Carriers of the TCF7L2 rs7903146 TT genotype have elevated levels of plasma glucose, serum proinsulin and plasma gastric inhibitory polypeptide (GIP) during a meal test.

AIMS/HYPOTHESIS: The transcription factor 7-like 2 (TCF7L2) rs7903146 T allele associates with type 2 diabetes in several populations, possibly mediated via decreased incretin secretion and/or action and altered beta and alpha cell function. We aimed to study circulating levels of glucose, proinsulin, insulin, C-peptide, glucagon, glucagon-like peptide-1 (GLP-1), glucagon-like peptide-2 (GLP-2) and gastric inhibitory polypeptide (GIP) among individuals carrying the high-risk rs7903146 TT genotype and low-risk CC genotype following a meal test. METHODS: A meal challenge was performed in 31 glucose-tolerant men (age 54 ± 7 years and BMI 26 ± 3 kg/m²) with rs7903146 TT genotype and 31 glucose-tolerant age- and BMI-matched men with CC genotype (age 53 ± 6 years and BMI 26 ± 3 kg/m²). Serum proinsulin, insulin, C-peptide and plasma glucose, glucagon, GLP-1, GLP-2 and GIP were obtained 0, 15, 30, 45, 60, 75, 90, 105, 120, 135, 150, 180, 210, and 240 min after ingestion of a standardised breakfast meal. RESULTS: An elevated incremental AUC for plasma glucose was observed among TT genotype carriers (CC carriers 21.8 ± 101.9 mmol/l × min vs TT carriers 97.9 ± 89.2 mmol/l × min, p = 0.001). TT carriers also had increased AUCs for proinsulin (CC carriers 6,030 ± 3,001 pmol/l × min vs TT carriers 6,917 ± 4,820 pmol/l × min, p = 0.03), C-peptide (CC carriers 397.6 ± 131.9 nmol/l × min vs TT carriers 417.1 ± 109.3 nmol/l × min, p = 0.04) and GIP (CC carriers 12,310 ± 3,840 pmol/l × min vs TT carriers 14,590 ± 5,910 pmol/l × min, p = 0.004). CONCLUSIONS/INTERPRETATION: Middle-aged normoglycaemic individuals carrying the rs7903146 TCF7L2 risk TT genotype show early signs of dysregulated glucose metabolism, decreased processing of proinsulin and elevated GIP secretion following a meal challenge.

Validation of methods for measurement of insulin secretion in humans in vivo.

To detect and understand the changes in beta-cell function in the pathogenesis of type 2 diabetes, an accurate and precise estimation of prehepatic insulin secretion rate (ISR) is essential. There are two common methods to assess ISR, the deconvolution method (by Eaton and Polonsky)-considered the "gold standard"-and the combined model (by Vølund et al.). The deconvolution method is a 2-day method, which generally requires separate assessment of C-peptide kinetics, whereas the combined model is a single-day method that uses insulin and C-peptide data from a single test of interest. The validity of these mathematical techniques for quantification of insulin secretion have been tested in dogs, but not in humans. In the present studies, we examined the validity of both methods to recover the known infusion rates of insulin and C-peptide mimicking ISR during an oral glucose tolerance test. ISR from both the combined model and the deconvolution method were accurate, i.e., recovery of true ISR was not significantly different from 100%. Furthermore, both maximal and total ISRs from the combined model were strongly correlated to those obtained by the deconvolution method (r = 0.89 and r = 0.82, respectively). These results indicate that both approaches provide accurate assessment of prehepatic ISRs in type 2 diabetic patients and control subjects. A simplified version of the deconvolution method based on standard kinetic parameters for C-peptide (Van Cauter et al.) was compared with the 2-day deconvolution method, and a close agreement was found for the results of an oral glucose tolerance test. We also studied whether C-peptide kinetics are influenced by somatostatin infusion. The decay curves after bolus injection of exogenous biosynthetic human C-peptide, the kinetic parameters, and the metabolic clearance rate were similar whether measured during constant peripheral somatostatin infusion or without somatostatin infusion. Assessment of C-peptide kinetics can be performed without infusion of somatostatin, because the endogenous insulin concentration remains constant. Assessment of C-peptide kinetics with and without infusion of somatostatin results in nearly identical secretion rates for insulin during an oral glucose tolerance test.

Monokine antagonism is reduced in patients with IDDM.

Interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF-alpha) have been implicated as immune effector molecules in the pathogenesis of insulin-dependent diabetes mellitus (IDDM). Recently, an increased frequency of the A1/A1 genotype of an IL-1 receptor antagonist (IL-1Ra) gene polymorphism was observed in patients with IDDM. Therefore, we investigated plasma IL-1Ra and soluble TNF p55 receptor (TNFsRp55) levels in 18 men with recent-onset IDDM, 10 men with long-standing IDDM, and 35 age-matched healthy men. No differences in plasma IL-1Ra were found among the three groups. However, when the plasma IL-1Ra levels in the subjects with IDDM and the control subjects were analyzed according to IL-1Ra genotypes, we found a 30% lower level of plasma IL-1Ra in subjects with IDDM carrying the A1/A1 genotype compared with the levels in those carrying the A1/A2 genotype (372 +/- 40 vs. 530 +/- 54 ng/l, respectively, P = 0.025). In contrast, no significant association was seen between plasma IL-1Ra and IL-1Ra genotype in the control subjects. The TNFsRp55 level in heparinized plasma was 17% lower in patients with IDDM than in control subjects (3.93 +/- 0.22 vs. 4.72 +/- 0.24 micrograms/l, respectively, P = 0.038). These findings could not be explained by metabolic derangement in the IDDM patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Decrease in beta-cell mass leads to impaired pulsatile insulin secretion, reduced postprandial hepatic insulin clearance, and relative hyperglucagonemia in the minipig.

Most insulin is secreted in discrete pulses at an interval of approximately 6 min. Increased insulin secretion after meal ingestion is achieved through the mechanism of amplification of the burst mass. Conversely, in type 2 diabetes, insulin secretion is impaired as a consequence of decreased insulin pulse mass. beta-cell mass is reported to be deficient in type 2 diabetes. We tested the hypothesis that decreased beta-cell mass leads to decreased insulin pulse mass. Insulin secretion was examined before and after an approximately 60% decrease in beta-cell mass achieved by a single injection of alloxan in a porcine model. Alloxan injection resulted in stable diabetes (fasting plasma glucose 7.4 +/- 1.1 vs. 4.4 +/- 0.1 mmol/l; P < 0.01) with impaired insulin secretion in the fasting and fed states and during a hyperglycemic clamp (decreased by 54, 80, and 90%, respectively). Deconvolution analysis revealed a selective decrease in insulin pulse mass (by 54, 60, and 90%) with no change in pulse frequency. Rhythm analysis revealed no change in the periodicity of regular oscillations after alloxan administration in the fasting state but was unable to detect stable rhythms reliably after enteric or intravenous glucose stimulation. After alloxan administration, insulin secretion and insulin pulse mass (but not insulin pulse interval) decreased in relation to beta-cell mass. However, the decreased pulse mass (and pulse amplitude delivered to the liver) was associated with a decrease in hepatic insulin clearance, which partially offset the decreased insulin secretion. Despite hyperglycemia, postprandial glucagon concentrations were increased after alloxan administration (103.4 +/- 6.3 vs. 92.2 +/- 2.5 pg/ml; P < 0.01). We conclude that an alloxan-induced selective decrease in beta-cell mass leads to deficient insulin secretion by attenuating insulin pulse mass, and that the latter is associated with decreased hepatic insulin clearance and relative hyperglucagonemia, thereby emulating the pattern of islet dysfunction observed in type 2 diabetes.

Proinsulin immunoreactivity in recent-onset IDDM: the significance of insulin antibodies and insulin autoantibodies.

