Molecular and Cellular Effects of C-peptide—New Perspectives on an Old Peptide

New results present C-peptide as a biologically active peptide hormone in its own right. Although C-peptide is formed from proinsulin and cosecreted with insulin, it is a separate entity with biochemical and physiological characteristics that differ from those of insulin. There is direct evidence of stereospecific binding of C-peptide to a cell surface receptor, which is different from those for insulin and other related hormones. The C-peptide binding site is most likely a G–protein–coupled receptor. The association constant for C-peptide binding is approximately 3 × 10⁹M^-1. Saturation of the binding occurs already at a concentration of about 1 nM, which explains why C-peptide effects are not observed in healthy subjects. Binding of C-peptide results in activation of Ca²⁺ and MAPK-dependent pathways and stimulation of Na⁺,K⁺-ATPase and eNOS activities. The latter 2 enzymes are both deficient in several tissues in type 1 diabetes. There is some evidence that C-peptide, and insulin may interact synergistically on the insulin signaling pathway. Clinical evidence suggests that replacement of C-peptide, together with regular insulin therapy, may be beneficial in patients with type 1 diabetes and serve to retard or prevent the development of long-term complications.     

Type 1 Diabetic Neuropathy and C-peptide

The most common microvascular diabetic complication, diabetic peripheral polyneuropathy (DPN), affects type 1 diabetic patients more often and more severely. In recent decades, it has become increasingly clear that perpetuating pathogenetic mechanisms, molecular, functional, and structural changes and ultimately the clinical expression of DPN differ between the two major types of diabetes. Impaired insulin/C-peptide action has emerged as a crucial factor to account for the disproportionate burden affecting type 1 patients. C-peptide was long believed to be biologically inactive. However, it has now been shown to have a number of insulin-like glucoseindependent effects. Preclinical studies have demonstrated dose-dependent effects on Na⁺,K⁺-ATPase activity, endothelial nitric oxide synthase (eNOS), and endoneurial blood flow. Furthermore, it has regulatory effects on neurotrophic factors and molecules pivotal to the integrity of the nodal and paranodal apparatus and modulatory effects on apoptotic phenomena affecting the diabetic nervous system. In animal studies, C-peptide improves nerve conduction abnormalities, prevents nodal degenerative changes, characteristic of type 1 DPN, promotes nerve fiber regeneration, and prevents apoptosis of central and peripheral nerve cell constituents. Limited clinical trials have confirmed the beneficial effects of C-peptide on autonomic and somatic nerve function in patients with type 1 DPN. Therefore, evidence accumulates that replacement of C-peptide in type 1 diabetes prevents and even improves DPN. Large-scale food and drug administration (FDA)-approved clinical trials are necessary to make this natural substance available to the globally increasing type 1 diabetic population.     

C-peptide corrects endoneurial blood flow but not oxidative stress in type 1 BB/Wor rats

Oxidative stress and neurovascular dysfunction have emerged as contributing factors to the development of experimental diabetic neuropathy (EDN) in streptozotocin-diabetic rodents. Additionally, depletion of C-peptide has been implicated in the pathogenesis of EDN, but the mechanisms of these effects have not been fully characterized. The aims of this study were therefore to explore the effects of diabetes on neurovascular dysfunction and indexes of nerve oxidative stress in type 1 bio-breeding Worcester (BB/Wor) rats and type 2 BB Zucker-derived (ZDR)/Wor rats and to determine the effects of C-peptide replacement in the former. Motor and sensory nerve conduction velocities (NCVs), hindlimb thermal thresholds, endoneurial blood flow, and indicators of oxidative stress were evaluated in nondiabetic control rats, BB/Wor rats, BB/Wor rats with rat II C-peptide replacement (75 nmol C-peptide.kg body wt(-1).day(-1)) for 2 mo, and diabetes duration-matched BBZDR/Wor rats. Endoneurial perfusion was decreased and oxidative stress increased in type 1 BB/Wor rats. C-peptide prevented NCV and neurovascular deficits and attenuated thermal hyperalgesia. Inhibition of nitric oxide (NO) synthase, but not cyclooxygenase, reversed the C-peptide-mediated effects on NCV and nerve blood flow. Indexes of oxidative stress were unaffected by C-peptide. In type 2 BBZDR/Wor rats, neurovascular deficits and increased oxidative stress were unaccompanied by sensory NCV slowing or hyperalgesia. Therefore, nerve oxidative stress is increased and endoneurial perfusion decreased in type 1 BB/Wor and type 2 BBZDR/Wor rats. NO and neurovascular mechanisms, but not oxidative stress, appear to contribute to the effects of C-peptide in type 1 EDN. Sensory nerve deficits are not an inevitable consequence of increased oxidative stress and decreased nerve perfusion in a type 2 diabetic rodent model.     

