Immunological and biological properties of iodoinsulin labeled with one or less atoms of iodine per molecule

99mTc-insulin: labeling, biodistribution and scintiimaging in animals

Porcine insulin was labeled with 99mTc by direct tin reduction. More than 95% labeling efficiency was obtained on paper chromatography in saline and methyl ethyl ketone. The stability of the labeled compound was confirmed by paper chromatography at 3 h post-labeling and by human serum albumin (HSA) challenge. PAGE pattern indicated no change in the electrophoretic behavior and the molecular size of insulin after the labeling procedure. Biodistribution in rats shows that kidney took up the maximum amount of 99mTc-insulin; maxima being maintained throughout 24 h post-injection. Liver and intestine were the other organs with significant uptake; the rest localizing little or negligible radioactivity. Most of the radioactivity was excreted via the renal pathway into urine. Scintiimages conformed to the biodistribution data. The results of this study present the potential of 99mTc-labeling of insulin by a simple method.

Hyperinsulinemia after pancreatic transplantation. Prediction by a novel computer model and in vivo verification

OBJECTIVE

The authors evaluated systemic venous insulin release as a cause of the hyperinsulinemia (HNS) associated with pancreatic transplantation (PTX) with respect to the mechanism and metabolic consequences.

SUMMARY BACKGROUND DATA

Many investigators believe the postoperative anatomy associated with common PTX techniques to be the sole cause of the two- to threefold posttransplantation HINS. However, this concept remains to be conclusively proved and characterized quantitatively.

METHODS

The authors used three approaches to achieve their objectives. First, a computer model was generated based on established data concerning blood flow and tissue insulin extraction to determine whether it was mathematically possible for HINS to be caused by systemic insulin release. Second, HINS clamps were applied to normal dogs using the Andres clamp technique to quantify the in vivo differences in peripheral insulin levels and the metabolic consequences of systemic versus portal insulin infusion. Third, prolonged insulin half-life was evaluated as a possible mechanism of HINS from systemic insulin release by determination of biexponential rates of plasma disappearance from an endogenous pulse of insulin in surgically induced dog models of systemic and portal insulin release.

RESULTS

First, the computer model calculated a 1.4- to 2.9-fold increase in peripheral venous insulin levels with systemic versus portal insulin release, verifying mathematically the concept of HINS resulting from systemic insulin release. Second, the actual systemic insulin infusion produced a 1.3- to 1.4-fold increase in peripheral venous insulin levels compared with portal infusion (p < 0.05). No significant differences in hepatic glucose output, total glucose disposal, or glucose infusion requirements were seen. Third, although the basal insulin level was twofold higher in the surgically induced animal models with systemic insulin release (p < 0.003), there were no differences in biexponential insulin clearance parameters.

CONCLUSIONS

The HINS produced by systemic insulin release did not significantly alter glucose metabolism and was not the result of altered peripheral insulin clearance parameters. In vivo systemic venous insulin infusion studies produce HINS, but not to the degree calculated by mathematic modeling or that occurs after clinical PTX, making it likely that other factors also play a role in the HINS after PTX.

Comparison of the continuously calculated fractional splanchnic extraction of insulin with its fractional disappearance using a new double-tracer technique

These studies were designed to calculate the fractional disappearance rate (FDR) and splanchnic extraction of insulin in response to an exogenous (intraperitoneal) input of insulin. A double-tracer technique using insulin tritiated on both the A1 and B1 positions was introduced for the measurement of hepatic extraction. The A1 tracer, not previously characterized in vivo, was compared in terms of its kinetics with H3-B1-insulin and unlabeled insulin. The metabolic clearance rates (MCR) of the three insulins were identical, as were the decay curves of the two tracers. To measure splanchnic insulin extraction, one tracer was infused systemically to evaluate the FDR of insulin, and the second was infused into the splanchnic circulation (superior mesenteric artery) and its peripheral appearance was calculated. Splanchnic extraction was determined from the difference between this rate of appearance and the rate of infusion of the mesenteric tracer. After intraperitoneal insulin injection, insulin levels increased to peaks of 549 +/- 93 microU/mL (portal vein) and 473 +/- 99 microU/mL (inferior vena cava) and decreased to basal levels over 3 hours. The FDR decreased from 0.295 +/- 0.051 min-1 to 0.125 +/- 0.026 min-1, and splanchnic extraction decreased from 0.534 +/- 0.06 to 0.232 +/- 0.088. The latter returned to near-basal values more rapidly than did the FDR. In conclusion, the kinetics of insulin both in and out of the steady state have been shown to be nonlinear through physiological insulin concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Posthepatic rate of appearance of insulin: measurement and validation in the nonsteady state