OBJECTIVE: To study the natural history of fasting proinsulin immunoreactivity (PIM) during the first 30 months of IDDM and its relationship to fasting C-peptide and insulin antibodies. RESEARCH DESIGN AND METHODS: An incidence cohort of 204 consecutive newly diagnosed IDDM patients were followed prospectively, having blood drawn for measurements at diagnosis and at 1, 3, 6, 9, 12, 18, 24, and 30 months. A sensitive enzyme-linked immunosorbent assay was used for the determination of PIM. RESULTS: All patients had detectable fasting PIM in plasma at diagnosis, with a median value and interquartile range of 3.5 pmol/l (2.2-6.2). The median PIM level increased during the first months of IDDM to reach a peak at 9-12 months (9.9-10.3 pmol/l). PIM then declined gradually to 5.6 pmol/l (1.9-13.5) at 30 months without reaching baseline. PIM at each time point was widely scattered in a skewed log-normal distribution without signs of bimodality. After the onset of insulin treatment, median insulin antibody level increased and declined in a similar pattern. Both PIM and antibody level were significantly higher in children and adolescents compared with adults. However, stepwise multiple regression analysis showed that age was only of minor importance for the PIM variation during the study period. Insulin antibody level and fasting C-peptide were the major determinants at 3-30 months, accounting for approximately 40% of the variation (R2). Blood glucose was of minor importance, and insulin dose, HbA1c, and BMI were of no importance. The correlation between fasting PIM and fasting C-peptide improved (R2 doubled) if the insulin antibody level was accounted for. Further, the slope of the correlation curve between PIM and C-peptide increased threefold when antibody binding was > 4%. At diagnosis, insulin autoantibodies could be detected in 19% of the patients. Their presence predicted higher proinsulin at 1-3 months, a higher insulin dose the 1st year, and higher levels of insulin antibodies later in the study. CONCLUSIONS: Circulating insulin antibodies may affect the level of PIM in IDDM, probably by adding a pool of IgG-bound PIM thereby increasing half-life and plasma concentration. This may explain why C-peptide and PIM levels do not change in concert during the 1st years of IDDM. Unlike C-peptide, PIM can not therefore quantitate beta-cell secretion unless the presence of insulin antibodies is ruled out.

Serum growth hormone-binding protein in obesity: effect of a short-term, very low calorie diet and diet-induced weight loss.

GH-binding protein (GHBP) is increased in obesity. It is not known whether the increase in GHBP is reversible with weight loss or modulated by acute changes in nutritional intake. To address these questions, we measured GHBP in 18 obese subjects [body mass index (BMI), 40.9 +/- 1.1 kg/m2 (mean +/-SEM)] before and after an average weight loss of 30.3 +/- 4.6 kg and in 18 age- and sex matched normal subjects (BMI, 23.0 +/- 0.4 kg/m2) and studied the effects of a very low calorie diet over 4 days in 5 normal subjects and a subgroup of obese subjects before (n = 6) and after (n = 5) weight loss. GHBP was elevated in the obese subjects compared to levels in age- and sex-matched normal controls (1.48 +/- 0.1 vs. 0.53 +/- 0.1 nmol/L; P < 0.0001). GHBP was positively correlated to BMI and waist circumference (r = 0.71; P < 0.00001 and r = 0.73; P < 0.00001, respectively). In addition, GHBP was positively correlated to insulin as well as proinsulin levels (r = 0.60; P < 0.001 and r = 0.55; P < 0.001, respectively). After diet-induced massive weight loss, GHBP levels were restored to normal in obese subjects (BMI, 27.8 +/- 1.4 kg/m2). Multiple stepwise regression analysis revealed that changes in waist circumference and abdominal sagittal diameter during weight loss were the major determinants of and accounted for 54% of the fall in GHBP levels. Neither insulin nor proinsulin was an independent predictor. No changes were observed in GHBP in normal, obese, or reduced weight obese subjects after 4 days of a very low calorie diet, although mean insulin levels fell significantly in the normal subgroup as well as in the obese subgroup studied after weight loss. In summary, GHBP levels are elevated in obesity, are restored to normal by massive weight loss, and are unaffected by short term hypocaloric feeding. We conclude that GHBP may be regulated by the same or closely related factors that regulate fat mass and abdominal fat mass in particular, but not by insulin or acute changes in nutrition.

An unusual type of familial lipodystrophy.

A mother and her daughter with a novel type of familial partial lipodystrophy were studied. Both had atrophy of fat in the face, chest, and upper and lower limbs and abdominal obesity caused by intraabdominal fat accumulation. The mother had severe insulin resistance and impaired glucose tolerance, whereas the daughter had normal glucose tolerance and normal insulin sensitivity. Both had metabolic rates about 30% above normal levels, but normal thyroid function and plasma lipids.

Lack of stimulation of 24-hour growth hormone release by hypocaloric diet in obesity.

