulisine, are new insulin types with amino acid modifications that enhance their subcutaneous absorption, resulting in a faster onset and shorter action duration. Ultra-rapid analogs, like faster aspart and ultra-rapid lispro, offer even shorter onset of action, providing better meal-time flexibility. The Technosphere insulin offers an inhaled route for prandial insulin delivery. The prandial insulins can be incorporated into basal-bolus, basal plus, or prandial-only regimens or delivered through insulin pumps. Human regular insulin, aspart, lispro, and faster aspart are recommended for management of hyperglycemia during pregnancy. Ongoing research is focused on refining prandial insulin replacement and exploring newer delivery methods. The article provides a comprehensive overview of various prandial insulin options and their clinical applications in the management of diabetes.

Single-chain insulin analogs threaded by the insulin receptor αCT domain

Insulin is a mainstay of therapy for diabetes mellitus, yet its thermal stability complicates global transportation and storage. Cold-chain transport, coupled with optimized formulation and materials, prevents to some degree nucleation of amyloid and hence inactivation of hormonal activity. These issues hence motivate the design of analogs with increased stability, with a promising approach being single-chain insulins (SCIs), whose C domains (foreshortened relative to proinsulin) resemble those of the single-chain growth factors (IGFs). We have previously demonstrated that optimized SCIs can exhibit native-like hormonal activity with enhanced thermal stability and marked resistance to fibrillation. Here, we describe the crystal structure of an ultrastable SCI (C-domain length 6; sequence EEGPRR) bound to modules of the insulin receptor (IR) ectodomain (N-terminal α-subunit domains L1-CR and C-terminal αCT peptide; "microreceptor" [μIR]). The structure of the SCI-μIR complex, stabilized by an Fv module, was determined using diffraction data to a resolution of 2.6 Å. Remarkably, the αCT peptide (IR-A isoform) "threads" through a gap between the flexible C domain and the insulin core. To explore such threading, we undertook molecular dynamics simulations to 1) compare threaded with unthreaded binding modes and 2) evaluate effects of C-domain length on these alternate modes. The simulations (employing both conventional and enhanced sampling simulations) provide evidence that very short linkers (C-domain length of -1) would limit gap opening in the SCI and so impair threading. We envisage that analogous threading occurs in the intact SCI-IR complex-rationalizing why minimal C-domain lengths block complete activity-and might be exploited to design novel receptor-isoform-specific analogs.

Facile synthesis of insulin fusion derivatives through sortase A ligation

Insulin derivatives such as insulin detemir and insulin degludec are U.S. Food and Drug Administration (FDA)-approved long-acting insulin currently used by millions of people with diabetes. These derivatives are modified in C-terminal B29 lysine to retain insulin bioactivity. New and efficient methods for facile synthesis of insulin derivatives may lead to new discovery of therapeutic insulin. Herein, we report a new method using sortase A (SrtA)-mediated ligation for the synthesis of insulin derivatives with high efficiency and functional group tolerance in the C-terminal B chain. This new insulin molecule (Ins-SA) with an SrtA-recognizing motif can be conjugated to diverse groups with N-terminal oligoglycines to generate new insulin derivatives. We further demonstrated that a new insulin derivative synthesized by this SrtA-mediated ligation shows strong cellular and in vivo bioactivity. This enzymatic method can therefore be used for future insulin design and development.

Contribution of TyrB26 to the Function and Stability of Insulin: STRUCTURE-ACTIVITY RELATIONSHIPS AT A CONSERVED HORMONE-RECEPTOR INTERFACE

Crystallographic studies of insulin bound to receptor domains have defined the primary hormone-receptor interface. We investigated the role of Tyr(B26), a conserved aromatic residue at this interface. To probe the evolutionary basis for such conservation, we constructed 18 variants at B26. Surprisingly, non-aromatic polar or charged side chains (such as Glu, Ser, or ornithine (Orn)) conferred high activity, whereas the weakest-binding analogs contained Val, Ile, and Leu substitutions. Modeling of variant complexes suggested that the B26 side chains pack within a shallow depression at the solvent-exposed periphery of the interface. This interface would disfavor large aliphatic side chains. The analogs with highest activity exhibited reduced thermodynamic stability and heightened susceptibility to fibrillation. Perturbed self-assembly was also demonstrated in studies of the charged variants (Orn and Glu); indeed, the Glu(B26) analog exhibited aberrant aggregation in either the presence or absence of zinc ions. Thus, although Tyr(B26) is part of insulin's receptor-binding surface, our results suggest that its conservation has been enjoined by the aromatic ring's contributions to native stability and self-assembly. We envisage that such classical structural relationships reflect the implicit threat of toxic misfolding (rather than hormonal function at the receptor level) as a general evolutionary determinant of extant protein sequences.

Human insulin analogues modified at the B26 site reveal a hormone conformation that is undetected in the receptor complex

The structural characterization of the insulin-insulin receptor (IR) interaction still lacks the conformation of the crucial B21-B30 insulin region, which must be different from that in its storage forms to ensure effective receptor binding. Here, it is shown that insulin analogues modified by natural amino acids at the TyrB26 site can represent an active form of this hormone. In particular, [AsnB26]-insulin and [GlyB26]-insulin attain a B26-turn-like conformation that differs from that in all known structures of the native hormone. It also matches the receptor interface, avoiding substantial steric clashes. This indicates that insulin may attain a B26-turn-like conformation upon IR binding. Moreover, there is an unexpected, but significant, binding specificity of the AsnB26 mutant for predominantly the metabolic B isoform of the receptor. As it is correlated with the B26 bend of the B-chain of the hormone, the structures of AsnB26 analogues may provide the first structural insight into the structural origins of differential insulin signalling through insulin receptor A and B isoforms.