C-Peptide Prevents Hippocampal Apoptosis in Type 1 Diabetes

To explore mechanisms underlying central nervous system (CNS) complications in diabetes, we examined hippocampal neuronal apoptosis and loss, and the effect of C-peptide replacement in type 1 diabetic BB/W rats. Apoptosis was demonstrated after 8 months of diabetes, by DNA fragmentation, increased number of apoptotic cells, and an elevated ratio of Bax/Bcl-x_L, accompanied by reduced neuronal density in the hippocampus. No apoptotic activity was detected and neuronal density was unchanged in 2-month diabetic hippocampus, whereas insulin-like growth factor (IGF) activities were impaired. In type 1 diabetic BB/W rats replaced with C-peptide, no TdT-mediated dUTP nick-end labeling (TUNEL)- positive cells were shown and DNA laddering was not evident in hippocampus at either 2 or 8 months. C-peptide administration prevented the preceding perturbation of IGF expression and reduced the elevated ratio of Bax/Bcl-x_L. Our data suggest that type 1 diabetes causes a duration-dependent programmed cell death of the hippocampus, which is partially prevented by C-peptide.     

C-peptide binding to human cell membranes: importance of Glu27

In addition to its established role in proinsulin folding, C-peptide has a function in regulation of cellular activity. The 31-residue peptide influences renal, vascular, and metabolic functions in patients with insulin-dependent diabetes mellitus. Binding to cells has been demonstrated for C-peptide, which can be displaced by its C-terminal pentapeptide. We have now used fluorescence correlation spectroscopy to investigate structural requirements on the pentapeptide part for C-peptide binding. All pentapeptide residues, E(27)GSLQ(31), were individually replaced with Ala and the capacity of the resulting peptides to displace rhodamine-labelled full-length human C-peptide from human renal tubular cell membranes was determined. This showed that Glu27 is essential for displacement, while replacement of Gly28 with Ala has little effect, and replacement of any of the three most C-terminal residues had intermediate effects. Morevover, free Glu displaces full-length C-peptide to about 50%, while free Ala, C-peptide(1-26), and the truncated pentapeptide, corresponding to the tetrapeptide G(28)SLG(31), have no displacing capacity. The peptides EVARQ (corresponding to the rat C-terminal pentapeptide) and ELGGGPGAG (corresponding to positions 11-19 of human C-peptide) do not displace human C-peptide. These results indicate that Glu27 of C-peptide is critically involved in binding to cellular targets.     

C-peptide improves adenosine-induced myocardial vasodilation in type 1 diabetes patients

