Diabetic ketoacidosis

This article reviews the pathogenesis of diabetic ketoacidosis (DKA) and provides a rational approach to the management of patients with DKA. The therapeutic discussion includes the use of low-dose insulin, no bicarbonate, or phosphate therapy on the majority of DKA patients, based on numerous prospective randomized studies. The article also discusses controversial subjects such as the use of hypotonic versus isotonic saline with and without colloids, hydration of subjects before insulin therapy, and admission of DKA patients to the general ward versus emergency ward versus intensive care unit. A concise, simple protocol is also presented as a suitable reference for management of patients with DKA.

Genetic and immunological markers of insulin dependent diabetes in Black Americans

ICA and GAD65 autoantibody profiles and HLA-DR and DQ analysis were performed on 43 Black juvenile onset IDDM patients and 34 unrelated Black controls from Tennessee, USA. 75% of patients were positive for GAD65 autoantibodies but only 53% had ICA; 39% both ICA and GAD65 antibodies. The strongest HLA association was with the DR3 haplotype DRB1*03 DQA1*0501 DQB1*0201 (63% of patients v 12% of controls RR = 13.0, p < 0.00002). DRB1*04 DQA1*0301 DQB1*0302, associated with IDDM in Caucasians but rare in Negroids, occurred in 27% of patients and 6% of controls (RR = 5.9, p < 0.04). All patients carried DQB1*0302 or DQB1*0201. DQB1*0602 was significantly reduced in patients (2.4% v 41%, RR = 0.036, p < 0.008) and DRB1*1501 was absent in patients (0% v 35%). The frequency of GAD65 autoantibodies in Black American IDDM patients is comparable to that in Caucasians; however ICA positivity is reduced. GAD65 antibodies may therefore be a more sensitive serological test to identify individuals in the Black American general population for markers associated with increased risk of developing IDDM. Current screening methods for predicting preclinical IDDM in Caucasians relies on a combination of immune and HLA markers of IDDM; studies of these markers in the Black Americans will make it possible to extend these options to additional genetically diverse populations.

Evidence for a defect in insulin metabolism in hyperandrogenic women with polycystic ovarian syndrome

It has been well established that the hypertestosteronemia of patients with polycystic ovarian syndrome (PCO) is associated with hyperinsulinemia and insulin resistance. We have recently noted a disparity between serum levels of insulin and C-peptide in certain hypertestosteronemic women with PCO and hypothesized a possible association between testosterone and insulin metabolism. Therefore, we have studied insulin clearance (baseline steady-state ratios of C-peptide to insulin) in 15 obese PCO women, 12 weight-matched controls (OC), and nine lean controls (LC), and examined the interactions of testosterone and insulin metabolism by examining the correlations between testosterone and insulin clearance and by studying the direct in vitro actions of testosterone on T-lymphocyte insulin binding and degradation. We found that the C-peptide to insulin ratio at baseline and T-lymphocyte insulin degradation of the PCO group were twofold below the LC and OC values. Basal C-peptide to insulin ratios and insulin-degradative activities were significantly and negatively interrelated (r = .56, P < .01), and both of these parameters were highly correlated (P < .01) with basal testosterone levels (r = .49 for basal C-peptide to insulin and r = -.61 for insulin degradation). In experiments where testosterone was added to cell cultures, insulin degradation was impaired in a biphasic fashion. We conclude that (1) elevated testosterone levels may contribute to impairments in insulin metabolism, and (2) the hyperinsulinemia of hyperandrogenic women may occur in part from defects in insulin clearance and peripheral tissue insulin degradation.