Obesity is associated with a marked reduction in the spontaneous secretion of GH. To investigate the effect of acute alterations in calorie intake on GH release, 24-hr spontaneous GH release was measured during habitual calorie intake as well as during a short term, very low calorie diet (VLCD) in 6 obese subjects, 5 obese subjects after weight loss, and 5 normal, age- and sex-matched control subjects. Integrated 20-min samples were obtained over 24-h on two occasions in each subject using a constant blood withdrawal technique. In addition, basal levels of serum insulin-like growth factor-I (IGF-I), IGF-binding protein-1 (IGFBP-1), IGF-binding protein-3 (IGFBP-3), insulin, pro-insulin, and blood glucose were measured during habitual energy intake as well as during the hypocaloric diet. Twenty-four-hour GH release profiles and IGFBP-1 were decreased, and insulin as well as proinsulin levels were elevated in obese subjects compared to those in normal age- and sex-matched controls. No differences between obese subjects and normal controls were present regarding IGF-I, IGFBP-3, or IGF-I/IGFBP-3 molar ratio. In the last 24 h during the 96-h VLCD, an increase in 24-h GH release and basal IGFBP-1 levels and a decrease in basal insulin levels occurred in the normal controls, whereas no such changes were observed in the obese subjects. After caloric restriction 24-hr GH release, IGFBP-1 levels and insulin levels were similar in control subjects and obese subjects after weight loss. This suggests a reversible defect in GH release, rather than a persistent preexisting disorder. It is hypothesized that enhanced bioavailability of IGF-I, acting in concert with elevated proinsulin and insulin levels, may account for the lack of stimulation of 24-hr GH release by the hypocaloric diet in obese subjects. We conclude that the increase in 24-h spontaneous GH release and IGFBP-1 levels observed in normal subjects during the last 24 h of a 96-h VLCD is abolished in obese subjects. The lack of short term hypocaloric stimulation of spontaneous GH release may promote the retention of body fat and perpetuate the obese state.

No influence of proinsulin and insulin on plasma levels of plasminogen activator inhibitor type 1 and tissue plasminogen activator in young women before and during intake of contraceptive steroids.

Clinical observations in patients predisposed to cardiovascular disorders and recent experimental observations suggest that proinsulin and insulin participate in the regulation of fibrinolysis in vivo. In the present study, we examined if proinsulin and insulin affect the constitutive (fasting) secretion of plasminogen activator inhibitor type 1 (PAI-1) and tissue plasminogen activator (t-PA) in young healthy women (N = 17). We also measured the antigen concentrations of PAI-1 and t-PA during slow and fast changes in proinsulin and insulin levels induced by oral (OGTT) and intravenous (IVGTT) glucose tolerance tests. The assessments were performed before and after 6 months of treatment with contraceptive steroids, which have a well-defined influence on the fibrinolytic variables. We observed no consistent correlations between fasting values of proinsulin, insulin, PAI-1, and t-PA either before or during hormonal treatment. Before hormonal treatment, PAI-1 and t-PA antigen levels decreased (P < .05) during the hyperproinsulinemia and hyperinsulinemia induced by the OGTT and IVGTT. After hormonal intake for 6 months, a decrease only in t-PA concentrations during the OGTT was observed despite similar proinsulin and insulin responses to the glucose loads. Our findings suggest that proinsulin and insulin have no influence on the regulation of plasma levels of PAI-1 and t-PA in young healthy women, irrespective of intake of contraceptive steroids.

PIP: In Denmark, clinicians conducted clinical and metabolic evaluations on 17 healthy women, 21-26 years old, within the last 10 days of their menstrual cycle preceding intake with a triphasic oral contraceptive (OC) (ethinyl estradiol + norgestimate) and during the last 7 days of the sixth period of OC treatment. They aimed to examine the effect of proinsulin and insulin on fasting secretion of plasminogen activator inhibitor type 1 (PAI-1) and tissue plasminogen activator (t-PA). OCs have a well-defined effect on plasma levels of PAI-1 and t-PA. The clinical researchers also studied the antigen concentrations of PAI-1 and t-PA during slow and fast changes in proinsulin and insulin levels induced by oral and intravenous glucose tolerance tests. They did not find consistent correlations between fasting values of proinsulin, insulin, PAI-1, and t-PA either before or during OC treatment. During the glucose tolerance test induced hyperproinsulinemia and hyperinsulinemia and before OC treatment, PAI-1 and t-PA antigen levels fell (p 0.05). After 6 months of OC treatment, t-PA levels fell only during the oral glucose tolerance test (p 0.05) even though proinsulin and insulin responded similarly to the glucose loads. These findings suggest that neither proinsulin nor insulin regulate plasma levels of PAI-1 and t-PA in young healthy women regardless of OC use status.

Improved postprandial glycaemic control with insulin Aspart in type 2 diabetic patients treated with insulin.

