Equilibrium binding assay and kinetic characterization of insulin antibodies.
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Binding Sites, Antibody↗
Biomedical subjects
A H Rubenstein
Publications and source records attributed to A H Rubenstein.
Measurment of rhesus monkey (Macaca mulatta) apolipoprotein B in serum by radioimmunoassay: comparison of immunoreactivities of rhesus and human low density lipoproteins.
A sensitive and specific double antibody radio-immunoassay for the major apolipoprotein (apoB) of rhesus (Macaca mulatta) serum very low density lipoprotein (VLDL) and low density lipoprotein (LDL) is described. The anti-serum was raised to LDL (d 1.030-1.040 g/ml) and the LDL(2) (d 1.020-1.050 g/ml) was labeled with (125)I by the chloramine-T or iodine monochloride method. The assay, which was sensitive to 0.02-0.5 micro g of LDL(2), had an inter-assay coefficient of variation of 4.5%. This assay was successfully used to measure apoB in the whole serum and low density lipoproteins of control monkeys maintained on a standard Purina monkey chow (PMC) diet and of three groups of monkeys fed atherogenic diets: an "average American diet," a 25% peanut oil and 2% cholesterol-supplemented PMC diet, and a 25% coconut oil and 2% cholesterol-supplemented PMC diet. The control monkeys (n = 13) had a serum cholesterol of 146 +/- 28 mg/dl and an apoB of 50 +/- 18 mg/dl. In the monkeys maintained on the atherogenic diets the serum apoB was elevated: 103 +/- 28 mg/dl (American), 102 +/- 35 mg/dl (peanut oil), and 312 +/- 88 mg/dl (coconut oil). The values for serum total cholesterol were 333 +/- 65 mg/dl (American), 606 +/- 212 mg/dl (peanut oil), and 864 +/- 233 mg/dl (coconut oil) and were elevated relative to controls (P < 0.001). For each of the diets, total serum cholesterol correlated with serum apoB (P < 0.001). The slopes of the regression lines of serum apoB vs. cholesterol for the monkeys on the PMC, American, and coconut oil diets were similar (m = 0.531, 0.401, and 0.359, respectively), but differed from that of monkeys on the peanut oil diet (m = 0.121). The immunoreactivities of rhesus and human LDL were compared using specific antisera raised against these antigens. In homologous assay systems, monkey and human LDL exhibited unique immunological determinants. The same results were obtained with the delipidated preparations of the two LDLs using antisera raised against either monkey or human apoB. Crossover studies using a heterologous tracer with each anti-serum resulted in the selection of a specific population of antibodies directed against antigenic sites shared by these two LDL species.
Animals↗
Measurement of serum proinsulin-like material: cross-reactivity of porcine and human proinsulin in the insulin radioimmunoassay.
The variation in cross-reactivity of human and porcine insulins and proinsulins was determined with a number of insulin antisera. The relative immunoreactivity of insulin and proinsulin with these antisera ranged from a ratio of 1.08 to 5.7 on a molar basis. The displacement curves of human insulin and proinsulin were not parallel, and the relationship of porcine and human proinsulin varied with different antisera. Sera with varying concentrations of PLM (proinsulin and its intermediate components) and serum PLM and insulin fractions (separated by gel filtration) were measured in two assays using the antisera which reacted most differently with human insulin and proinsulin standards. With the use of these two antisera, measurement of serum PLM against a human insulin standard gave different results, which were only partially corrected by reading the values from a human proinsulin standard. The magnitude of the serum IRI differences with these antisera was related to the proinsulin/insulin ratio in each sample. These results indicate the necessity for each laboratory to critically evaluate the reaction of their insulin antiserum with human proinsulin. In addition, measurement of serum PLM in terms of a insulin standard will give different results depending upon the particular antiserum used.
Animals↗
Factitious hypoglycemia. Diagnosis by measurement of serum C-peptide immunoreactivity and insulin-binding antibodies.
In seven patients with factitious hypoglycemia due to the surreptitious injection of insulin, we made the diagnosis by measurements of plasma insulin and C-peptide immunoreactivity (in seven patients), facilitated by the finding of circulating insulin-binding antibodies (in two patients). The simultaneous demonstration of low plasma glucose, high immunoreactive insulin and suppressed C-peptide immunoreactivity represents a triad of results pathognomonic of exogenous insulin administration. Determination of plasma free C-peptide and free insulin permitted patients with high titers of insulin antibodies, including those with a history of insulin-treated diabetes, to be studied and diagnosed in a way similar to that in subjects who had no circulating insulin antibodies.
Adenoma, Islet Cell↗
Circulating proinsulin in patients with maturity onset diabetes.