Insulin analogs for the treatment of diabetes mellitus: therapeutic applications of protein engineering

The engineering of insulin analogs represents a triumph of structure-based protein design. A framework has been provided by structures of insulin hexamers. Containing a zinc-coordinated trimer of dimers, such structures represent a storage form of the active insulin monomer. Initial studies focused on destabilization of subunit interfaces. Because disassembly facilitates capillary absorption, such targeted destabilization enabled development of rapid-acting insulin analogs. Converse efforts were undertaken to stabilize the insulin hexamer and promote higher-order self-assembly within the subcutaneous depot toward the goal of enhanced basal glycemic control with reduced risk of hypoglycemia. Current products either operate through isoelectric precipitation (insulin glargine, the active component of Lantus(®); Sanofi-Aventis) or employ an albumin-binding acyl tether (insulin detemir, the active component of Levemir(®); Novo-Nordisk). To further improve pharmacokinetic properties, modified approaches are presently under investigation. Novel strategies have recently been proposed based on subcutaneous supramolecular assembly coupled to (a) large-scale allosteric reorganization of the insulin hexamer (the TR transition), (b) pH-dependent binding of zinc ions to engineered His-X(3)-His sites at hexamer surfaces, or (c) the long-range vision of glucose-responsive polymers for regulated hormone release. Such designs share with wild-type insulin and current insulin products a susceptibility to degradation above room temperature, and so their delivery, storage, and use require the infrastructure of an affluent society. Given the global dimensions of the therapeutic supply chain, we envisage that concurrent engineering of ultra-stable protein analog formulations would benefit underprivileged patients in the developing world.

Engineering of insulin receptor isoform-selective insulin analogues

Background

The insulin receptor (IR) exists in two isoforms, A and B, and the isoform expression pattern is tissue-specific. The C-terminus of the insulin B chain is important for receptor binding and has been shown to contact the IR just adjacent to the region where the A and B isoforms differ. The aim of this study was to investigate the importance of the C-terminus of the B chain in IR isoform binding in order to explore the possibility of engineering tissue-specific/liver-specific insulin analogues.

Methodology/Principal Findings

Insulin analogue libraries were constructed by total amino acid scanning mutagenesis. The relative binding affinities for the A and B isoform of the IR were determined by competition assays using scintillation proximity assay technology. Structural information was obtained by X-ray crystallography. Introduction of B25A or B25N mutations resulted in analogues with a 2-fold preference for the B compared to the A isoform, whereas the opposite was observed with a B25Y substitution. An acidic amino acid residue at position B27 caused an additional 2-fold selective increase in affinity for the receptor B isoform for analogues bearing a B25N mutation. Furthermore, the combination of B25H with either B27D or B27E also resulted in B isoform-preferential analogues (2-fold preference) even though the corresponding single mutation analogues displayed no differences in relative isoform binding affinity.

Conclusions/Significance

We have discovered a new class of IR isoform-selective insulin analogues with 2–4-fold differences in relative binding affinities for either the A or the B isoform of the IR compared to human insulin. Our results demonstrate that a mutation at position B25 alone or in combination with a mutation at position B27 in the insulin molecule confers IR isoform selectivity. Isoform-preferential analogues may provide new opportunities for developing insulin analogues with improved clinical benefits.

Adult neural stem/progenitor cells reduce NMDA-induced excitotoxicity via the novel neuroprotective peptide pentinin

Although the potential of adult neural stem cells to repair damage via cell replacement has been widely reported, the ability of endogenous stem cells to positively modulate damage is less well studied. We investigated whether medium conditioned by adult hippocampal stem/progenitor cells altered the extent of excitotoxic cell death in hippocampal slice cultures. Conditioned medium significantly reduced cell death following 24 h of exposure to 10 microM NMDA. Neuroprotection was greater in the dentate gyrus, a region neighboring the subgranular zone where stem/progenitor cells reside compared with pyramidal cells of the cornis ammonis. Using mass spectrometric analysis of the conditioned medium, we identified a pentameric peptide fragment that corresponded to residues 26-30 of the insulin B chain which we termed 'pentinin'. The peptide is a putative breakdown product of insulin, a constituent of the culture medium, and may be produced by insulin-degrading enzyme, an enzyme expressed by the stem/progenitor cells. In the presence of 100 pM of synthetic pentinin, the number of mature and immature neurons killed by NMDA-induced toxicity was significantly reduced in the dentate gyrus. These data suggest that progenitors in the subgranular zone may convert exogenous insulin into a peptide capable of protecting neighboring neurons from excitotoxic injury.

Enhancing the activity of a protein by stereospecific unfolding: conformational life cycle of insulin and its evolutionary origins

A central tenet of molecular biology holds that the function of a protein is mediated by its structure. An inactive ground-state conformation may nonetheless be enjoined by the interplay of competing biological constraints. A model is provided by insulin, well characterized at atomic resolution by x-ray crystallography. Here, we demonstrate that the activity of the hormone is enhanced by stereospecific unfolding of a conserved structural element. A bifunctional beta-strand mediates both self-assembly (within beta-cell storage vesicles) and receptor binding (in the bloodstream). This strand is anchored by an invariant side chain (Phe(B24)); its substitution by Ala leads to an unstable but native-like analog of low activity. Substitution by d-Ala is equally destabilizing, and yet the protein diastereomer exhibits enhanced activity with segmental unfolding of the beta-strand. Corresponding photoactivable derivatives (containing l- or d-para-azido-Phe) cross-link to the insulin receptor with higher d-specific efficiency. Aberrant exposure of hydrophobic surfaces in the analogs is associated with accelerated fibrillation, a form of aggregation-coupled misfolding associated with cellular toxicity. Conservation of Phe(B24), enforced by its dual role in native self-assembly and induced fit, thus highlights the implicit role of misfolding as an evolutionary constraint. Whereas classical crystal structures of insulin depict its storage form, signaling requires engagement of a detachable arm at an extended receptor interface. Because this active conformation resembles an amyloidogenic intermediate, we envisage that induced fit and self-assembly represent complementary molecular adaptations to potential proteotoxicity. The cryptic threat of misfolding poses a universal constraint in the evolution of polypeptide sequences.

Insulin structure and function

Throughout much of the last century insulin served a central role in the advancement of peptide chemistry, pharmacology, cell signaling and structural biology. These discoveries have provided a steadily improved quantity and quality of life for those afflicted with diabetes. The collective work serves as a foundation for the development of insulin analogs and mimetics capable of providing more tailored therapy. Advancements in patient care have been paced by breakthroughs in core technologies, such as semisynthesis, high performance chromatography, rDNA-biosynthesis and formulation sciences. How the structural and conformational dynamics of this endocrine hormone elicit its biological response remains a vigorous area of study. Numerous insulin analogs have served to coordinate structural biology and biochemical signaling to provide a first level understanding of insulin action. The introduction of broad chemical diversity to the study of insulin has been limited by the inefficiency in total chemical synthesis, and the inherent limitations in rDNA-biosynthesis and semisynthetic approaches. The goals of continued investigation remain the delivery of insulin therapy where glycemic control is more precise and hypoglycemic liability is minimized. Additional objectives for medicinal chemists are the identification of superagonists and insulins more suitable for non-injectable delivery. The historical advancements in the synthesis of insulin analogs by multiple methods is reviewed with the specific structural elements of critical importance being highlighted. The functional refinement of this hormone as directed to improved patient care with insulin analogs of more precise pharmacology is reported.