Enhanced adrenocortical activity as a contributing factor to diabetes in hyperandrogenic women

The high incidence of non-insulin-dependent diabetes mellitus (NIDDM) in women with polycystic ovarian syndrome (PCO) is believed to occur secondary to the insulin resistance associated with their androgenicity. In the present study, we have examined the interrelationships between glucose tolerance, androgenicity, and various in vivo and in vitro parameters of insulin sensitivity in 11 obese PCO patients with NIDDM, 14 PCO patients without diabetes, and 14 weight-matched controls. Both groups of PCO patients were hypertestosteronemic, hyperinsulinemic, and insulin-resistant when compared with a group of weight-matched controls. However, PCO patients with NIDDM differed from those without diabetes in that they had elevated basal and corticotropin-stimulated adrenal steroids (cortisol, dehydroepiandrosterone [DHEA], dehydroepiandrosterone sulfate [DHEAS]). The hyperglycemia of our diabetic patients was not related to their elevated testosterone levels or to their degree of insulin resistance, but was significantly and positively correlated with adrenal hypersecretion, which in turn was associated with postreceptor defects in insulin action. These findings would suggest that enhanced adrenocortical activity may be an important factor underlying the development of NIDDM in women with PCO.

Case report: amelioration of insulin resistance in diabetes with dehydroepiandrosterone

In hyperandrogenic females, the ratio of dehydroepiandrosterone (DHEA) to testosterone may be an important determinant of insulin sensitivity. This study involved changes in insulin sensitivity and glucose metabolism with therapeutic manipulation of DHEA (S)/testosterone in a female patient with non-insulin-dependent diabetes and hyperandrogenism. Therapeutic intervention included 1-month treatment with 0.25 mg dexamethasone at bedtime and 1-month dexamethasone + DHEA. Insulin sensitivity and glucose tolerance were assessed before and after each treatment regimen by examining: 1) fasting and oral glucose tolerance test glucose and insulin levels, 2) hypoglycemic response to intravenous insulin, and 3) erythrocyte insulin receptor binding. With dexamethasone alone, DHEAS, testosterone, and their ratio were reduced with a concomitant increase (30%) in oral glucose tolerance test insulin levels and a decrease (33%) in erythrocyte insulin binding. With DHEA + dexamethasone, the ratio of DHEAS/testosterone increased 16-fold along with a marked improvement in insulin sensitivity, as determined by a more than 30% reduction in fasting and oral glucose tolerance test insulin levels, a threefold stimulation of the rate of glucose disappearance with intravenous insulin, and a 30% increase in insulin binding. DHEA improved insulin sensitivity and reduced fasting and oral glucose tolerance test glucose levels and ameliorated the diabetic state. The ratio of DHEAS/testosterone is an important regulator of insulin sensitivity and glucose tolerance and that DHEA therapy may be beneficial in the treatment of certain forms of insulin resistance.

Opposing actions of dehydroepiandrosterone and testosterone on insulin sensitivity. In vivo and in vitro studies of hyperandrogenic females

It has been hypothesized that the androgens testosterone and dehydroepiandrosterone (DHEA) may have opposing actions on insulin sensitivity. To test this hypothesis, we selected patients with polycystic ovary syndrome (PCO) and hypertestosteronemia and a group of individuals with adrenal hyperplasia (AH) and elevated DHEA and studied their 1) insulin and glucose responses to a 75-g oral glucose tolerance test, 2) insulin resistance by hypoglycemic responses to a standard dose of intravenous (IV) insulin, and 3) insulin binding and pyruvate dehydrogenase (PDH) responsiveness to insulin in phytohemagglutinin (PHA)-activated T lymphocytes. PCO patients exhibited elevated basal and glucose-challenged insulin levels and had blunted hypoglycemic responses to IV insulin. Conversely, AH patients had hypoglycemic responses to IV insulin significantly greater than and basal and glucose-challenged insulin levels lower than the PCO patients and weight-matched control subjects. In vitro, T-lymphocyte insulin binding of the PCO patients was 40-60% below control values; in AH patients, insulin binding and PDH insulin sensitivity were above those of the control subjects. Testosterone levels in all study subjects were negatively correlated to T-lymphocyte insulin binding and positively correlated to basal insulin, insulin area under the curve (AUC), and insulin-glucose indices. DHEA levels were positively correlated to insulin binding and inversely related to basal insulin, insulin AUC, and insulin-glucose indices. In all instances, the parameters of insulin sensitivity were more strongly correlated to individuals' ratios of DHEA to testosterone than to either of these androgens alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of adrenal and gonadal androgens in insulin action and metabolism