The contribution of proinsulin to the total serum immunoreactive insulin (IRI) was measured in 59 patients with maturity onset diabetes (23 being treated with diet alone and 36 with oral sulfonylurea agents) and compared to that in 44 control subjects. The percentage of proinsulin was increased in 11 patients and correlated with plasma glucose, but not with IRI. There was no difference between the drug-treated group and diet-treated group, or between patients taking different sulfonylurea agents. Sequential studies in one patient showed normalization of the proportion of proinsulin following lowering of the plasma glucose level. It is probably that the increased circulating proportion of proinsulin in hyperglycemic diabetic patients is secondary to beta cell exhaustion with release of less mature granules.
Adult↗
Control of juvenile diabetes mellitus and its relationship to endogenous insulin secretion as measured by C-peptide immunoreactivity.
Proinsulin is converted to insulin and C-peptide in the pancreatic beta-cells; the latter two peptides are secreted in equimolar concentrations. Thus measurements of C-peptide immunoreactivity may provide a means of assessing residual pancreatic function in insulin-treated diabetic patients. Thirty-five patients with a mean (+/- SE) age of 13.4 +/- .6 years who had diabetes mellitus for 4.8 +/- .3 years were included in this study. Glucose and CPR were measured in the fasting state and one hour after 1 gm/kg (maximum 50 gm) of oral and glucose. Patients were assigned to one of two groups on the basis of adequate or poor control of diabetes. Twenty-five of the 35 (71%) patients had evidence of endogenous beta-cell function, i.e., CPR greater than 0.5 ng/ml. CPR levels over 0.5 ng/ml were present in a significantly (p less than 0.05) greater number of patients with diabetes of less than 5 years duration (19/21) than in those with diabetes greater than 5 years duration (6/14). Only one patient showed a rise in CPR after the glucose load. All patients with CPR greater than 2.0 ng/ml were in the adequately controlled groups, but there were patients with CPR less than 2.0 ng/ml in both adequately and poorly controlled groups. Because the CPR value includes both C-peptide and antibody-bound proinsulin, separate determination of free C-peptide was done in 30 patients. These results confirmed the conclusions based on CPR estimation. Although growth hormone values were higher in patients in the poorly controlled group, there was no correlation between hGH and CPR. We conclude that residual insulin secretion in diabetic patients may facilitate good control, but that low CPR values and hence absent beta-cell reserve is not always associated with poor control.
Adolescent↗
Secretion by glucagonomas of a possible glucagon precursor.
Five patients with glucagonomas had elevated plasma levels of total glucagon immunoreactivity. Gel filtrations of these plasma samples on Bio-Gel P30 columns showed that most of the immunoreactivity eluted in the 3,500-(true glucagon) and 9,000-dalton fractions. After the administration of alpha cell effectors, changes in total glucagon immunoreactivity were seen which were accounted for primarily by the 3,500-dalton species, but there were also changes in the 9,000-dalton moiety. Venous effluent plasma from tumors of two subjects contained elevated concentrations of glucagon immunoreactivity in both fractions. When material from both the 3,500- and 9,000-dalton peaks were serially diluted in a glucagon immunoassay, parallel displacement curves were found, suggesting that both have similar or identical antigenic determinants. Thus, with conversion to a neoplastic state, alpha cells of glucagonomas, much like beta cells of insulinomas, may secrete an increased amount of a larger, 9,000-mol wt glucagon species which may be a prohormone.
Adult↗
The role of the liver in glucagon metabolism.
Total plasma immunoreactive pancreatic glucagon (IRG) was measured in samples taken simultaneously from the proximal portal vein and superior vena cava of 26 healthy rats. The portal-peripheral ratio of IRG was 2.80+/-0.25, the portal-peripheral difference (Delta) 124+/-15 pg/ml, and percentage extraction 58+/-3. Gel filtration of paired portal and peripheral vein samples showed that reduction in the 3,500-dalton IRG component (glucagon) in peripheral samples accounted for almost all the differences, there being minimal and inconsistent changes in the high molecular weight (>40,000) fraction. The portal-peripheral ratio of the 3,500-dalton glucagon was 5.24+/-1.10, the portal-peripheral difference 130+/-33 pg/ml, and the percentage extraction 81+/-5. To study the transhepatic differences in the 9,000-dalton "proglucagon-like" material, the experiment was repeated in nine rats 24 h after bilateral nephrectomy, a procedure which increases plasma levels of this fraction. The portal-peripheral ratio for plasma IRG in these rats was 1.48+/-0.12, the portal-peripheral difference 140+/-29 pg/ml, and percentage extraction 28+/-5. Gel filtration revealed no consistent differences between portal and peripheral concentrations of the 9,000- and >40,000-dalton components, which comprised 40 and 13%, respectively, of the mean IRG level of 492+/-35 pg/ml. In contrast, there were marked differences between portal and peripheral levels of the 3,500-dalton component the ratio being 3.42+/-0.63, the portal-peripheral difference 182+/-32 pg/ml, and percentage extraction 64+/-5. Similar studies in a healthy dog, in which species there are significant circulating levels of the 9,000-dalton IRG component, confirmed the selective hepatic extraction of the 3,500-dalton fraction. We conclude that the various IRG fractions are metabolized differently by the liver, and that portal-peripheral ratios based on direct assay of plasma IRG will vary depending on the percentage glucagon immunoreactivity in each fraction; the greater the combined contribution of fractions other than the 3,500-dalton component to total plasma IRG, the lower will be the ratio. Because of the heterogeneity of circulating IRG and significant differences in the metabolism of its various components, gel filtration of plasma samples is necessary for precise quantitation of the hepatic uptake of each particular fraction.
