Insulin C peptide

Plasma C-peptide. Insulin secretion in insulin-treated diabetics cannot be assessed by the measurement of plasma insulin since the insulin given therapeutically will also be measured in the assay. However, insulin and its associated connecting-peptide (or C-peptide) are secreted by the islet cells in equimolar amounts (Fig. 2)... 

Inbibin (activin), beta A Inhibin (activin), beta B Inbibin (activin), beta C Inhibin (activin), beta E Inhibin, alpha Insulin C-peptide Insulin, A chain Insulin, B chain Insulin-like growth factor lA Insulin-like growth factor II Inter-alpha trypsin inhibitor, HI Inter-alpha trypsin inhibitor, H2 Inter-alpha trypsin inhibitor, H4 Inter-alpha trypsin inhibitor, L Interferon alpha Interferon beta Interferon gamma Interleukin-1 beta Interleukin-10 Interleukin-12, alpha Interleukin-12, beta Interleukin-1 receptor antagonist Interleukin-2 Interleukin-4... 

Figure 42-12. Structure of human proinsulin. Insulin and C-peptide molecules are connected at two sites by dipeptide links. An initial cleavage by a trypsin-like enzyme (open arrows) followed by several cleavages by a car-boxypeptidase-like enzyme (solid arrows) results in the production of the heterodimeric (AB) insulin molecule (light blue) and the C-peptide.

The human pancreas secretes about 40—50 units of insulin daily, which represents about 15—20% of the hormone stored in the B cells. Insidin and the C-peptide (see Figure 42—12) are normally secreted in equimolar amounts. Stimuh such as glucose, which provokes insidin secretion, therefore trigger the processing of proinsidin to insidin as an essential part of the secretory response. 

G Gwinup, AN Elias, ES Domurat. Insulin and C-peptide levels following oral administration of insulin in intestinal-enzyme protected capsules. Gen Pharmacol 22 143-246, 1991. 

Bendayan, M. (1989) Ultrastructural localization of insulin and C-peptide antigenic sites in rat pancreatic B cell obtained by applying the quantitative high-resolution protein A-gold approach. Am. J. Anat. 185, 205-216. 

Proinsulin is proteolytically processed in the coated secretory granules, yielding mature insulin and a 34-amino acid connecting peptide (C peptide, Figure 11.1). The C peptide is further proteolytically modified by removal of a dipeptide from each of its ends. The secretory granules thus contain low levels of proinsulin, C peptide and proteases, in addition to insulin itself. The insulin is stored in the form of a characteristic zinc-insulin hexamer, consisting of six molecules of insulin stabilized by two zinc atoms. 

Although a high degree of homology is evident between insulins from various species, the same is not true for proinsulins, as the C peptide sequence can vary considerably. This has therapeutic implications, as the presence of proinsulin in animal-derived insulin preparations can potentially elicit an immune response in humans. 

An alternative method (developed in the Eli Lilly research laboratories), entails inserting a nucleotide sequence coding for human proinsulin into recombinant E. coli. This is followed by purification of the expressed proinsulin and subsequent proteolytic excision of the C peptide in vitro. This approach has become more popular, largely due to the requirement for a single fermentation and subsequent purification scheme. Such preparations have been termed human insulin prb ... 

P cells of the pancreatic islets in combination with atoms of zinc, but when required to regulate blood glucose concentration, the prohormone is cleaved and functional insulin is released into the circulation along with the C-peptide. This example of post-translational processing is mediated by peptidases which are contained in the vesicles along with the proinsulin. The fusion of the secretory vesicles with the cell membrane and activation of the peptidase prior to exocytosis of the insulin are prompted by an influx of calcium ions into the P-cell in response to the appropriate stimulus. Similarly, catecholamines are synthesized and held within the cell by attachment to proteins called chromogranins. 

Structurally insulin is a small peptide, with a molecular mass of around 5500 and composed of two subunits, denoted a and (3 chains. Insulin is synthesized as a single peptide, Proinsulin and stored within the pancreatic p-cells. At the moment of secretion, pro-insulin is cleaved, releasing C-peptide and functional insulin in to the blood circulation (Figure 4.22). 