Insulin glulisine complementing basal insulins: a review of structure and activity

The advancement in protein engineering offers targeted development of insulin analogs that display either faster absorption kinetics or longer time-action profiles compared with human insulin and, therefore, more closely mimic endogenous insulin secretion. Insulin glulisine (3(B)Lys29(B) Glu-human insulin) is a new fast-acting analog that provides absorption and onset of action more rapidly with a shorter duration of action compared with regular human insulin, and thus better resembles physiologic mealtime insulin requirements. Insulin glulisine has been designed to exhibit intrinsic stability while maintaining rapid deployment of insulin monomers. Pharmacokinetic and pharmacodynamic profiling of insulin glulisine in healthy subjects and patients with type 1 and type 2 diabetes not only confirms the rapid absorption and fast action of insulin glulisine compared with human insulin, but also provides evidence that the unique drug formulation may offer additional benefits. Insulin glulisine complements insulin glargine (21(A)-Gly30(Ba)-L-Arg-30(Bb)-L-Arg-human insulin), the first long-acting basal insulin analog that displays a smoothed time-action profile with a 24-h duration of action. Together these analogs offer patients a more physiologic approach to insulin replacement.

How Insulin Binds: the B-Chain α-Helix Contacts the L1 β-Helix of the Insulin Receptor

Binding of insulin to the insulin receptor plays a central role in the hormonal control of metabolism. Here, we investigate possible contact sites between the receptor and the conserved non-polar surface of the B-chain. Evidence is presented that two contiguous sites in an alpha-helix, Val(B12) and Tyr(B16), contact the receptor. Chemical synthesis is exploited to obtain non-standard substitutions in an engineered monomer (DKP-insulin). Substitution of Tyr(B16) by an isosteric photo-activatable derivative (para-azido-phenylalanine) enables efficient cross-linking to the receptor. Such cross-linking is specific and maps to the L1 beta-helix of the alpha-subunit. Because substitution of Val(B12) by larger side-chains markedly impairs receptor binding, cross-linking studies at B12 were not undertaken. Structure-function relationships are instead probed by side-chains of similar or smaller volume: respective substitution of Val(B12) by alanine, threonine, and alpha-aminobutyric acid leads to activities of 1(+/-0.1)%, 13(+/-6)%, and 14(+/-5)% (relative to DKP-insulin) without disproportionate changes in negative cooperativity. NMR structures are essentially identical with native insulin. The absence of transmitted structural changes suggests that the low activities of B12 analogues reflect local perturbation of a "high-affinity" hormone-receptor contact. By contrast, because position B16 tolerates alanine substitution (relative activity 34(+/-10)%), the contribution of this neighboring interaction is smaller. Together, our results support a model in which the B-chain alpha-helix, functioning as an essential recognition element, docks against the L1 beta-helix of the insulin receptor.

A possible physiological function and the tertiary structure of a 4-kDa peptide in legumes

Previously, we isolated a 4-kDa peptide capable of binding to a 43-kDa receptor-like protein and stimulating protein kinase activity of the 43-kDa protein in soybean. Both of them were found to localize in the plasma membranes and cell walls. Here, we report the physiological effects of 4-kDa peptide expressed transiently in the cultured carrot and bird's-foot trefoil cells transfected with pBI 121 plasmid containing the 4-kDa peptide gene. At early developmental stage, the transgenic callus grew rapidly compared to the wild callus in both species. Cell proliferation of in vitro cultured nonembryogenic carrot callus was apparently affected with the 4-kDa peptide in the medium. Complementary DNAs encoding the 4-kDa peptide from mung bean and azuki bean were cloned by PCR and sequenced. The amino-acid sequences deduced from the nucleotide sequences are homologous among legume species, particularly, the sites of cysteine residues are highly conserved. This conserved sequence reflects the importance of intradisulfide bonds required for the 4-kDa peptide to perform its function. Three dimensional structure of the 4-kDa peptide determined by NMR spectroscopy suggests that this peptide is a T-knot scaffold containing three beta-strands, and the specific binding activity to the 43-kDa protein and stimulatory effect on the protein phosphorylation could be attributed to the spatial arrangements of hydrophobic residues at the solvent-exposed surface of two-stranded beta-sheet of 4-kDa peptide. The importance of these residues for the 4-kDa peptide to bind to the 43-kDa protein was indicated by site-directed mutagenesis. These results suggest that the 4-kDa peptide is a hormone-like peptide and the 43-kDa protein is involved in cellular signal transduction of the peptide.

Chiral mutagenesis of insulin's hidden receptor-binding surface: structure of an allo-isoleucine(A2) analogue

The hydrophobic core of vertebrate insulins contains an invariant isoleucine residue at position A2. Lack of variation may reflect this side-chain's dual contribution to structure and function: Ile(A2) is proposed both to stabilize the A1-A8 alpha-helix and to contribute to a "hidden" functional surface exposed on receptor binding. Substitution of Ile(A2) by alanine results in segmental unfolding of the A1-A8 alpha-helix, lower thermodynamic stability and impaired receptor binding. Such a spectrum of perturbations, although of biophysical interest, confounds interpretation of structure-activity relationships. To investigate the specific contribution of Ile(A2) to insulin's functional surface, we have employed non-standard mutagenesis: inversion of side-chain chirality in engineered monomer allo-Ile(A2)-DKP-insulin. Although the analogue retains native structure and stability, its affinity for the insulin receptor is impaired by 50-fold. Thus, whereas insulin's core readily accommodates allo-isoleucine at A2, its activity is exquisitely sensitive to chiral inversion. We propose that the Ile(A2) side-chain inserts within a chiral pocket of the receptor as part of insulin's hidden functional surface.