In a previous report we found extreme hyperinsulinemia associated with high testosterone levels in patients with polycystic ovarian syndrome (PCO) and normal insulin levels in a small group of patients with elevated dehydroepiandrosterone (DHEA). From these observations, we hypothesized that DHEA and testosterone may have opposing actions on insulin sensitivity. To test this hypothesis, we studied insulin sensitivity in vivo and in vitro in a) obese PCO women with elevated testosterone, b) obese patients with adult-onset adrenal hyperplasia (AH) and high levels of DHEA, c) weight-matched obese controls, and d) lean controls. Insulin sensitivity was determined by insulin responses to a standard OGTT, hypoglycemic responses to an IV insulin tolerance test (ITT), red blood cell (RBC) insulin binding and receptor kinase activity, and phytohemagglutinin (PHA)-activated T-lymphocyte (T-cell) insulin binding and PDH insulin sensitivity. In PCO patients, we found that basal and glucose-challenged insulin levels were significantly greater than, and hypoglycemic responses to IV insulin, significantly lower than, weight-matched control values. However, AH patients had insulin values significantly below, and hypoglycemic responses significantly above, those of the weight-matched controls. Their values were, in fact, comparable to those observed for the lean control subjects. Similar findings were observed with insulin binding and PDH insulin sensitivity. Insulin sensitivity in all study subjects was found to be negatively correlated to testosterone and positively correlated to DHEA and, more significantly, to the ratio of DHEA/testosterone. These data would suggest that, in females, DHEA and T may have opposing actions on insulin sensitivity. We conclude that in females insulin sensitivity in vivo and in vitro is modulated, at least in part, by the ratios of DHEA to testosterone.

Pseudonormoglycemia in diabetic ketoacidosis with elevated triglycerides

A 24-year-old newly diagnosed male patient with diabetes presented with diabetic ketoacidosis (DKA) (pH 7.16, HCO3 6.0) and extreme hypertriglyceridemia (239.35 mmol/L). The diagnosis of DKA was delayed because of the apparent depression of the true serum glucose (to 11 mmol/L). He was treated with intravenous (IV) insulin and rehydration, which normalized his pH, HCO3, and triglyceride levels. To the authors' knowledge, this is both the highest triglyceride level recorded and the first report of a high triglyceride level as the apparent cause of a factitiously low glucose level.

Bimodal correlation between the circulating insulin level and the production rate of dehydroepiandrosterone: positive correlation in controls and negative correlation in the polycystic ovary syndrome with acanthosis nigricans

The objective of this study was to define the relationship among the circulating insulin level (IRI), the MCR of dehydroepiandrosterone (MCR-D), and the production rate of DHEA (PR-D) in 10 women with the polycystic ovary syndrome and acanthosis nigricans (PCOS-AN). Seven normal weight and 10 obese women served as controls. Measurement of the MCR-D and PR-D was accomplished by iv saline infusion (0.9% NaCl solution) on day 1 as a control and a 4-h iv dehydroepiandrosterone (unlabeled) infusion (1 mg/h) was performed on day 2. Mean MCR-D was more than 2-fold higher in the obese controls compared to the normal weight controls. However, the plasma concentration of DHEA(PC-D) was not significantly different in the normal and obese control women, since the PR-D was increased proportionately to the MCR-D. The MCR-D and the PR-D were coupled through insulin in the control women, and their correlation coefficients with IRI were positive and identical (0.77 and 0.73, respectively). In contrast, IRI was negatively correlated with PR-D in the PCOS-AN women. Also, unlike the control women, there was minimal or no change in the MCR-D across a broad range of IRI in the PCOS-AN women. Thus, the MCR-D and PR-D were not coupled in these women.