Patients with type 1 (insulin-dependent) diabetes show reduced skeletal muscle blood flow and coronary vasodilatory function despite intensive insulin therapy and good metabolic control. Administration of proinsulin C-peptide increases skeletal muscle blood flow in these patients, but a possible influence of C-peptide on myocardial vasodilatory function in type 1 diabetes has not been investigated. Ten otherwise healthy young male type 1 diabetic patients (Hb A1c 6.6%, range 5.7-7.9%) were studied on two consecutive days during normoinsulinemia and euglycemia in a double-blind, randomized, crossover design, receiving intravenous infusion of C-peptide (5 pmol.kg-1.min-1) for 120 min on one day and saline infusion on the other day. Myocardial blood flow (MBF) was measured at rest and during adenosine administration (140 microg.kg-1.min-1) both before and during the C-peptide or saline infusions by use of positron emission tomography and [15O]H2O administration. Basal MBF was not significantly different in the patients compared with an age-matched control group, but adenosine-induced myocardial vasodilation was 30% lower (P < 0.05) in the patients. During C-peptide administration, adenosine-stimulated MBF increased on average 35% more than during saline infusion (P < 0.02) and reached values similar to those for the healthy controls. Moreover, as evaluated from transthoracal echocardiographic measurements, C-peptide infusion resulted in significant increases in both left ventricular ejection fraction (+5%, P < 0.05) and stroke volume (+7%, P < 0.05). It is concluded that short-term C-peptide infusion in physiological amounts increases the hyperemic MBF and left-ventricular function in type 1 diabetic patients.     

C-peptide,Na+,K+-ATPase,and Diabetes

Na⁺,K⁺-ATPase is an ubiquitous membrane enzyme that allows the extrusion of three sodium ions from the cell and two potassium ions from the extracellular fluid. Its activity is decreased in many tissues of streptozotocin-induced diabetic animals. This impairment could be at least partly responsible for the development of diabetic complications. Na⁺,K⁺-ATPase activity is decreased in the red blood cell membranes of type 1 diabetic individuals, irrespective of the degree of diabetic control. It is less impaired or even normal in those of type 2 diabetic patients. The authors have shown that in the red blood cells of type 2 diabetic patients, Na⁺,K⁺-ATPase activity was strongly related to blood C-peptide levels in non–insulin-treated patients (in whom C-peptide concentration reflects that of insulin) as well as in insulin-treated patients. Furthermore, a gene-environment relationship has been observed. The alpha-1 isoform of the enzyme predominant in red blood cells and nerve tissue is encoded by the ATP1A1 gene.Apolymorphism in the intron 1 of this gene is associated with lower enzyme activity in patients with C-peptide deficiency either with type 1 or type 2 diabetes, but not in normal individuals. There are several lines of evidence for a low C-peptide level being responsible for low Na⁺,K⁺-ATPase activity in the red blood cells. Short-term C-peptide infusion to type 1 diabetic patients restores normal Na⁺,K⁺-ATPase activity. Islet transplantation, which restores endogenous C-peptide secretion, enhances Na⁺,K⁺-ATPase activity proportionally to the rise in C-peptide. This C-peptide effect is not indirect. In fact, incubation of diabetic red blood cells with C-peptide at physiological concentration leads to an increase of Na⁺,K⁺-ATPase activity. In isolated proximal tubules of rats or in the medullary thick ascending limb of the kidney, C-peptide stimulates in a dose-dependent manner Na⁺,K⁺-ATPase activity. This impairment in Na⁺,K⁺-ATPase activity, mainly secondary to the lack of C-peptide, plays probably a role in the development of diabetic complications. Arguments have been developed showing that the diabetesinduced decrease in Na⁺,K⁺-ATPase activity compromises microvascular blood flow by two mechanisms: by affecting microvascular regulation and by decreasing red blood cell deformability, which leads to an increase in blood viscosity. C-peptide infusion restores red blood cell deformability and microvascular blood flow concomitantly with Na⁺,K⁺-ATPase activity. The defect in ATPase is strongly related to diabetic neuropathy. Patients with neuropathy have lower ATPase activity than those without. The diabetes-induced impairment in Na⁺,K⁺-ATPase activity is identical in red blood cells and neural tissue. Red blood cell ATPase activity is related to nerve conduction velocity in the peroneal and the tibial nerve of diabetic patients. C-peptide infusion to diabetic rats increases endoneural ATPase activity in rat. Because the defect in Na⁺,K⁺-ATPase activity is also probably involved in the development of diabetic nephropathy and cardiomyopathy, physiological C-peptide infusion could be beneficial for the prevention of diabetic complications.     