Non-standard insulin design: structure-activity relationships at the periphery of the insulin receptor

The design of insulin analogues has emphasized stabilization or destabilization of structural elements according to established principles of protein folding. To this end, solvent-exposed side-chains extrinsic to the receptor-binding surface provide convenient sites of modification. An example is provided by an unfavorable helical C-cap (Thr(A8)) whose substitution by favorable amino acids (His(A8) or Arg(A8)) has yielded analogues of improved stability. Remarkably, these analogues also exhibit enhanced activity, suggesting that activity may correlate with stability. Here, we test this hypothesis by substitution of diaminobutyric acid (Dab(A8)), like threonine an amino acid of low helical propensity. The crystal structure of Dab(A8)-insulin is similar to those of native insulin and the related analogue Lys(A8)-insulin. Although no more stable than native insulin, the non-standard analogue is twice as active. Stability and affinity can therefore be uncoupled. To investigate alternative mechanisms by which A8 substitutions enhance activity, multiple substitutions were introduced. Surprisingly, diverse aliphatic, aromatic and polar side-chains enhance receptor binding and biological activity. Because no relationship is observed between activity and helical propensity, we propose that local interactions between the A8 side-chain and an edge of the hormone-receptor interface modulate affinity. Dab(A8)-insulin illustrates the utility of non-standard amino acids in hypothesis-driven protein design.

Inactive conformation of an insulin despite its wild-type sequence

The peptide group between residues B24 and B25 of insulin was replaced by an ester bond. This modification only in the backbone was meant to eliminate a structurally important H-bond between the amide proton of B25 and the carbonyl oxygen of A19, and consequently to enhance detachment of the C-terminal B-chain from the body of the molecule, exposing the underlying A-chain. According to a model derived from the effects of side-chain substitutions, main-chain shortening, and crosslinking, this conformational change is prerequisite for receptor binding. Contrary to the expectation that increased flexibility would increase receptor binding and activity, depsi-insulin ([B24-B25 CO-O]insulin) has turned out be only 3-4% potent. In search of an explanation for this observation, the solution structure of depsi-insulin was determined by two-dimensional 1H-NMR spectroscopy. It was found that the loss of the B25-A19 H-bond does not entail detachment of the C-terminal B-chain. On the contrary, it is overcompensated by a gain in hydrophobic interaction achieved by insertion of the Phe B25 side chain into the molecule's core. This is possible because of increased rotational freedom in the backbone owing to the ester bond. Distortion of the B20-B23 turn and an altered direction of the distal B-chain are consequences that also affect self-association. The exceptional position of the B25 side chain is thus the key feature of the depsi-insulin structure. Being buried in the interior, it is not available for guiding the interaction with the receptor, a crucial role attributed to it by the model. This seems to be the main reason why the structure of depsi-insulin represents an inactive conformation.

The role of the C-terminus of the insulin B-chain in modulating structural and functional properties of the hormone

Within the scope of structure-function studies on the proteohormone insulin, the role of the C-terminal segment B26-B30 for self-association and receptor interaction was analyzed. Insulin derivatives with modifications in the region B26-B30 were synthesized by trypsin-catalyzed coupling reactions of des-(B23-B30)-insulin with synthetic peptides. The peptides were obtained by Fmoc solid-phase peptide synthesis. Insulins with multiple amino acid-->glycine substitutions were examined to distinguish between the influence of the side chains and the influence of the main chain in positions B27-B30 on the self-association of the hormone. The analogues [GlyB27,B28,B29,B30]insulin and [GlyB27,B28,B30]insulin exhibit relative receptor affinities of 80% and self-associate. The successive extension of [AlaB26]des-(B27-B30)-insulin-B26-amide (relative receptor binding 273%) with amino acids corresponding to the native sequence B27-B30 showed the influence of the length of the B-chain on receptor affinity: the extension by B27-threonine amide reduces receptor binding to 71%, all further prolongations have only small effects on the binding. The effect of the B28-side chain on main-chain conformation, self-association and receptor binding was examined with [XB28]des-(B29-B30)-insulin-B28-amides (X = Phe, Gly, D-Pro). While the glycine and D-proline analogues (relative binding 104 and 143%, respectively) retain the self-association properties typical of insulin, [PheB28]des-(B29-B30)-insulin-B28-amide (relative binding 50%) shows diminished self-association. The backbone-modified insulin derivative [SarB26]des-(B27-B30)-insulin-B26-amide (sarcosine = N-methylglycine) exhibits an unexpectedly high receptor affinity of 1100% which demonstrates that the B26-amide hydrogen of the native hormone is not important for receptor binding.

Structure-function relationships of des-(B26-B30)-insulin

In order to study the role of the amino acid in position B25 and its environment in shortened insulins, a series of analogues was prepared with the following modifications: 1, Stepwise shortening of the B-chain including replacements of TyrB26 and ThrB27 by glycine; 2, substitutions at the carboxamide nitrogen of des-(B26-B30)-insulin-B25-amide by apolar, polar or charged residues of various chain lengths; 3, replacement of PheB25 by asparagine-amide, phenylalaninol or a series of alkyl and aralkyl residues. Trypsin-catalyzed semisyntheses were performed with Boc-protected or unprotected des-octapeptide-(B23-B30)-insulin and synthetic peptides. Relative receptor binding and in vitro bioactivity of [AsnB25]-des-(B26-B30)-insulin-B25-amide was 227 and 292% (on insulin), other activities ranged between 1 and ca. 200%. We make the following conclusions. An L-amino acid is essential in position B25. The B25-carbonyl and NH groups favour high binding and "superpotency", but are not indispensible for receptor contacts. For high affinity receptor interaction, the planarity at the C gamma-atom and the distance of B25-side-chain branching in position B25 are important, but an aromatic ring is not necessary.

Determination of sulfated peptides by differential iodination

A sequential approach was developed to label tyrosine sulfate and peptides containing tyrosine sulfate selectively. Amino acids and peptides containing tyrosine and tyrosine sulfate were first iodinated using chloramine-T-method. Reaction products were determined by RP-HPLC. Mono- and biiodination of tyrosine and several model peptides was achieved within 120 s incubation time. Iodination of free tyrosine sulfate and sulfated cholecystokinin26-33 was less than 5%. After desulfation of the reaction products with 1 N HCl successful radioiodination of desulfated tyrosine was carried out whereas tyrosine did incorporate radioactive iodine only 10%. As shown by RP-HPLC specific labeling of tyrosine sulfate containing peptides with 125iodine was achieved.