To assess the accuracy with which insulin appearance rates in the peripheral circulation can be measured out of steady state, seven conscious dogs were simultaneously infused with somatostatin and insulin at known variable rates. Tritiated insulin was infused concurrently at a constant rate. Insulin rates of appearance were estimated continuously on the basis of a two-compartment model for systemic insulin kinetics. The calculations were performed assuming that insulin kinetics were linear (tracer data not used) and nonlinear or time varying (tracer data used to assess the variation). The average error in areas under the curve was -3.5 +/- 2.5 and 27.0 +/- 14.2% when nonlinear or linear kinetics were assumed. The maximal errors when linearity was assumed was 39.9 +/- 11.3% and decreased to 16.3 +/- 2.6% when the tracer data was used to account for changes in the fractional removal rate of insulin. The accuracy of the linear estimates improved as the fractional removal rate remained closer to constant. These data suggest that a priori assumptions should not be made on the linearity of the insulin system in a given experimental situation.

Insulin degradation in vivo: a high-performance liquid chromatographic analysis

The metabolism of insulin in vivo was investigated using an isocratic reversed-phase high-performance liquid chromatographic (RP-HPLC) method. After intravenous injection of A14-[125I]insulin into normals, eight labelled insulin derivatives were found in plasma (peaks 1-8). Two of them (peaks 1 and 7) showed an elution pattern identical with those of reference [125I]monoiodotyrosine and intact A14-[125I]insulin, respectively. Of the other six peaks, five (2-6) eluted before and one (peak 8) after insulin. This pattern was highly reproducible in terms of capacity factors and peak heights. Radioactivity separated by RP-HPLC was further characterized for its trichloroacetic acid precipitability and immunoprecipitability. Fractions corresponding to peaks 4-6 and 8, which showed an immunoprecipitability higher than 50%, were pooled in order to obtain sufficient radioactivity and were found to be insulin separated by Sephadex G-50 chromatography, containing in its structure, after sulphitolysis, intact A-chain and to be partially rebindable to monocyte insulin receptors. These data demonstrate that in blood, products of insulin metabolism circulate which retain a part of the immunological and biological properties of the hormone. These products are clearly separated from one another and from intact insulin by RP-HPLC, suggesting that the appropriate use of this technique may allow a further and more accurate qualitative and quantitative characterization of in vivo insulin metabolism in physiological and pathological conditions.

Iodinated derivatives of the hormone avian pancreatic polypeptide

Reaction of avian pancreatic polypeptide with an iodine monochloride reagent at both pH 4 and pH 7.5 results in the differential modification of the four tyrosine residues in this peptide hormone. A total of 19 distinct iodinated derivatives were isolated by reverse-phase high-performance liquid chromatography, and their sites of iodination were characterized by both tryptic mapping and leucine aminopeptidase techniques coupled with HPLC. The pH 4 reaction produced 16 derivatives which, overall, represented substantial iodination at each tyrosine residue, whereas the pH 7.5 reaction was more directed, producing only 7 derivatives. Iodination at the C-terminal tyrosineamide 36 predominated at both pH values, and diiodo-Tyr 36 was found in the majority of the pH 7.5 derivatives. The relative of the four tyrosine residues with ICl were as follows: at pH 7.5, Tyr 36 much greater than Tyr 21 much greater than Tyr 27 greater than Tyr 7; at pH 4, Tyr 36 greater than Tyr 27 greater than Tyr 7 greater than Tyr 21.

Improved preparation of semisynthetic PheB1-tritiated insulin

Semisynthetic [PheB1-3H]insulin was prepared. Each intermediate in the synthesis was characterised by HPLC, polyacrylamide gel electrophoresis and end group determination by dansylation. The end product had a specific activity of up to 59 Ci/mmol. Semisynthetic insulin proved indistinguishable from natural insulin in its chemical and immunological properties and was shown to be biologically fully active by in vivo and in vitro assays.