Honey and diabetes mellitus: Obstacles and challenges – Road to be repaired

Background and objectiveSince ancient times, honey has been used due to its nutritional and therapeutic value. The role of honey has been acknowledged in the scientific literature however, its use has been controversially discussed and has not been well accepted in modern medicine especially for diabetic patients. This study aimed to investigate the role of honey in diabetic patients.MethodsIn this study, we identified 107 research articles from data based search engines including “PubMed”, “ISI-Web of Science”, “Embase” and “Google Scholar”. The research papers were selected by using the primary key-terms including “Honey”, “Honey bee” and “Diabetes Mellitus”. The research documents in which “Honey” and “Diabetes Mellitus” were debated are included. After screening, we reviewed 66 papers and finally we selected 35 studies which met the inclusion criteria and the remaining documents were excluded.ResultsThis study investigated the preclinical, clinical, human and animal model studies on honey and diabetes mellitus and found that honey decreases the fasting serum glucose, increases the sting C-peptide and 2-h postprandial C-peptide. Although, there is a dearth of data and literature also contrary discussed the use of honey in diabetic patients.ConclusionHoney decreases the fasting serum glucose, increases fasting C-peptide and 2-h postprandial C-peptide. Honey had low glycemic index and peak incremental index in diabetic patients. The use of honey in diabetic patients still has obstacles and challenges and needs more large sample sized, multi-center clinical controlled studies to reach better conclusions.     

Role of C-peptide in human physiology

The C-peptide of proinsulin is important for the biosynthesis of insulin but has for a long time been considered to be biologically inert. Data now indicate that C-peptide in the nanomolar concentration range binds specifically to cell surfaces, probably to a G protein-coupled surface receptor, with subsequent activation of Ca(2+)-dependent intracellular signaling pathways. The association rate constant, K(ass), for C-peptide binding to endothelial cells, renal tubular cells, and fibroblasts is approximately 3. 10(9) M(-1). The binding is stereospecific, and no cross-reaction is seen with insulin, proinsulin, insulin growth factors I and II, or neuropeptide Y. C-peptide stimulates Na(+)-K(+)-ATPase and endothelial nitric oxide synthase activities. Data also indicate that C-peptide administration is accompanied by augmented blood flow in skeletal muscle and skin, diminished glomerular hyperfiltration, reduced urinary albumin excretion, and improved nerve function, all in patients with type 1 diabetes who lack C-peptide, but not in healthy subjects. The possibility exists that C-peptide replacement, together with insulin administration, may prevent the development or retard the progression of long-term complications in type 1 diabetes.     

Human C-peptide Dose Dependently Prevents Early Neuropathy in the BB/Wor-rat

In order to explore the neuroprotective and crossspecies activities of.C-peptide on type 1 diabetic neuropathy, spontaneously diabetic BB/W-rats were given increasing doses of human recombinant Cpeptide (hrC-peptide). Diabetic rats received 10, 100, 500, or 1000 μg of hrC-peptide/kg body weight/ day from onset of diabetes. After 2 months of hrC-peptide administration, 100 μg and greater doses completely prevented the nerve conduction defect, which was associated with a significant but incomplete prevention of neural Na⁺/K⁺-ATPase activity in diabetic rats with 500 μg or greater C-peptide replacement. Increasing doses of hrC-peptide showed increasing prevention of early structural abnormalities such as paranodal swelling and axonal degeneration and an increasing frequency of regenerating sural nerve fibers. We conclude that hrC-peptide exerts a dose dependent protection on type 1 diabetic neuropathy in rats and that this effect is probably mediated by the partially conserved sequence of the active C-terminal pentapeptide     