Animals↗
Heterogeneity of circulating C-peptide.
Serum C-peptide levels vary when measured with different immunoassay systems. In order to assess the factors contributing to this finding, serum C-peptide was measured in two assays utilizing different antisera, but the same standards and labeled peptide. The antisera were characterized with synthetic C-peptide fragments and their reactivities towards some of these fragments differed. The results of dilution and recovery tests and stability of the C-peptide during storage showed differences between the two assays. Gel filtration experiments indicated heterogeneity within the major C-peptide peak, and, in addition, a smaller peak of lower molecular weight material was present in some sera. Although degradation of serum C-peptide may occur during storage or with freezing and thawing, fragments of C-peptide may also be secreted or arise during in vivo metabolism.
Animals↗
Abnormalities in circulating beta cell peptides in chronic renal failure: comparison of C-peptide, proinsulin and insulin.
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Adult↗
Determination of free and total insulin and C-peptide in insulin-treated diabetics.
Serum levels of free and total insulin as well as total C-peptide immunoreactivity (C-peptide and proinsulin) and C- peptide were measured in insulin-treated diabetics with circulating insulin antibodies by the addition of polyethylene glycol (PEG) before and after acidification. PEG resulted in complete precipitation of insulin antibodies from serum and made it possible to measure free insulin in the supernatant. Incubation of serum at 37 degrees C. for two hours before addition of PEG resulted in values for free insulin that probably resembled the in-vivo levels most closely. The same method could also be used to remove proinsulin bound to circulating insulin antibodies and permitted the measurement of C-peptide in the supernatant. Clinical studies using this approach indicate that combined measurements of serum free and total insulin and C-peptide provide information that is helpful in understanding the contribution of endogenous and exogenous insulin to the course and metabolic control of insulin-requiring diabetic patients.
Antibodies↗
Quantitation of human pancreatic beta-cell function by immunoassay of C-peptide in urine.
Human proinsulin connecting peptide (C-peptide) was measured by immunoassay in urine from 25 normal subjects, 18 patients with diabetes mellitus, and 34 patients with various degrees of renal insufficiency. Assay validation studies showed that pancreatic C-peptide was quantitatively recovered when added to urine. Fractionation of urine by gel filtration indicated that most endogenous C-peptide eluted in fractions that corresponded to the C-peptide standard. In 34 nondiabetic subjects with normal kidney function or various renal diseases, C-peptide clearance was independent of creatinine clearance over a range of 6 to 190 ml./min. Urine C-peptide clearance (5.1 +/- 0.6 ml./min.) is greater than that of insulin (1.1 +/- 0.2 ml./min.), and the total quantity of C-peptide excreted in the urine per day represents 5 per cent of pancreatic secretion, as against only 0.1 per cent of secreted insulin. Healthy subjects excreted 36 +/- 4 mug. C-peptide per 24 hours, while this value in juvenile-onset diabetics was only 1.1 +/- 0.5 mug. Adult-onset diabetics excreted 24 +/- 7 mug./24 hr., the range overlapping the excretory rates of both normal subjects and juvenile-onset diabetics. Two insulin-requiring adult-onset diabetics showed significant beta-cell reserve during the course of acute infections. These results suggest that urine C-peptide provides a useful means of assessing beta-cell secretory capacity over a period of time and is especially advantageous when frequent blood sampling is not feasible.
Acute Kidney Injury↗
Abnormalities of endogenous glucagon and insulin in unstable diabetes.