Measurement using tracers of steady-state turnover and metabolic clearance of insulin in dogs

In nine conscious dogs, the steady-state metabolic clearance rate (MCR) and the systemic appearance rate (Ra) of insulin were determined by the tracer dilution method. [3H-PheB1]insulin ([3H]insulin) was infused as a tracer from time 0 at a constant rate. After tracer equilibration was attained, unlabeled porcine insulin was infused at variable constant rates (10.6-279 mU/min) with somatostatin (0.3 microgram . kg-1 . min-1) to suppress endogenous insulin secretion. Glucose was infused to prevent hypoglycemia. Tritiated and immunoreactive insulin (IRI) concentrations were determined in plasma samples after extraction on a C-18 reverse-phase column. Tracer-determined basal Ra of insulin was 2.39 +/- (SE) 0.61 mU/min. The calculated steady-state Ra of insulin for plasma IRI from 20 to 2,300 microU/ml showed good agreement with insulin infusion rates. The mean ratio of these rates was 0.973 +/- 0.018. The MCR of insulin under basal conditions was 29.9 +/- 3.4 ml . kg-1 . min-1, and it decreased with increasing insulin concentrations. It is concluded that 1) insulin turnover rates can be measured accurately using [3H]insulin as a tracer and 2) insulin kinetics are nonlinear.

Impaired insulin degradation in a patient with insulin resistance and acanthosis nigricans

The kinetics of plasma insulin were studied in a 14 year old girl with the syndrome of insulin resistance and acanthosis nigricans. The clearance of plasma insulin was found to be strikingly reduced (135 ml/min . m2 versus 456 +/- 22 in 17 normal control subjects), whereas the basal systemic insulin delivery rate was increased about 10-fold (25.5 mU/min . m2 versus 2.6 +/- 0.3 in normal subjects). Thus, reduced insulin clearance and excessive posthepatic delivery of the hormone both contributed to the severe fasting hyperinsulinemia (218 microunits/ml) associated with the other clinical features of the syndrome (glucose intolerance, primary amenorrhea, polycystic ovaries, hirsutism). Following ovarian wedge resection, insulin clearance rose to 264 ml/min . m2, and insulin delivery fell to 9.8 microunits/ml min . m2. The resulting abatement of the patient's hyperinsulinism (fasting plasma insulin = 37 microunits/ml) was accompanied by the appearance of menses, normalization of glucose tolerance, and amelioration of the acanthosis. The improvement in menstrual function and acanthosis, however, was not sustained. This case provides evidence for interdependence of insulin action and insulin degradation in humans.

Modeling of plasma disappearance of unlabeled insulin in man

Fifty-two portal and 68 peripheral, brief infusions of unlabeled insulin were given to ambulant, nondiabetic patients. After intraportal insulin infusion (5--50 mU/kg), plasma clearance rate (PCR, dose/area of the incremental plasma insulin concentrations) decreased with increasing dose, varying from 32 to 14 ml-min-1-kg-1 at normoglycemia. After peripheral insulin infusion (5--30 mU/kg), PCR (mean value 15 ml-min-1-kg-1) showed no certain dose-dependence, but transfer rate constants and distribution volumes did. Despite a detectable reentry of insulin from one or more extravascular pools to the plasma pool, transfer rate constants or distribution volumes could not be accurately determined. The shortcomings of conventional noncompartmental and compartmental models did not appear to be due to the dose-dependence demonstrated. Instead, the limitations of these models were caused mainly by the difficulty of defining a proper base-line concentration and, in particular, by the imprecision of the experimental data, indicating that it will be difficult to find more appropriate models from data obtained with unlabeled insulin.

Kinetic analysis of plasma insulin disappearance in nonketotic diabetic patients and in normal subjects. A tracer study with 125I-insulin

The studies so far reported on the metabolic clearance rate of insulin in human diabetes mellitus have given conflicting results, probably because they have been conducted on few patients and have used a variety of experimental techniques and data treatments. We investigated the kinetics of insulin distribution and degradation in 35 normal subjects and in 42 nonketotic, nonobese, overtly diabetic patients, of whom 26 were above 40 yr old and 16 were 40 yr old or less at diagnosis. The design of the study combined (a) the use of a tracer to perturb minimally the steady state and to avoid glucose infusion; (b) the preparation of purified [(125)I]-monoiodoinsulin, which has a metabolic behavior similar to that of native insulin; and (c) noncompartmental analysis of the plasma immunoprecipitable (125)I-insulin disappearance curves, which were recorded for 2 h after pulse i.v. injection of the tracer.Metabolic clearance rate was found to be similar in diabetics (404+/-18 ml/min.m(2), mean+/-SEM) and in normals (420+/-14), although the latter-onset patients had slightly, if not significantly, lower metabolic clearance rate values than the earlier-onset diabetics (385+/-19 and 443+/-36, respectively). The initial distribution volume of the hormone also did not significantly differ in diabetics and normals and was similar to plasma volume. The reentry rate into the initial distribution volume of the hormone and the total, plasma-equivalent distribution volume of insulin were both significantly raised in diabetics (251+/-12 ml/min.m(2) and 10.3+/-0.5 liters/m(2)) in comparison with normals (195+/-8 and 7.5+/-0.3). The posthepatic delivery rate of insulin was found to be slightly raised in later-onset diabetics (194+/-20 mU/h.m(2)), but somewhat reduced in earlier-onset diabetics (133+/-15) in comparison with normals (172+/-14); these differences reflected the different basal plasma insulin concentrations in these three groups. Chronic treatment with oral hypoglycemic drugs, age, duration of the disease, and degree of metabolic control appeared to have only little effect on the kinetics of insulin.On the basis of these results, we conclude that insulin-independent adult diabetics show, already in the fasting state, a combination of insulin resistance and insulin deficiency and a derangement in insulin distribution, the precise significance of which is uncertain.