Sensitivity of pyruvate dehydrogenase to insulin in activated T lymphocytes. Lack of responsiveness to insulin in patients with polycystic ovarian disease and diabetes

Using phytohemagglutinin-activated T lymphocytes, we studied possible mechanisms responsible for insulin resistance in patients with polycystic ovarian disease (PCO) and acanthosis nigricans (AN) by examining insulin binding to erythrocytes and activated T lymphocytes and T-lymphocyte pyruvate dehydrogenase (PDH) responsiveness to insulin in three groups. These groups of subjects consisted of six PCO-AN patients with normal glucose tolerance, six PCO-AN patients with mild non-insulin-dependent diabetes mellitus (NIDDM), and six weight-matched control subjects. We found that insulin binding to both erythrocytes and activated T lymphocytes was significantly lower in PCO and PCO-NIDDM patients than control subjects but did not differ between the PCO groups. Insulin binding to erythrocytes and T lymphocytes varied inversely with basal insulin. In activated T lymphocytes of PCO-NIDDM patients, PDH responsiveness to both submaximal and maximal insulin concentrations was impaired, the extent of which varied in proportion to their degree of carbohydrate intolerance. In contrast, PDH responsiveness to maximal amounts of insulin in T lymphocytes of PCO patients without NIDDM was similar to the weight-matched control subjects. These data may suggest that lesions at the level of the receptor are primarily responsible for insulin resistance in patients with PCO but that both receptor and postreceptor defects (i.e., PDH responsiveness to insulin) contribute to the insulin-resistant state of PCO patients with NIDDM.

Identification of insulin intermediates and sites of cleavage of native insulin by insulin protease from human fibroblasts

We have studied the time sequence degradation of native insulin by insulin protease from human fibroblast using multiple steps involving purification of the products by high performance liquid chromatography, determination of peak composition by amino acid sequence analysis, and confirmation of structure by mass spectrometry and thus elucidated the sites of cleavage of insulin by human insulin protease. We observed that as early as 0.5 min of incubation, three major new peptide peaks, intact insulin, and four smaller peptide peaks can be detected. The major peptides are portions of the insulin molecule, with the amino ends of the A and B chains or the carboxyl ends of the A and B chains still connected by disulfide bonds. Peptide peak I is A1-13-B1-9. Peptide peak II is A1-14-B1-9. Peptide peak III is A14-21-B14-30. The smaller peptide peaks are A14-21-B17-30, A15-21-B14-30, A15-21-B10-30, and A14-21-B10-30. The major peptide bond cleavage sites therefore consist of A13-14, A14-15, B9-10, B13-14, and B10-17. With longer incubation times, peptide peak II appears to lose the A14 tyrosine to form peptide peak I. This peptide I, which is the amino end of the A and B chains, is not further degraded even after 1.5 h of incubation. With longer incubation times, the peptides containing the carboxyl ends of the A and B chains are further degraded to form products from cleavage at the A18-19, B14-15, B25-26, and a small amount of A19-20, B10-11, and B24-25 cleavage and the emergence of 2-5-amino acid peptide chains, tyrosine, alanine, histidine, and leucine-tyrosine. We conclude, based on the three-dimensional structure of insulin, that human insulin protease recognizes the alpha-helical regions around leucine-tyrosine bonds and that final degradation steps to small peptides do not require lysosomal involvement.

Diabetic ketoacidosis and hyperosmolar hyperglycemic nonketotic coma

Diabetic ketoacidosis and hyperosmolar hyperglycemic nonketotic coma are two of the most common acute complications of diabetes. The pathophysiologic changes that occur in both disease states represent an extreme example of the super-fasted state. The physiology of the fed and fasted state, evaluation, therapeutic issues, recommendations for therapy, immediate follow up care, and complications of therapy are reviewed for both syndromes.