The responses of glucagon, growth hormone, and insulin secretion to the oral administration of glucose and to the intravenous infusion of saline, arginine, and insulin were measured in seven patients who had stable diabetes, eight who had unstable diabetes, and seven healthy volunteers. Hyperglycemia suppressed secretion of glucagon in normal subjects but not in diabetics. The oral glucose and arginine infusion tests demonstrated partial preservation of insulin-secretory ability in stable diabetics and its virxual absence in unstable diabetics. Glucagon responses to arginine infusion were similar in all three groups. In response to hypoglycemia induced by insulin infusion, the concentrations of plasma glucagon increased in normal subjects and, to a lesser extent, in stable diabetics but increased in only two of the unstable diabetics. The impairment in glucagon response during hypoglycemia in diabetics correlated positively with the degree of diabetic instability and insulin deficiency during glucose and arginine testing. The severity of the insulin deficiency also correlated with the degree of diabetic instability. These findings support the hypothesis that inherent abnormalities of insulin and glucagon secretion may account for many of the clinical characteristics of unstable and stable diabetic patients.
Adult↗
Circulating glucagon. Plasma profiles and metabolism in health and disease.
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Adult↗
Clinical significance of circulating C-peptide in diabetes mellitus and hypoglycemic disorders.
Proinsulin is converted to insulin and C-peptide in the pancreatic in the pancreatic beta cells: the latter two peptides are secreted in equimolar concentrations. Thus, measurements of serum C-peptide provide a means of assessing pancreatic beta cell function in addition to that of insulin. This technique has proved particularly useful in insulin treated diabetic patients in whom the development of circulating insulin antibodies interferes with the radioimmunoassay of the hormone. The C-peptide assay has also been used to facilitate the diagnosis of various hypoglycemic conditions, including islet cell tumors and factitious injection of insulin. The extraction of C-peptide in the urine reflects average serum values over a period of time and urine C-peptide measurements are especially useful in children or individuals in whom repeated blood sampling is difficult.
Adenoma, Islet Cell↗
C-peptide suppression test for insulinoma.
During hypoglycemia induced by an infusion of porcine insulin, impaired suppression of endogenous insulin secretion as measured by C-peptide was demonstrated in 11 of 12 patients with insulinoma. During hypoglycemia (plasma glucose less than or equal to 40 mg/dl) the mean C-peptide immunoreactivity (CPR) of normal subjects was less than or equal to 1.2 ng/ml, whereas 11 of 12 insulinoma patients had a mean CPR of larger than or equal to 1.9 ng/ml. One patient showed normal CPR suppression by these criteria but may have shown impaired CPR suppression for glucose less than or equal to 30 mg/dl. Impaired CPR suppression during insulin-induced hypoglycemia may prove to be a useful test for insulinoma.
Adenoma, Islet Cell↗
Fractionation of human serum lipoproteins by single-spin gradient ultracentrifugation: quantification of apolipoproteins B and A-1 and lipid components.
A sensitive and reproducible method has been developed for separation of the major serum lipoproteins from 1 ml or less of human serum by isopycnic density gradient ultracentrifugation. The serum, applied to a step gradient (total volume 12.8 ml), was spun for 48 hr at 38,000 rpm at 10 degrees C and, in each of the fractions, apolipoproteins B and A-I were quantified by the respective radioimmunoassays. The markers for lipid distribution used were [4-(14)C]cholesterol and [U-(14)C]lecithin, each incubated with an aliquot of serum at 20 degrees C for 75 min prior to ultracentrifugation. In control sera, three main fractions, very low density (VLDL), low density (LDL), and high density (HDL) lipoproteins were clearly separated from a bottom fraction. Their flotational, electrophoretic, and chemical properties were in good agreement with those reported for the corresponding lipoproteins separated by conventional ultracentrifugation. Both apo B and apo A-I were fully recovered. Essentially all of the apo B was found in VLDL (9.3 +/- 3.5%) and LDL (87 +/- 4.6%); of the apo A-I, 81.0 +/- 5.7% was in HDL and the remainder (17.0 +/- 5.8%) was in the bottom fraction. The peak activities of [(14)C]cholesterol coincided with the peak of apo B in both LDL and VLDL, and with the peak of apo A-I in HDL. The results with the radiolabeled cholesterol were in good agreement with those obtained by chemical analyses. Carbon 14-labeled lecithin, although fully recovered, was not an accurate marker of phospholipid distribution because, under our experimental conditions, a significant amount of the lecithin was converted into its lyso derivative. The mechanism of the conversion was not established; it appeared to be unrelated to the activities of either lecithin-cholesterol acyl transferase or a Ca(2+)-dependent phospholipase. Besides its validity in the study of control sera, our method also proved successful in the separation of the serum lipoproteins of the few patients with dyslipoproteinemia (abetalipoproteinemia and familial hypercholesterolemia) who were examined. However, the applicability of the method to all dyslipoproteinemias was not assessed. Taken together, the results indicate that the single-spin method could be useful in clinical studies as a complement to other established techniques.
Abetalipoproteinemia↗
Circulating serum C-peptide. A brief review of diagnostic implications.
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Adenoma, Islet Cell↗