Implications of replacing peptide bonds in the COOH-terminal B chain domain of insulin by the psi (CH2-NH) linker

To evaluate more thoroughly the importance of main-chain structure and flexibility in ligand interactions with the insulin receptor, we undertook to synthesize analogues with reduced peptide bonds in the COOH-terminal B chain domain of the hormone (a stable, but adjustable beta-strand region). By use of solid-phase, solution-phase and semisynthetic methods, analogues were prepared in which ArgB22 of des-octapeptide(B23-B30)-insulin was extended by the sequences Gly-Phe-psi (CH2-NH)-Phe-NH2, Gly-Gly-psi(CH2-NH)-Phe-Phe-NH2, Gly-Phe-psi (CH2-NH)-Phe-Phe-Thr-Pro-Ala-Thr-OH, and Gly-Phe-Phe-psi (CH2-NH)-Phe-Thr-Pro-Ala-Thr-OH, and were studied with respect to their abilities both to interact with the hepatocyte insulin receptor and to form soluble anion-stabilized hexamers in the presence of Co2+ and phenol. Additional analogues of des-pentapeptide(B26-B30)-insulin were also examined. Overall, our results show that, whereas all analogues retain considerable ability to form organized metal ion-coordinated complexes in solution, the reduction of peptide bonds both proximal and distal to the critical side chain of PheB25 results in analogues with severely diminished receptor binding potency. We conclude that the peptide carbonyls from both PheB24 and PheB25 are important for insulin-receptor interactions and that the structural organization of the region when insulin is bound to its receptor differs from that occurring during simple monomer-monomer and higher-order interactions of the hormone.

Contribution of the B16 and B26 tyrosine residues to the biological activity of insulin

We report the synthesis and biological evaluation of five insulin analogues in which one or both of the B-chain tyrosine residues have been substituted. [B16 Phe]insulin and [B16 Trp]insulin display a very modest reduction in potency (c. 65%) relative to porcine insulin; [B26 Phe]insulin is less active (30-50%), and the doubly substituted [B16 Phe, B26 Phe]insulin displays still lower potency (c. 35%). The further substitution of Asp for B10 His in [B16 Phe, B26 Phe]insulin raises its activity to approximately twofold greater than natural insulin, an increase of approximately fivefold over the parent compound. We conclude that the bulk and/or aromaticity of the amino acid residue at position B16, but not its hydrogen-bonding capacity, contributes to the biological activity of the hormone. We further conclude that hydrogen-bonding capacity or special side-chain packing characteristics are required at the B26 position for insulin to display high biological activity.

Acute metabolic actions of des-(B27-B30)-insulin and related analogues in adult rats

Metabolic potencies of the destetrapeptide insulin analogues des-(B27-B30)-insulin, des-(B27-B30)-insulin-B26-amide, [ThrB26] des-(B27-B30)-insulin-B26-amide and [GluB26] des-(B27-B30)-insulin-B26-amide were studied in anaesthetized adult rats and in primary cultures of rat hepatocytes and compared with that of the native hormone. Hypoglycaemic effects following intravenous bolus injection of insulin or analogues were similar, as were the stimulatory actions on total body glucose disposal during euglycaemic clamping. In these latter studies a maximal stimulation in the range 16-20 mg glucose/kg per hour was observed and identical half-maximally effective serum concentrations for all peptides of about 1 pmol/ml were obtained. Analogue actions on individual peripheral tissues estimated by the uptake of 2-deoxyglucose were not different from those of insulin. In hepatocyte cultures the stimulatory action of destetrapeptide analogues on glycogenesis and on aminoisobutyric acid transport was indistinguishable from that of native insulin, with identical half-maximally effective concentrations. These data demonstrate that des-(B27-B30)-insulin and related destetrapeptide analogues have high biological activity. Since the truncated non-amidated analogue appeared to be monomeric in solution, this peptide could be a candidate for an insulin preparation potentially showing rapid absorption from subcutaneous tissue.

Three-dimensional solution structure of an insulin dimer. A study of the B9(Asp) mutant of human insulin using nuclear magnetic resonance, distance geometry and restrained molecular dynamics

The solution structure of the B9(Asp) mutant of human insulin has been determined by two-dimensional 1H nuclear magnetic resonance spectroscopy. Thirty structures were calculated by distance geometry from 451 interproton distance restraints based on intra-residue, sequential and long-range nuclear Overhauser enhancement data, 17 restraints on phi torsional angles obtained from 3JH alpha HN coupling constants, and the restraints from 17 hydrogen bonds, and the three disulphide bridges. The distance geometry structures were optimized using restrained molecular dynamics (RMD) and energy minimization. The average root-mean-square deviation for the best 20 RMD refined structures is 2.26 A for the backbone and 3.14 A for all atoms if the less well-defined N and C-terminal residues are excluded. The helical regions are better defined, with root-mean-square deviation values of 1.11 A for the backbone and 2.03 A for all atoms. The data analysis and the calculations show that B9(Asp) insulin, in water solution at the applied pH (1.8 to 1.9), is a well-defined dimer with no detectable difference between the two monomers. The association of the two monomers in the solution dimer is relatively loose as compared with the crystal dimer. The overall secondary and tertiary structures of the monomers in the 2Zn crystal hexamer is found to be preserved. The conformation-averaged NMR structures obtained for the monomer is close to the structure of molecule 1 in the hexamer of the 2Zn insulin crystal. However, minor, but significant deviations from this structure, as well as from the structure of monomeric insulin in solution, exist and are ascribed to the absence of the hexamer and crystal packing forces, and to the presence of monomer-monomer interactions, respectively. Thus, the monomer in the solution dimer shows a conformation similar to that of the crystal monomer in molecular regions close to the monomer-monomer interface, whereas it assumes a conformation similar to that of the solution structure of monomeric insulin in other regions, suggesting that B9(Asp) insulin adopts a monomer-like conformation when this is not inconsistent with the monomer-monomer arrangement in the dimer.

Comparison of subcutaneously administered soluble insulin and des-(B26-B30)-insulin-B25-amide in rabbit, pig and healthy man

Subcutaneous injections of soluble human insulin and of the monomeric insulin analogue des-(B26-B30)-insulin-B25-amide were given to fasted rabbits, pigs and healthy man. The time course of blood glucose decline and the rate of appearance of the hormones in the peripheral circulation were studied for doses of 0.35 and 0.7 nmol/kg. In rabbits an identical time course with early glucose nadirs between 45 and 60 min and hormone peaks at about 20 min were observed for both hormones. Similar results were obtained for insulin and the analogue in pigs. When injected into humans, slightly earlier peaks of the analogue at 30 to 45 min were measured compared to insulin peaks at 60 min. In addition, a trend towards faster decline in blood glucose could be demonstrated after analogue injection. In contrast to monomeric insulin analogues produced by recombinant DNA technology, s.c. injections of des-(B26-B30)-insulin-B25-amide do not appear to meet mealtime insulin requirements better than soluble insulin.