C-peptide and Central Nervous System Complications in Diabetes

Substantial evidence collected from clinical data and experimental studies has indicated that CNS is not spared from diabetes complications. Impairments in CNS function are well documented in both type 1 and type 2 diabetic patients as well as in various animal models of diabetes, in terms of alterations in cognition, neuropsychology, neurobehavior, electrophysiology, structure, neurochemistry and apoptotic activities. These data suggest that primary diabetic encephalopathy exists as a definable diabetic complication. The mechanisms underlying this CNS complication are not clear. Experimental studies have suggested that neuronal apoptosis may play an important role in neuronal loss and impaired cognitive function. In diabetes multiple factors are responsible for neuronal apoptosis, such as a perturbed IGF system, hyperglycemia and the aging process itself. Recent data suggest that insulin/C-peptide deficiency may exert an eminent role. Administration of C-peptide partially corrects the perturbed IGF system in the brain and prevents neuronal apoptosis in hippocampus of type 1 diabetes. In neuroblastoma SH-SY5Y cells C-peptide provides a dose-dependent stimulation on cell proliferation and an anti-apoptotic effect as well. These studies provide a basis for administration of C-peptide as a potentially effective therapy for type 1 diabetes.     

Effects of C-peptide on Microvascular Blood Flow and Blood Hemorheology

Beside functional and structural changes in vascular biology, alterations in the rheologic properties of blood cells mainly determines to an impaired microvascular blood flow in patients suffering from diabetes mellitus. Recent investigations provide increasing evidence that impaired C-peptide secretion in type 1 diabetic patients might contribute to the development of microvascular complications. C-peptide has been shown to stimulate endothelial NO secretion by activation of the Ca²⁺ calmodolin regulated enzyme eNOS. NO himself has the potency to increase cGMP levels in smooth muscle cells and to activate Na⁺ K⁺ ATPase activity and therefore evolves numerous effects in microvascular regulation. In type 1 diabetic patients, supplementation of C-peptide was shown to improve endothelium dependent vasodilatation in an NO-dependent pathway in different vascular compartments. In addition, it could be shown that C-peptide administration in type 1 diabetic patients, results in a redistribution of skin blood flow by increasing nutritive capillary blood flow in favour to subpapillary blood flow. Impaired Na⁺ K⁺ ATPase in another feature of diabetes mellitus in many cell types and is believed to be a pivotal regulator of various cell functions. C-peptide supplementation has been shown to restore Na⁺ K⁺ATPase activity in different cell types during in vitro and in vivo investigations. In type 1 diabetic patients, C-peptide supplementation was shown to increase erythrocyte Na⁺ K⁺ATPase activity by about 100%. There was found a linear relationship between plasma C-peptide levels and erythrocyte Na⁺ K⁺ATPase activity. In small capillaries, microvascular blood flow is increasingly determined by the rheologic properties of erythrocytes. Using laser-diffractoscopie a huge improvement in erythrocyte deformability could be observed after C-peptide administration in erythrocytes of type 1 diabetic patients. Inhibition of the Na⁺ K⁺ATPase by Obain completely abolished the effect of C-peptide on erythrocyte deformability. In conclusion, C-peptide improves microvascular function and blood flow in type 1 diabetic patients by interfering with vascular and rheological components of microvascular blood flow.     

Gastric inhibitory polypeptide: the neglected incretin revisited

After the ingestion of fat- and glucose-rich meals, gut hormones are secreted into the circulation in order to stimulate insulin secretion. This so-called "incretin effect" is primarily conferred by Glucagon-like peptide 1 (GLP-1) and Gastric Inhibitory Polypeptide (GIP). In contrast to GLP-1, GIP has lost most of its insulinotropic effect in type 2 diabetic patients. In addition to its main physiological role in the regulation of endocrine pancreatic secretion, GIP exerts various peripheral effects on adipose tissue and lipid metabolism, thereby leading to increased lipid deposition in the postprandial state. In some animal models, an influence on gastrointestinal functions has been described. However, such effects do not seem to play an important role in humans. During the last years, the major line of research has focussed on GLP-1, due to its promising potential for the treatment of type 2 diabetes mellitus. However, the physiological importance of GIP in the regulation of insulin secretion has been shown to even exceed that of GLP-1. Furthermore, work from various groups has provided evidence that GIP contributes to the pathogenesis of type 2 diabetes to a considerable degree. Recent data with modified GIP analogues further suggested a possibility of therapeutic use in the treatment of type 2 diabetes. Thus, it seems worthwhile to refocus on this important and-sometimes-neglected incretin hormone. The present work aims to review the physiological functions of GIP, to characterize its role in the pathogenesis of type 2 diabetes, and to discuss possible clinical applications and future perspectives in the light of new findings.     

