Insulin C-peptide and

EQppen AD, Cerini F, Vadas L, Stocklin R, Vu L, Offord RE, et al. Development of an isotope dilution assay for precise determination of insulin, C-peptide, and proinsulin levels in non-diabetic and type II diabetic individuals with comparison to immunoassay. 

C-peptide is cosecreted with insulin by the pancreatic P-cells as a by-product of the enzymatic cleavage of proinsulin to insulin. C-peptide and insulin are secreted into the portal circulation in equimolar concentrations. ... 

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. 

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). 

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. 

A 33-year-old nurse with unexplained hypoglycemia had only small increases in insulin and C-peptide, and glibenclamide was found in her serum (47). However, she denied using it and did not want psychiatric therapy. 

In 17 patients, in whom fasting blood samples were taken immediately before transplantation and at 1 and 3 months after transplantation for measurement of HbA, insulin, C-peptide, free fatty aids, lipids, urea, and creatinine, the incidence of diabetes mellitus was high (47%)... 

With this assay, basal insulin values of 5-15 pU/mL (30-90 pmol/L) are found in normal humans, with a peak rise to 60-90 U/mL (360-540 pmol/L) during meals. Similar assays for measuring all of the known hormones of the endocrine pancreas (including C-peptide and proinsulin) have been developed. 

Clinical Utility of Measuring Insulin, Proinsulin, C-Peptide, and Glucagon... 

Box 25-1 lists the clinical conditions in which hormones that regulate glucose, namely insulin, proinsulin, C-peptide, and glucagon, have been measured. Although there is interest in the possible clinical value of measurement of the concentrations of insulin and its precursors, the assays are useful primarily for research purposes. There is no role for routine testing for insulin, proinsulin, or C-peptide in patients with diabetes mellitus. It must be emphasized that the diagnostic criteria for diabetes mellitus do not include measurements of hormones, which remain predominantly research tools. 

High proinsulin concentrations are usually noted in patients with benign or malignant j3 ceil tumors of the pancreas. Most patients with fl-cell tumors have increased insuhn, C-peptide, and proinsulin concentrations, but occasionally only proinsulin is increased because the tumors have defective conversion of proinsulin to insulin. Despite its low biological activity, proinsuHn production may be adequate to produce hypoglycemia. In addition, a rare form of familial hyperproinsulinemia, produced by impaired conversion to insulin, has been described. Measurement of proinsulin can... 

The laboratory diagnosis of diabetes is made exclusively by the demonstration of hyperglycemia. Other assays, such as the OGTT, contribute to the classification and characterization. Although other tests (e.g., C-peptide and insulin analysis) have been proposed to assist in the diagnosis and classification of the disease, these do not at present have a role outside of research studies. ... 

Ward WK, Paquette TL, Frank BH, Porte D, Jr. A sensitive radioimmunoassay for human proinsuhn, with sequential use of antisera to C-peptide and insulin. Clin Chem 1986 32 728-33. 

Species variations in primary stmctnre are also important in medicine, as illustrated by the comparison of human, beef, and pork insulin. Insulin is one of the hormones that are highly conserved between species, with very few amino acid substitutions and none in the regions that affect activity. Insulin is a polypeptide hormone of 51 amino acids that is composed of two polypeptide chains (Fig. 6.13). It is synthesized as a single polypeptide chain, but is cleaved in three places before secretion to form the C peptide and the active insulin molecule containing the A and B chains. The folding of the A and B chains into the correct three-dimensional structure is promoted by the presence of one intrachain and two interchain disulfide bonds formed by cysteine residues. The invariant residues consist of the cysteine residues engaged in disulfide bonds and the residues that form the surface of the insulin molecule that binds to the insulin receptor. The amino acid substitutions in bovine and porcine insulin (shown in blue in Fig. 6.13.) are not in amino acids that affect its activity. Consequently, bovine and pork insulin were used for many years for the treatment of diabetes mellitus. However, even with only a few different amino acids, some patients developed an immune response to these insulins. 

Fig 26.10. Cleavage of proinsulin to insulin. Proinsulin is converted to insulin by proteolytic cleavage, which removes the C-peptide and a few additional amino acid residues. Cleavage occurs at the arrows. From Murray RK, et al. Harper s Biochemistry, 23rd Ed. Stanford, CT Appleton Lange, 1993 560. 

Gorczyca M., Augart C., and Budnik V. 1993. Insulin-like peptide and insulin-like receptor are localized at neuromuscular junctions in Drosophila. J. Neurosci. 13 3692-3704. 

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. 

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