Structure and evolution of insulins: implications for receptor binding

Insulin is a member of a family of hormones, growth factors and neuropeptides which are found in both vertebrates and invertebrates. A common 'insulin fold' is probably adopted by all family members. Although the specificities of receptor binding are different, there is a possibility of co-evolution of polypeptides and their receptors.

Structure and dynamics of des-pentapeptide-insulin in solution: the molten-globule hypothesis

Structures of insulin in different crystal forms exhibit significant local and nonlocal differences, including correlated displacement of elements of secondary structure. Here we describe the solution structure and dynamics of a monomeric insulin analogue, des-pentapeptide-(B26-B30)-insulin (DPI), as determined by two-dimensional NMR spectroscopy and distance geometry/restrained molecular dynamics (DG/RMD). Although the solution structure of DPI exhibits a general similarity to its crystal structure, individual DG/RMD structures in the NMR ensemble differ by rigid-body displacements of alpha-helices that span the range of different crystal forms. These results suggest that DPI exists as a partially folded state formed by coalescence of distinct alpha-helix-associated microdomains. The physical reality of this model is investigated by comparison of the observed two-dimensional nuclear Overhauser enhancement (NOE) spectroscopy (NOESY) spectrum with that predicted from crystal and DG/RMD structures. The observed NOESY spectrum contains fewer tertiary contacts than predicted by any single simulation, but it matches their shared features; such "ensemble correspondence" is likely to reflect the effect of protein dynamics on observed NOE intensities. We propose (i) that the folded state of DPI is analogous to that of a compact protein-folding intermediate rather than a conventional native state and (ii) that the molten state is the biologically active species. This proposal (the molten-globule hypothesis) leads to testable thermodynamic predictions and has general implications for protein design.

Mutations at the dimer, hexamer, and receptor-binding surfaces of insulin independently affect insulin-insulin and insulin-receptor interactions

Mutagenesis of the dimer- and hexamer-forming surfaces of insulin yields analogues with reduced tendencies to aggregate and dramatically altered pharmacokinetic properties. We recently showed that one such analogue, HisB10----Asp, ProB28----Lys, LysB29----Pro human insulin (DKP-insulin), has enhanced affinity for the insulin receptor and is useful for studying the structure of the insulin monomer under physiologic solvent conditions [Weiss, M. A., Hua, Q. X., Lynch, C. S., Frank, B. H., & Shoelson, S. E. (1991) Biochemistry 30, 7373-7389]. DKP-insulin retains native secondary and tertiary structure in solution and may therefore provide an appropriate baseline for further studies of related analogues containing additional substitutions within the receptor-binding surface of insulin. To test this, we prepared a family of DKP analogues having potency-altering substitutions at the B24 and B25 positions using a streamlined approach to enzymatic semisynthesis which negates the need for amino-group protection. For comparison, similar analogues of native human insulin were prepared by standard semisynthetic methods. The DKP analogues show a reduced tendency to self-associate, as indicated by 1H-NMR resonance line widths. In addition, CD spectra indicate that (with one exception) the native insulin fold is retained in each analogue; the exception, PheB24----Gly, induces similar perturbations in both native insulin and DKP-insulin backgrounds. Notably, analogous substitutions exhibit parallel trends in receptor-binding potency over a wide range of affinities: D-PheB24 greater than unsubstituted greater than GlyB24 greater than SerB24 greater than AlaB25 greater than LeuB25 greater than SerB25, whether the substitution was in a native human or DKP-insulin background. Such "template independence" reflects an absence of functional interactions between the B24 and B25 sites and additional substitutions in DKP-insulin and demonstrates that mutations in discrete surfaces of insulin have independent effects on protein structure and function. In particular, the respective receptor-recognition (PheB24, PheB25), hexamer-forming (HisB10), and dimer-forming (ProB28, LysB29) surfaces of insulin may be regarded as independent targets for protein design. DKP-insulin provides an appropriate biophysical model for defining structure-function relationships in a monomeric template.

The solution structure of a monomeric insulin. A two-dimensional 1H-NMR study of des-(B26-B30)-insulin in combination with distance geometry and restrained molecular dynamics

The solution conformation of des-(B26-B30)-insulin (DPI) has been investigated by 1H-NMR spectroscopy. A set of 250 approximate interproton distance restraints, derived from two-dimensional nuclear Overhauser enhancement spectra, were used as the basis of a structure determination using distance geometry (DG) and distance-bound driven dynamics (DDD). Sixteen DG structures were optimized using energy minimization (EM) and submitted to short 5-ps restrained molecular dynamics (RMD) simulations. A further refinement of the DDD structure with the lowest distance errors was done by energy minimization, a prolonged RMD simulation in vacuo and a time-averaged RMD simulation. An average structure was obtained from a trajectory generated during 20-ps RMD. The final structure was compared with the des-(B26-B30)-insulin crystal structure refined by molecular dynamics and the 2-Zn crystal structure of porcine insulin. This comparison shows that the overall structure of des-(B26-B30)-insulin is retained in solution with respect to the crystal structures with a high flexibility at the N-terminal part of the A chain and at the N-terminal and C-terminal parts of the B chain. In the RMD run a high mobility of Gly A1, Asn A21 and of the side chain of Phe B25 is noticed. One of the conformations adopted by des-(B26-B30)-insulin in solution is similar to that of molecule 1 (Chinese nomenclature) in the crystal structure of porcine insulin.