Figure 4.22 Insulin is produced from pro insulin by proteolysis. Active hormone and inactive C-peptide are co-secreted...

The primary structure of a protein is its amino acid sequence. During the biosynthesis of insulin in the pancreas, a continuous peptide chain with 84 residues is first synthesized—proinsu/in (see p.160). After folding of the molecule, the three disulfide bonds are first formed, and residues 31 to 63 are then proteolytically cleaved releasing the so-called C peptide. The molecule that is left over (1) now consists of two peptide chains, the A chain (21 residues, shown in yellow) and the B chain (30 residues, orange). One of the disulfide bonds is located inside the A chain, and the two others link the two chains together. 

Diagnosis of hypoglycaemia requires that blood glucose concentration be measured before treatment. If the cause is uncertain, samples can be taken to measure the concentrations of insulin, insulin antibodies, and pancreatic C-peptide before the administration of glucose. 

However additional peculiarities are found in some preparations. Glimepiride results in lower insulin and C-peptide levels with similar glucose control, probably due its greater extrapancreatic effect on insulin target tissues. Gliclazide has a lower secondary failure rate than glyburide or glipizide. 

Human proinsulin has been synthesized in homogeneous solution from 11 protected fragments using azide coupling.1151 The difficulties with insoluble intermediates were sufficiently overcome to allow the 86-residue peptide to be synthesized. The product was de-protected, converted into the 5-sulfonate, and then reduced and reoxidized to form the three disulfide bonds. The product was extensively purified and analyzed, and shown to be pure proinsulin. This product could then be converted into insulin by the use of endopeptidases I and II from pancreatic (3-cell granules, together with carboxypeptidase H, which removed the four basic residues 31, 32, 64, and 65, and split out C-peptide.1 6 ... 

Insulin is synthesized by the pancreatic P cells in the form of proinsulin (5.113), in which a connecting C-peptide consisting of amino acids 31-60 joins the A chain at A through an Arg-Lys, to connect with B ° through an Arg-Arg fragment. In this way... 

Insulin therefore consists of two peptide chains that are connected by two disulfide bonds, since the C-peptide is cleaved off. There are some species-specific differences in the amino acid sequence of the hormone. X-ray diffraction studies have shown that insulin occurs as a hexameric protein containing two Zn atoms. The dimers are first held by four hydrogen bonds and a hydrophobic bond along the sequence in the form of an antiparallel P sheet. The dimers then bind by interaction of the B -Ala, B -Leu, and B -Val residues. The core of the hexamer contains water. 

Beta (B) cell 75 Insulin, C-peptide, proinsulin, amylin... 

Several tests use glucagon to diagnose endocrine disorders. In patients with type 1 diabetes mellitus, a classic research test of pancreatic beta-cell secretory reserve uses 1 mg of glucagon administered as an intravenous bolus. Because insulin-treated patients develop circulating anti-insulin antibodies that interfere with radioimmunoassays of insulin, measurements of C-peptide are used to indicate beta-cell secretion. 

FIGURE 23-5 Insulin. Mature insulin is formed from its larger precursor preproinsulin by proteolytic processing. Removal of a 23 amino acid segment (the signal sequence) at the amino terminus of preproinsulin and formation of three disulfide bonds produces proinsulin. Further proteolytic cuts remove the C peptide from proinsulin to produce mature insulin, composed of A and B chains. The amino acid sequence of bovine insulin is shown in Figure 3-24. 

The 24-residue prepeptide, the 29-residue C-peptide and basic residues 31,32, 64, and 65 are cut from the peptide upon conversion to insulin as indicated by the small arrows. Some amino acid residues are identified using the one-letter codes. See Fig. 7-17 for details of insulin structure. 

Additional polypeptide sequences are necessary for proper peptide chain folding (e.g., C peptide of insulin). 

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