The effect on postprandial blood glucose control of an immediately pre-meal injection of the rapid acting insulin analogue Aspart (IAsp) was compared with that of human insulin Actrapid injected immediately or 30 minutes before a test meal in insulin-treated type 2 diabetic patients with residual beta-cell function. In a double-blind, double dummy crossover design, patients attended three study days where the following insulin injections in combination with placebo were given in a random order: IAsp (0.15 IU/kg body weight) immediately before the meal, or insulin Actrapid (0.15 IU/kg) immediately (Act0) or 30 minutes before (Act-30) a test meal. We studied 25 insulin-requiring type 2 diabetic patients, including 14 males and 11 females, with a mean age of 59.7 years (range, 43-71), body mass index 28.3 kg/m2 (range, 21.9-35.0), HbA1c 8.5% (range, 6.8-10.0), glucagon-stimulated C-peptide 1.0 nmol/l (range, 0.3-2.5) and diabetes duration 12.5 years (range, 3.0-26.0). Twenty-two patients completed the study. A significantly improved postprandial glucose control was demonstrated with IAsp as compared to Act0, based on a significantly smaller postprandial blood glucose excursion (IAsp, 899 +/- 609 (SD) mmol/l.min versus Act0, 1102 +/- 497 mmol/l min, p < 0.01) and supported by a significantly lower maximum serum glucose concentration (Cmax) up to 360 min after dosing (IAsp, 10.8 +/- 2.2 mmol/l vs. Act0, 12.0 +/- 2.4 mmol/l, p < 0.02). No difference was demonstrated in glucose endpoints between IAsp, administered with a meal and Actrapid injected 30 minutes before the meal (AUCglucose IAsp, 899 +/- 609 mmol/l min vs. Act-30, 868 +/- 374 mmol/l min; Cmax IAsp, 10.8 +/- 2.2 mmol/l vs. Act-30, 11.1 +/- 1.8 mmol/l). No concerns about the safety of IAsp were raised. Immediate pre-meal administration of the rapid-acting insulin analogue Aspart in patients with type 2 diabetes resulted in an improved postprandial glucose control compared to Actrapid injected immediately before the meal, but showed similar control compared to Actrapid injected 30 minutes before the meal. These results indicate that the improved glucose control previously demonstrated with insulin Aspart compared to human insulin in healthy subjects and type 1 diabetic patients also applies to insulin-treated type 2 diabetic patients.

Quantification of beta-cell function during IVGTT in Type II and non-diabetic subjects: assessment of insulin secretion by mathematical methods.

AIMS/HYPOTHESIS: We compared four methods to assess their accuracy in measuring insulin secretion during an intravenous glucose tolerance test in patients with Type II (non-insulin-dependent) diabetes mellitus and with varying beta-cell function and matched control subjects. METHODS: Eight control subjects and eight Type II diabetic patients underwent an intravenous glucose tolerance test with tolbutamide and an intravenous bolus injection of C-peptide to assess C-peptide kinetics. Insulin secretion rates were determined by the Eaton deconvolution (reference method), the Insulin SECretion method (ISEC) based on population kinetic parameters as well as one-compartment and two-compartment versions of the combined model of insulin and C-peptide kinetics. To allow a comparison of the accuracy of the four methods, fasting rates and amounts of insulin secreted during the first phase (0-10 min) and the second phase (10-180 min) were calculated. RESULTS: All secretion responses from the ISEC method were strongly correlated to those obtained by the Eaton deconvolution method (r = 0.83-0.92). The one-compartment combined model, however, showed a high correlation to the reference method only for the first-phase insulin response (r = 0.78). The two-compartment combined model failed to provide reliable estimates of insulin secretion in three of the control subjects and in two patients with Type II diabetes. The four methods were accurate with respect to mean basal and first-phase secretion response. The one-compartment and two-compartment combined models were less accurate in measuring the second-phase response. CONCLUSION/INTERPRETATION: The ISEC method can be applied to normal, obese or Type II diabetic patients. In patients with deviating kinetics of C-peptide the Eaton deconvolution method is the method of choice while the one-compartment combined model is suitable for measuring only the first-phase insulin secretion.

Insulin secretion rates estimated by two mathematical methods in pancreas-kidney transplant recipients.