C-peptide as a Therapy for Kidney Disease: A Systematic Review and Meta-Analysis

C-peptide has intrinsic biological activity and may be renoprotective. We conducted a systematic review to determine whether C-peptide had a beneficial effect on renal outcomes. MEDLINE, EMBASE, and the Cochrane Central Databases were searched for human and animal studies in which C-peptide was administered and renal endpoints were subsequently measured. We identified 4 human trials involving 74 patients as well as 18 animal studies involving 35 separate experiments with a total of 641 animals. In humans, the renal effects of exogenously delivered C-peptide were only studied in type 1 diabetics with either normal renal function or incipient nephropathy. Pooled analysis showed no difference in GFR (mean difference, -1.36 mL/min/1.73 m2, p = 0.72) in patients receiving C-peptide compared to a control group, but two studies reported a reduction in glomerular hyperfiltration (p<0.05). Reduction in albuminuria was also reported in the C-peptide group (p<0.05). In diabetic rodent models, C-peptide led to a reduction in GFR (mean difference, -0.62 mL/min, p<0.00001) reflecting a partial reduction in glomerular hyperfiltration. C-peptide also reduced proteinuria (mean difference, -186.25 mg/day, p = 0.05), glomerular volume (p<0.00001), and mesangial matrix area (p<0.00001) in diabetic animals without affecting blood pressure or plasma glucose. Most studies were relatively short-term in duration, ranging from 1 hour to 3 months. Human studies of sufficient sample size and duration are needed to determine if the beneficial effects of C-peptide seen in animal models translate into improved long-term clinical outcomes for patients with chronic kidney disease. (PROSPERO CRD42014007472)     

Lower 24-hour growth hormone levels in type I diabetics responding to thyrotropin-releasing hormone (TRH).

The aim of the study was the evaluation of growth hormone secretion under physiologic conditions in two groups of type I diabetics: responding and nonresponding to TRH stimulation. Both groups matched for age and metabolic control of diabetes were studied during 24-hours and after GHRH stimulation. The whole diabetic group (n = 18) showed circadian rhythm of GH secretion with mesor value of 4.03 micrograms/l. TRH-responders had lower mesor GH value than TRH-nonresponders: 3.53 vs. 5.32, p < 0.05. GH response to GHRH was almost identical in both groups. C-peptide level was lower in TRH-responders: 0.16 vs. 0.56 microgram/l, p < 0.05. No correlation was found between growth hormone response and HbA1 and C-peptide levels. It is concluded that type I diabetics responding to TRH stimulation are characterized by lower mean 24-hour GH levels and lower C-peptide values.     

Metabolic factors affecting residual beta cell function assessed by C-peptide secretion in patients with newly diagnosed type 1 diabetes.