Disposition of the phenylalanine B25 side chain during insulin-receptor and insulin-insulin interactions

By the semisynthesis of both full-length insulin analogues and their des-pentapeptide-(B26-B30)-alpha-carboxamide counterparts, we have examined the importance of the electronic character and bulk of the position B25 side chain both in directing insulin interaction with its receptor on isolated canine hepatocytes and in determining the ability of insulin to self-associate in solution. Analogues include those in which PheB25 was replaced by cyclohexyl-Ala; Tyr; p-nitro-, p-fluoro-, p-iodo-, or p-amino-Phe; or p-amino-Phe in which the aromatic amino function had been acylated by the acetyl, hexanoyl, decanoyl, or 1-adamantanoyl group. Our findings identify that (a) the beta-aromatic side chain at position B25 is indeed critical for high-affinity ligand-receptor interactions, (b) neither electron withdrawal from nor electron donation to the beta-aromatic ring perturbs ligand-receptor interactions in major ways, (c) considerable latitude is allowed the placement of linear or polycyclic apolar mass at the para position in p-amino-PheB25-substituted analogues with respect both to receptor binding affinity and to biological activity in vivo, and (d) para apolar mass at position B25 is readily accommodated during the self-association of insulin monomers, as assessed by analytical tyrosine radioiodination and spectroscopic analysis of analogue complexes with Co2+ and Co3+. These findings are discussed in terms of a model for insulin-receptor interactions at the cell membrane in which the position B25 side chain defines the edge of intermolecular contact.

Two-dimensional NMR studies of Des-(B26-B30)-insulin: sequence-specific resonance assignments and effects of solvent composition

Des-pentapeptide-insulin (DPI), a monomeric analogue which lacks the C-terminal five residues of the B-chain, provides a tractable model for 2D-NMR studies of insulin under a variety of solvent conditions. In this paper we present the sequential assignment of DPI at pH 1.8 and 25 degrees C in 10% deuterated DMSO/90% H2O; the chemical shifts are in general similar to those recently described in the absence of an organic cosolvent [1], in 20% acetic acid [2] and (for intact insulin) in 35% acetonitrile [3]. Under each of these solvent conditions qualitative analysis of the 2D-NMR data indicates that the major elements of secondary structure observed in the crystal state (three alpha-helices and B-chain beta-turn) are retained in solution. However, there is disagreement in the literature regarding the stability of the insulin fold, as monitored by amide-proton exchange rates and long-range nuclear Overhauser enhancements [1-3]. In contrast to a previous study [1], we observe slowly exchanging amide resonances (in freshly prepared D2O solutions) and nonlocal NOEs under each of the solvent conditions described, implying the existence of a stably folded secondary structure and hydrophobic core. The slowly-exchanging resonances are assigned to the central alpha-helix of the B-chain, the ends of the adjoining beta-turn, and the two A-chain alpha-helices. Qualitative analysis of long-range NOEs indicates that the major features of the crystal state are retained under these solvent conditions.

Structure-function relationships of insulin receptor interactions in cultured mouse astrocytes

Insulin receptor binding and effects upon uridine incorporation into nucleic acid were studied and compared to 3 insulin analogues, DPI, DPI-amide and DOI, in primary cultures of mouse astrocytes. From half-maximal inhibition of [125I]monoiodoinsulin binding (0.14 nM), the relative binding affinities of DPI, DPI-amide and DOI were 20%, 80% and 4% as compared with the native insulin, respectively. IC50 values were 158.5 nM for insulin, 199.5 nM for DPI-amide, 794.3 nM for DPI and 3980 nM for DOI. The corresponding relative potencies in stimulating [14C]uridine incorporation into TCA-insoluble material were 9% for DPI, 100% for DPI-amide and 1.1% for DOI. These findings are commensurate with data for these analogues in other insulin-sensitive tissues. It is concluded that the astrocyte insulin receptor has similar structural requirements to receptor in tissues outside the CNS, at least in terms of the involvement of the C-terminus of the insulin B-chain.

Two-dimensional NMR studies on des-pentapeptide-insulin. Proton resonance assignments and secondary structure analysis

The shortened analogue of insulin, des-(B26-B30)-pentapeptide insulin, has been characterized by two-dimensional 1H NMR. The 1H resonance assignments and the secondary structure in water solution are discussed The results indicate that the secondary structure in solution is very similar to that reported for the crystalline state. A high flexibility of both A and B chains is observed. Of the two conformations seen in the 2-Zn insulin crystals and indicated as molecules 1 and 2 (Chinese nomenclature), the structure of the analogue is more similar to that of molecule 1.

In vivo metabolic activity of des-(B26-B30)-insulin-B25-amide and related analogues in the rat

Metabolic potency of des-(B26-B30)-insulin-B25-amide, [TyrB25]des- (B26-B30)-insulin-B25-amide and [HisB25]des-(B26-B30)-insulin-B25-amide was studied in anaesthetized rats. Compared to insulin, full potency for des-(B26-B30)-insulin-B25-amide and an enhanced potency for both substituted analogues has been described previously on rat adipocytes in vitro. Hypoglycaemic effects following i.v. injection of all of these analogues were almost identical to those of native insulin with a half-maximal effective dose of approximately 3 nmol.kg-1. Stimulation of glucose metabolism during euglycaemic hyperinsulin-/analogueaemic clamp studies was indistinguishable from that of the native hormone with a maximal stimulation of approximately 19 mg.kg-1.min-1 and half-maximal effective hormone concentrations of approximately 1 pmol.ml-1. Analogue action on individual peripheral tissues estimated by the uptake of 2-deoxyglucose as well as stimulation of lipogenesis in epididymal fat was not different to that of insulin. These data demonstrate that C-terminal amidation of des-(B26-B30)-insulin results in a shortened molecule with full in vivo metabolic potency. When substituting phenylalanine in position B25 by tyrosine or histidine, the insulin-identical potency is preserved.

Biological activity of des-(B26-B30)-insulinamide and related analogues in rat hepatocyte cultures

Short-term and long-term biological activities were studied in adult rat hepatocytes cultured in the presence of the insulin analogues des-(B26-B30)-insulinamide, [TyrB25]des-(B26-B30)-insulinamide and [HisB25]des-(B26-B30)-insulinamide. When compared to insulin, full potency of des-(B26-B30)-insulinamide has been reported in rat adipocytes and an enhanced potency has been reported for the other analogues. Steady state binding characteristics of the analogues to hepatocytes were indistinguishable from those of native insulin with half-maximal binding occurring at concentrations of about 0.8 nmol/l. Half-maximal effects for the stimulation of glycolysis and inhibition of basal and glucagon-activated glycogenolysis required identical concentrations for insulin and all 3 analogues. Induction of the key glycolytic enzymes glucokinase and pyruvate kinase as well as the inhibition of glucagon-dependent induction of phosphenolpyruvate carboxy-kinase also required identical concentrations of insulin and the 3 analogues. These data confirm that in cultured hepatocytes the C-terminal amidation of des-(B26-B30)-insulin results in a molecule with full in vitro potency. In contrast to data obtained in adipocytes, the des-(B26-B30)-insulin-amidated analogues with tyrosine or histidine substitutions at position B25 are equally as potent as native insulin in eliciting biological responses in rat hepatocyte culture.