Divergent correlations of circulating dehydroepiandrosterone sulfate and testosterone with insulin levels and insulin receptor binding

We evaluated the insulin response to a standard oral glucose tolerance test (OGTT) and in vitro insulin binding to erythrocytes (RBC) in 26 women from 3 groups: Group NW, normal women (n = 11); Group DS, women (n = 9) with elevated serum DHEAS concentrations, greater than 400 micrograms/dl (greater than 10.84 mumol/L); and Group IR, women (n = 6) with elevated basal plasma insulin concentrations (IRI). There was a significant linear correlation between the area under the insulin response curve (IRI-AUC) and serum testosterone (T) (r = 0.78, p = 0.0001). Using stepwise multiple linear regression, IRI-AUC was characterized as a function of both serum T and DHEAS; positively with T and negatively with DHEAS. In vitro (n = 17), there was a positive correlation between RBC-insulin binding and serum DHEAS (r = 0.54, p = 0.029) and a negative correlation between RBC-binding and T (r = -0.57, p = 0.017). We conclude that DHEAS may enhance insulin binding and action and that DHEAS and T have divergent functional relationships with IRI. DHEAS and T may therefore exert opposing effects on insulin secretion and action.

Increased insulin binding of erythrocytes and insulin sensitivity in adrenal insufficiency

We have studied 125I-insulin binding to erythrocytes (RBC) in five patients with hypoadrenocortisolism, and compared to 17 normal subjects and in nine patients with Cushing's syndrome. In another study insulin sensitivity index (ISI) was measured by the IV insulin tolerance test in four patients with hypoadrenocortisolism (1.82 +/- 0.15 mg/dL/min), and compared to 19 normal subjects and 23 patients with Cushing's syndrome (1.56 +/- 0.1 mg/min/dL). Mean insulin binding in hypocortisolism was 17.9 +/- 0.7%, and was significantly higher (P less than .01) than in normal subjects (12.0 +/- 1.4%) and was significantly (P less than .001) decreased toward normal (11.8 +/- 1.47) during replacement therapy. Increased binding in untreated hypoadrenocortisolism was due to elevated high affinity site receptor concentration as compared to the treated patients (0.10 +/- 0.015 v 0.053 +/- 0.003 nmol/L,P less than .01). These results suggest that increased insulin binding in chronic hypoadrenocortisolism may be attributed to increased insulin binding to the receptor, which can revert to normal by replacement therapy. The role of increased insulin binding to increased insulin sensitivity in hypoadrenocortisolism is discussed.

Combined therapy of insulin and tolazamide decreases insulin requirement and serum triglycerides in obese patients with noninsulin-dependent diabetes mellitus

Insulin requirements, C-peptide levels, and serum lipids have been assessed in 12 obese, insulin-requiring (greater than 60 U/day) patients with type II diabetes mellitus, in a randomized crossover fashion with two treatment regimens: NPH alone and combined NPH and tolazamide, over a period of 3 months each, with maintenance of weight and glycemic control (HgA1, 2hpp and mean 24h glucose profile) at comparable levels. Serum cholesterol improved in both groups compared to their respective baseline values (p less than 0.05). In addition, serum triglyceride was lower (p less than 0.05) in the combined therapy as compared with NPH alone therapy. Insulin requirements were decreased by 23% (p less than 0.002) in the combined therapy group, without significant change in weight, glycemic control, or C-peptide levels. However, C-peptide increments in the combined therapy group were significantly higher than the baseline by 70% (p less than 0.02). NPH plus tolazamide therapy as compared with NPH alone lowers insulin requirement in obese, type II diabetic women without significant alteration in glycemic control, possibly by an increased tissue sensitivity to insulin, and decreases serum triglyceride levels.

Glyburide-induced hepatitis

Drug-induced hepatotoxicity, although common, has been reported only infrequently with sulfonylureas. For glyburide, a second-generation sulfonylurea, only two brief reports of hepatotoxicity exist. Two patients with type II diabetes mellitus developed an acute hepatitis-like syndrome soon after initiation of glyburide therapy. There was no serologic evidence of viral infection, and a liver biopsy sample showed a histologic pattern consistent with drug-induced hepatitis. Both patients recovered quickly after stopping glyburide therapy and have remained well for a follow-up period of 1 year. Glyburide can produce an acute hepatitis-like illness in some persons.