BACKGROUND: In recent onset of type 1 diabetes, the residual beta cell function, assessed by baseline and/or stimulated C-peptide secretion, can be a useful parameter to establish the extension of beta cell destruction. How metabolic parameters at diagnosis influence residual C-peptide secretion is not well established. PATIENTS AND METHODS: We analyzed 553 consecutive patients with recent onset (<4 weeks) of type 1 diabetes (250 females and 303 males, mean age 15+/-8 years). Baseline and stimulated C-peptide by i.v. glucagon were evaluated using a highly sensitive radio-immunoassay. Metabolic parameters including blood glucose, HbA1c, insulin dose, and BMI were also evaluated. RESULTS: Baseline and stimulated C-peptide were 0.26+/-0.22 and 0.47+/-0.38 nmol/l and correlated positively with age (p<0.001). There was no significant correlation between C-peptide and blood glucose at diagnosis. BMI was positively correlated with both baseline and stimulated C-peptide secretion (p<0.001). By contrast, HbA1c levels inversely correlated with both baseline and stimulated C-peptide secretion (p<0.001). CONCLUSION: In type 1 diabetes at diagnosis, baseline and stimulated C-peptide are higher in pubertal and young adult patients compared with pre-pubertal patients suggesting that such parameter can be used as an end point marker for studies aimed at protecting and/or restoring beta cells in patients with substantial beta cell function. High levels of HbA1c and lower BMI are dependent variables of C-peptide values.     

The Predictive Ability of C-Peptide in Distinguishing Type 1 Diabetes From Type 2 Diabetes: A Systematic Review and Meta-Analysis

ObjectiveThis systematic review and meta-analysis aimed to investigate the predictive ability of plasma connecting peptide (C-peptide) levels in discriminating type 1 diabetes (T1D) from type 2 diabetes (T2D) and to inform evidence-based guidelines in diabetes classification.MethodsWe conducted a holistic review and meta-analysis using PubMed, MEDLINE, EMBASE, and Scopus. The citations were screened from 1942 to 2021. The quality criteria and the preferred reporting items for systematic reviews and meta-analysis checklist were applied. The protocol was registered with the International Prospective Register of Systematic Reviews (PROSPERO) (CRD42022355088).ResultsA total of 23,658 abstracts were screened and 46 full texts reviewed. Of the 46 articles screened, 12 articles were included for the meta-analysis. Included studies varied by race, age, time, and proportion of individuals. The main outcome measure in all studies was C-peptide levels. A significant association was reported between C-peptide levels and the classification and diagnosis of diabetes. Furthermore, lower concentrations and the cutoff of <0.20 nmol/L for fasting or random plasma C-peptide was indicative of T1D. In addition, this meta-analysis revealed the predictive ability of C-peptide levels in discriminating T1D from T2D. Results were consistent using both fixed- and random-effect models. The I² value (98.8%) affirmed the variability in effect estimates was due to heterogeneity rather than sampling error among all selected studies.ConclusionPlasma C-peptide levels are highly associated and predictive of the accurate classification and diagnosis of diabetes types. A plasma C-peptide cutoff of ≤0.20 mmol/L is indicative of T1D and of ≥0.30 mmol/L in the fasting or random state is indicative of T2D.     

EDTA improves stability of whole blood C-Peptide and insulin to over 24 hours at room temperature

Introduction

C-peptide and insulin measurements in blood provide useful information regarding endogenous insulin secretion. Conflicting evidence on sample stability and handling procedures continue to limit the widespread clinical use of these tests. We assessed the factors that altered the stability of insulin and C-peptide in blood.

Methods

We investigated the impact of preservative type, time to centrifugation, storage conditions and duration of storage on the stability of C-peptide and insulin on three different analytical platforms.

Results

C-peptide was stable for at least 24 hours at room temperature in both centrifuged and whole blood collected in K⁺-EDTA and serum gel tubes, with the exception of whole blood serum gel, which decreased to 78% of baseline at 24 hours, (p = 0.008). Insulin was stable at room temperature for 24 hours in both centrifuged and whole blood collected in K⁺-EDTA tubes. In contrast insulin levels decreased in serum gel tubes both centrifuged and whole blood (66% of baseline, p = 0.01 and 76% of baseline p = 0.01, by 24 hours respectively). C-peptide and insulin remained stable after 6 freeze-thaw cycles.

Conclusions

The stability of C-peptide and insulin in whole blood K⁺-EDTA tubes negates the need to conform to strict sample handling procedures for these assays, greatly increasing their clinical utility.