B1-3,5-diiodotyrosine insulin: a valid tracer for insulin

Insulin, specifically substituted at the PheB1 position with 3,5-diiodotyrosine, has been tested in several biological and immunological systems. Immunoreactivity was assessed using antisera specific for different parts of the insulin molecule. Biological activity in vitro was estimated on isolated rat fat cells. In vivo bioactivity (hypoglycaemia) and metabolism (metabolic and urinary clearance rates, half-life, apparent distribution space) were measured by infusion of the material into greyhounds. The results indicated that this B1-labelled insulin preparation was biologically fully active and, unlike randomly labelled preparations of iodoinsulin, was metabolised with kinetics indistinguishable from those of the unlabelled hormone. We suggest that this material is a valid tracer for insulin, fulfilling the criteria of high specific activity and biological identity to the native hormone.

Determinant competition during the immune response to N-acyl derivatives of ox insulin in the Hartley guinea-pig

Subgroups of female Hartley guinea-pigs were immunized with N-carbamylated ox insulin, N-maleylated ox insulin, N-phthaloylated ox insulin, or with the crystalline ox insulin from which the N-acylated insulins had been prepared. The immunogens were administered in water-in-oil emulsions containing pertussis vaccine as adjuvant. Sera obtained 20 days after secondary immunization were assayed for their antibody titres to iodo-ox insulin and their insulin-binding capacities. The data were log transformed for statistical comparison. N-carbamylated ox insulin seemed to be as immunogenic as crystalline ox insulin and no specific carbamyl hapten antibody could be found. N-maleylated and N-phthaloylated ox insulins yelded significantly less antibody cross-reactingwith iodo-ox insulin, but produced a complementary quantity of specific maleyl and phthaloyl hapten antibody respectively. Thus it was shown that in the system used the immune response was partitioned between different determinants, ox insulin and its N-acylated derivatives being equipotent immunogens.

Aspects of the secondary antibody response to ox insulin in the Hartley guinea-pig; the use of chemically modified ox insulin to delineate the antigenic determinants of ox insulin

The use of a quinea-pig model to study the immunogenicity of the insulin molecule is presented. The Hartley guinea-pig has been shown consistently to form antibody to ox insulin, when given in a water-in-oil emulsion containing pertussis vaccine as adjuvant. After log transformation of standardized antibody titres to iodo-ox insulin, a valid statistical comparison of the antibody response to different ox insulin preparations could be made. Antibody cross-reacting with ox insulin, but not iodo-ox insulin, was also detected. The quantity of one type of antibody was complementary to the other, an observation compatible with determinant competition having occurred during the immune response. From the results of cross-reactivity experiments using N-triacylated ox insulins and human insulin, it was shown that antibody cross-reacting with iodo-ox insulin had most probably been produced to a localized area of the molecule.

Influence of pregnancy on the kinetics of insulin

The disappearance rate of intravenously injected insulin was investigated in the serum of 30 women during the third trimester of pregnancy and 6 to 8 weeks post partum, in order to determine whether pregnancy has an influence on insulin kinetics in human subjects. Both women with unimpaired glucose tolerance and those with latent diabetes were included in this study. The disappearance rate of exogenous serum insulin in pregnancy was characterized by a two-compartment model. Multivariate analyses of variance were used to determine whether the estimated parameters of this model during pregnancy differ from those obtained after the puerperium and whether the insulin kinetics are altered when carbohydrate metabolism is disturbed. The kinetics of insulin during pregnancy did not differ from those after pregnancy. Thus, hyperinsulinemia observed in pregnancy cannot be explained by a change in the insulin kinetics. It appears improbable that the insulin-degrading enzyme activities of the placenta participate in degradation of insulin circulating in the maternal blood. A connection between the decline of glucose tolerance during pregnancy and the kinetics of exogenous insulin could not be found.