Maintenance of normal circulating levels of delta 4-androstenedione and dehydroepiandrosterone in simple obesity despite increased metabolic clearance rates: evidence for a servo-control mechanism

To study the effect of obesity on the metabolism of adrenal androgens not bound to testosterone-estradiol-binding globulin, the MCRs of delta 4-androstenedione (A) and dehydroepiandrosterone (DHEA) were determined using constant infusion of unlabeled steroids to steady state in 8 normal weight and 19 obese nonhirsute eumenorrheic women. The blood production rates (PR) were calculated as the product of the MCR and the 24-h integrated serum concentrations (IC). The mean MCR and PR of A and DHEA were significantly higher in the obese women than in the normal weight women. There was, however, no difference in the mean IC of each androgen in the 2 groups. The MCR and PR of A and DHEA were each correlated with the body mass index (BMI; kilograms per m2). The MCR and PR of A and the MCR of DHEA were also correlated with the ratio of waist circumference to hip circumference (WHR). However, the PR of DHEA was not correlated with WHR. There was no correlation between the IC of either androgen and BMI or WHR. However, partial correlation analysis revealed that correction of the BMI for WHR resulted in a significant negative correlation between BMI and IC of A. We conclude that the MCR and PR of A and DHEA were increased in obese nonhirsute eumenorrheic women; there was a strong correlation between BMI and the MCR and PR of A and DHEA; upper segment obesity, as measured by WHR, was correlated with the MCR and PR of A and the MCR of DHEA, but not with the PR of DHEA; and circulating DHEA and A were maintained at normal levels in the obese eumenorrheic women despite an increase in the MCR, which suggests that a servo-mechanism is operative which registers the body size and adjusts the PR according to the MCR.

Reduction of hyperinsulinemia and insulin resistance by opiate receptor blockade in the polycystic ovary syndrome with acanthosis nigricans

We previously reported that circulating beta-endorphin levels are increased in obese hirsute women and that plasma immunoreactive insulin (IRI) levels are increased in proportion to the degree of hyperandrogenism in women with the polycystic ovary (PCO) syndrome. We, therefore, tested the hypothesis that endogenous opiates are at least partially responsible for the hyperinsulinemia and insulin resistance in this syndrome. In the first study, acute naloxone administration significantly reduced the plasma IRI response and IRI/glucose ratio in three euglycemic obese women with PCO and acanthosis nigricans (AN) and marked insulin resistance, but did not alter the glucose response. Naloxone had no effect on these parameters in the normal weight control subjects. In the second study, nalmefene, a new, orally active opiate antagonist, reduced IRI and the IRI/glucose ratio in four women with PCO-AN and marked hyperinsulinemia in a randomized, double blind, crossover protocol. We conclude that endogenous opiates are at least partially responsible for the hyperinsulinemia and insulin resistance in PCO-AN.

Bicarbonate therapy in severe diabetic ketoacidosis

Twenty-one adult patients with severe diabetic ketoacidosis entered a randomized prospective protocol in which variable doses of sodium bicarbonate, based on initial arterial pH (6.9 to 7.14), were administered to 10 patients (treatment group) and were withheld from 11 patients (control group). During treatment, there were no significant differences in the rate of decline of glucose or ketone levels or in the rate of increase in pH or bicarbonate levels in the blood or cerebrospinal fluid in either group. Similarly, there were no significant differences in the time required for the plasma glucose level to reach 250 mg/dL, blood pH to reach 7.3, or bicarbonate level to reach 15 meq/L. We conclude that in severe diabetic ketoacidosis (arterial pH 6.9 to 7.14), the administration of bicarbonate does not affect recovery outcome variables as compared with those in a control group.