A highly potent insulin: des-(B26-B30)-[AspB10,TyrB25-NH2]insulin(human)

An insulin analogue that embodies two distinct structural modifications, each of which independently increases insulin activity, has been synthesized and evaluated for biological activity. The analogue, des-(B26-B30)-[AspB10,TyrB25-NH2]insulin is the most potent insulin analogue yet described; it displays an 11- to 13-fold higher activity than natural insulin. The findings are discussed with regard to the receptor-binding domains of insulin.

Monomeric insulins obtained by protein engineering and their medical implications

The use of insulin as an injected therapeutic agent for the treatment of diabetes has been one of the outstanding successes of modern medicine. The therapy has, however, had its associated problems, not least because injection of insulin does not lead to normal diurnal concentrations of insulin in the blood. This is especially true at meal times when absorption from subcutaneous tissue is too slow to mimic the normal rapid increments of insulin in the blood. In the neutral solutions used for therapy, insulin is mostly assembled as zinc-containing hexamers and this self-association, which under normal physiological circumstances functions to facilitate proinsulin transport, conversion and intracellular storage, may limit the rate of absorption. We now report that it is possible, by single amino-acid substitutions, to make insulins which are essentially monomeric at pharmaceutical concentrations (0.6 mM) and which have largely preserved their biological activity. These monomeric insulins are absorbed two to three times faster after subcutaneous injection than the present rapid-acting insulins. They are therefore capable of giving diabetic patients a more physiological plasma insulin profile at the time of meal consumption.

An examination of the role of insulin dimerisation and negative cooperativity using the biological properties of the despentapeptide and deshexapeptide insulins

The C-terminus of the insulin B chain is essential for dimerisation and expression of negative cooperativity. In order to evaluate the possible physiological role of these phenomena, we have studied the properties in vivo and in vitro of despentapeptide insulin (B26-30 deleted), derived from beef insulin, and deshexapeptide insulin (B25-30 deleted), derived from pork insulin. These materials do not dimerise and have 15% and 0% retention of negative cooperativity respective. Lipogenesis potencies in rat adipocytes were: despentapeptide insulin 19.9 +/- 0.3%; deshexapeptide insulin 19.9 +/- 1.5%. Binding potencies in adipocytes were: despentapeptide insulin 22.6 +/- 7.8%; deshexapeptide insulin 13.2 +/- 3.3%. Metabolic clearance rates were reduced compared to insulin (insulin = 19.1 +/- 0.9; despentapeptide insulin = 9.7 +/- 0.8; deshexapeptide insulin = 6.4 +/- 0.6 ml . min-1 . kg-1 at plasma concentration 0.5 nmol/l). Hypoglycaemic potencies were reduced for both analogues (40% and 30%) when calculated on the basis of plasma concentration although both analogues and insulin were equally effective at lowering plasma glucose concentration in equimolar doses. Plasma half-disappearance time was prolonged (despentapeptide insulin = 7.3 +/- 0.5; deshexapeptide insulin = 9.1 +/- 0.2 min). Both analogues were full agonists and conformed to the general relationship between in vitro and in vivo properties seen with a wide range of modified insulins. They resemble other analogues with modifications which reduce receptor affinity without impairing dimerisation or negative cooperativity. The results do not support a physiological role for dimerisation or negative cooperativity.

Structure-function relationships of shortened [LeuB25]insulins, semisynthetic analogues of a mutant human insulin

Replacement of B25-phenylalanine by leucine in the insulin sequence causes marked inactivation. The effect of this sequence variation was studied here in des-(B26-30)-insulin. [LeuB25]des-(B26-30)-insulin and its B25-amide were prepared by trypsin-mediated semisynthesis from N-terminally protected des-(B23-30)-insulin and synthetic tripeptides. The relative lipogenic potency in isolated rat adipocytes was 8.0% for the truncated analogue with a free B25-carboxyl function, and 18.1% for the amidated analogue. Binding to cultured human IM-9 lymphocytes was 4% and 9%, respectively. Thus, both shortened insulins are markedly more active than [LeuB25]insulin. The PheB25----LeuB25 substitution in both the shortened and the full sequence has a moderate effect on the CD spectrum, indicating that the gross main chain conformation is largely retained in both molecules. Independent of the substitution an absolute increase of the circular dichroism is observed upon amidation of the B25-carboxyl group.

Self-association of des-(B26-B30)-insulin. The effect of Ca2+ and some other divalent cations

It has been confirmed by sedimentation equilibrium and sedimentation velocity experiments that des-(B26-B30)-insulin does not self-associate at neutral pH. Sedimentation equilibrium experiments at pH 7, 25 degrees C were conducted to investigate the effects of the structurally and physiologically important divalent cations Zn2+, Cd2+, Pb2+ and Ca2+ on the aggregation state of des-(B26-B30)-insulin (pig) in solution. It was found that all of these ions bring about association of this insulin analogue; Zn2+ and Cd2+ to a more marked degree than Pb2+ and Ca2+. The predominant species in solutions containing Zn2+ appear to be hexamers and hexameric aggregates, in those containing Cd2+, species up to and including tetramers, and in those containing Pb2+ and Ca2+, monomers and dimers of des-(B26-B30)-insulin appear to be the only species present. The possible significance of these findings, especially in relation to a role for Ca2+ in the action of insulin, is discussed.

Enzymatic semisynthesis of porcine despentapeptide (B26-30) insulin using unprotected desoctapeptide (B23-30) insulin as a substrate. Model studies

Unprotected porcine desoctapeptide(B23-30) insulin (DOPI) and the synthetic Gly-Phe-Phe were used as substrates for the trypsin-catalyzed synthesis of despentapeptide(B26-30) insulin (DPPI). The DPPI synthesis was accompanied by a moderate oligomerization and by the formation of a side produce which was identified as a DOPI derivative having an extra peptide bond between the Gly(A1) and Arg(B22) and which was named des(23-63) proinsulin (1). Despite side reactions, the conditions were found where the overall DPPI yields were comparable to those obtained via di-Boc DOPI, and these procedures were faster and simpler since the Boc protection and deprotection steps were omitted. The reaction progress was directly monitored by HPLC.