After pancreas-kidney transplantation, it is difficult to obtain an accurate estimate of the insulin secretion of the pancreas graft, since several pitfalls are involved using peripheral C-peptide and/or insulin measurements in this determination. In this study, the individual kinetic parameters of C-peptide and then the rates of insulin secretion were estimated by two mathematical methods, the deconvolution method and the "combined model" during slow (oral glucose) and fast (intravenous glucagon) changes in insulin secretion in six successful pancreas-kidney transplant recipients with systemic delivery of insulin (Px), six nondiabetic kidney-transplant recipients with portal insulin secretion (Kx), six nondiabetic controls (NS), and six C-peptide-negative insulin-dependent diabetes mellitus patients (IDDM). Decreased C-peptide clearance and basal and poststimulatory hyperinsulinemia were found in both Px and Kx compared with NS (P < 0.05). Similar glucose responses were observed after intravenous glucagon in all groups, whereas the responses after oral glucose were 30% higher in Px and Kx than in NS (P < 0.05). During oral glucose and after intravenous glucagon, both mathematical methods resulted in significantly lower maximal and incremental insulin secretion rates (ISR) in Px than in Kx (P < 0.05). In contrast, calculations of incremental ISR in NS and Px induced by the two beta-cell stimuli were about the same but significantly higher in Kx than in NS (P < 0.05). These results differed markedly from those obtained using peripheral measurements of insulin and C-peptide alone. In conclusion, when C-peptide clearance and insulin metabolism change, such as in pancreas-kidney transplant recipients, accurate evaluation of insulin secretion from the graft can be obtained only by using individual kinetics of the peptides before calculating the ISR. This study also clearly demonstrates that insulin secretion after pancreas transplantation is still defective.

Overnight inhibition of insulin secretion restores pulsatility and proinsulin/insulin ratio in type 2 diabetes.

Impaired insulin secretion in type 2 diabetes is characterized by decreased first-phase insulin secretion, an increased proinsulin-to-insulin molar ratio in plasma, abnormal pulsatile insulin release, and heightened disorderliness of insulin concentration profiles. In the present study, we tested the hypothesis that these abnormalities are at least partly reversed by a period of overnight suspension of beta-cell secretory activity achieved by somatostatin infusion. Eleven patients with type 2 diabetes were studied twice after a randomly ordered overnight infusion of either somatostatin or saline with the plasma glucose concentration clamped at approximately 8 mmol/l. Controls were studied twice after overnight saline infusions and then at a plasma glucose concentration of either 4 or 8 mmol/l. We report that in patients with type 2 diabetes, 1) as in nondiabetic humans, insulin is secreted in discrete insulin secretory bursts; 2) the frequency of pulsatile insulin secretion is normal; 3) the insulin pulse mass is diminished, leading to decreased insulin secretion, but this defect can be overcome acutely by beta-cell rest with somatostatin; 4) the reported loss of orderliness of insulin secretion, attenuated first-phase insulin secretion, and elevated proinsulin-to-insulin molar ratio also respond favorably to overnight inhibition by somatostatin. The results of these clinical experiments suggest the conclusion that multiple parameters of abnormal insulin secretion in patients with type 2 diabetes mechanistically reflect cellular depletion of immediately secretable insulin that can be overcome by beta-cell rest.

Highly sensitive enzyme immunoassay of proinsulin immunoreactivity with use of two monoclonal antibodies.

A highly sensitive two-site sandwich ELISA measuring total proinsulin immunoreactive material in serum or plasma was developed. The assay was based on two monoclonal antibodies, an anti-C-peptide antibody bound to a microtest plate and a biotin-labeled anti-insulin antibody. The detection limit (3 SD above zero value) in buffer was 0.05 pmol/L, corresponding to 0.25 pmol/L in human serum (diluted 1:5). The linear calibrator range was 0.05-20 pmol/L. Interassay CVs were 4.7% at a median (range) of 2.3 pmol/L (1.4-2.8 pmol/L, n = 8), 6.7% at 5.1 pmol/L (3.3-8.0 pmol/L, n = 8), and 8.7% at 10.0 pmol/L (8-12 pmol/L, n = 10). Mean analytical recovery of added human proinsulin (hPI) (2, 5, and 10 pmol/L) to serum was 84% (range 68-128%, n = 9). Human insulin and human C-peptide did not cross-react at 5000 and 10,000 pmol/L, respectively. The four major proinsulin conversion intermediates reacted 65-99%: split(32-33)hPI 74%, des-(31,32)hPI 65%, split(65-66)hPI 78%, and des(64,65)hPI 99%. All serum values from 38 fasting healthy subjects were above the detection limit: median (range) 4.0 (2.1-12.6) pmol/L.