Phytohemagglutinin (PHA) activated human T-lymphocytes: concomitant appearance of insulin binding, degradation and insulin-mediated activation of pyruvate dehydrogenase (PDH)

Binding and degradation of A14125I-Insulin as well as the effect of insulin on pyruvate dehydrogenase (PDH) activation were studied in non-stimulated and phytohemagglutinin (PHA)-stimulated thymic-derived lymphocytes (T-lymphocytes) of man under varying conditions of time, temperature, and cell concentration. The nonstimulated viable T-lymphocytes exhibited neither binding, degradation, nor PDH activation in response to insulin. With PHA stimulation, a time and temperature-dependent binding was noted in T-lymphocytes which paralleled the appearance of cell-associated insulin degrading activity. Concomitant with the emergence of insulin binding and degrading activities in these cells, PDH activation was observed which was responsive to as little as 5.0 microU/ml of insulin. We conclude that in PHA-activated T-lymphocytes of man the process of insulin binding and degradation is closely related to insulin sensitive activation of PDH. These activated cells may serve as a useful model in which to study insulin binding and processing, as well as effects of insulin on postreceptor events.

The effect of inhibitors of insulin processing on generation of insulin intermediate products from human fibroblast as detected by high performance liquid chromatography (HPLC)

To assess the role of various modulators of insulin processing on cell-associated A14-125I-insulin intermediates in human fibroblasts, we have studied the effect of N-ethylmaleimide (NEM), chloroquine, bacitracin, dansylcadavarine, and phenylarsine oxide on generation of these intermediate products with the use of HPLC. NEM completely inhibited generation of intermediate peaks or iodotyrosine. Chloroquine inhibited conversion of A14-125I-insulin to iodotyrosine by about 75 percent and the remaining A14-125I-insulin was not susceptible to acid wash. Bacitracin, dansylcadavarine, and phenylarsine oxide, on the other hand, stimulated formation of intermediate products with concomitant inhibition of iodotyrosine formation. We conclude that there are at least three components of insulin degradation in human fibroblasts. These include the sulfhydryl group inhibitor-sensitive, the intracellular chloroquine-sensitive, and membrane site inhibitor-sensitive components.

Characterization of insulin-degrading activity of intact and subcellular components of human fibroblasts

We have studied insulin degrading activity (IDA) in cultured human fibroblasts and assessed the effect of various inhibitors of insulin processing on IDA. To evaluate the role of three enzymes of insulin degradation (neutral protease, microsomal glutathione insulin transhydrogenase, and lysosomal acid protease), we subfractionated homogenized fibroblasts into membrane (and nuclei) cytosol, mitochondria, microsomes, and lysosomes. Greater than 90% of IDA was found to be present in the cytosolar fraction containing neutral protease. IDA in intact fibroblasts was completely inhibited by 1 mM N-ethylmaleimide and partially by 0.5 mM dansylcadaverine (75%), 0.5 mM chloroquine (48%), 1 mg/ml bacitracin (32%) and Trasylol (30%). Lidocaine (5 mM) and glucagon (10(-6)M) exhibited about 15% inhibition with minimal inhibition (7%) by nonsuppressible insulin-like activity. Study of similar inhibitors on subfractionated components indicated inhibition of cytosolar enzyme by N-ethylmaleimide (100%), glucagon (30%), chloroquine (41%), nonsuppressible insulin-like activity (30%), Lidocaine (25%), dansylcadaverine (16%), and bacitracin (11%). Incubation of ammonium sulfate-fractionated cytosolar enzyme at 37 C with A14-125I-insulin resulted in generation of two intermediate peaks as early as 1 min. These peaks could be identified by HPLC but not by molecular sieve chromatography. These intermediates exhibited less immunoprecipitability with antiinsulin antibody and receptor binding with liver membrane preparations than intact insulin. Further incubation of A14-125I-insulin with the cytosolar enzyme(s) resulted in reduction of these peaks as well as insulin and formation of 125Iodotyrosine peak. We conclude that human fibroblast is capable of metabolizing cell-associated A14-125I-insulin in a time- and temperature-dependent manner. This process is inhibited by various inhibitors of insulin processing. The bulk of IDA consists of soluble neutral protease(s) with properties similar to other more purified neutral insulin protease preparations. This fraction, similar to the intact fibroblast degrades insulin to two intermediates with similar molecular weight to that of intact insulin but with more hydrophilicity and less binding affinity to antiinsulin antibody and liver membrane than intact insulin.