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.

In vivo chloroquine-induced inhibition of insulin degradation in a diabetic patient with severe insulin resistance

We report on a 26-yr-old patient with an 11-yr history of insulin-dependent diabetes mellitus who exhibited insulin resistance with a requirement of up to 15,000 U of intravenous (i.v.) insulin/day. Attempts to diminish her insulin requirement by administration of sulfated insulin or Trasylol were unsuccessful, with the patient remaining resistant to subcutaneous (s.c.) and i.v. administration of pure pork insulin. Chloroquine phosphate therapy (500 mg twice a day) resulted in a decreased requirement for i.v. insulin (700 U/day as compared with the pretreatment requirement of 8400 U/day). Accelerated insulin degradation in s.c. fat tissue of the patient before treatment with chloroquine was demonstrated. This activity was decreased by 64% during chloroquine therapy. Inhibition of insulin degrading activity (IDA) during chloroquine therapy was associated with reductions in the leukocyte lysosomal enzymes alpha-galactosidase and hexosaminidase-A but not hexosaminidase-B and beta-glucuronidase. This study constitutes the first reported use of chloroquine for treatment of insulin resistance as a result of accelerated insulin degradation, and it provides evidence of the effectiveness of this agent in this rare condition.

Activation of pyruvate dehydrogenase complex (PDC) of rat heart mitochondria by glyburide

The effects of the second generation sulfonylurea, glyburide, on the pyruvate dehydrogenase multienzyme complex (PDC) of rat myocardial tissue were examined using rat ventricular slices and isolated mitochondria. Therapeutic concentrations (10(-7) to 10(-6)M) of glyburide produced a 30% increase in the decarboxylation of [1(-14)C] pyruvate by the PDC of ventricular tissue. Addition of glyburide to intact rat heart mitochondria stimulated activity of the PDC in a time- and concentration-dependent manner. Half-maximal stimulation of the enzyme occurred with 6 X 10(-5)M glyburide and maximal activation of the enzyme was achieved with 1 X 10(-4)M glyburide. At the height of stimulation, PDC activities were 6-fold greater than those observed under control conditions with succinate alone. When mitochondria were disrupted by sonication or freeze-thawing, glyburide produced no stimulation of pyruvate decarboxylation. We conclude that glyburide directly stimulates the decarboxylation of pyruvate by the PDC of the myocardium. Furthermore, the presence of intact mitochondria is necessary for the stimulatory action of glyburide on the PDC.

Activation of pyruvate dehydrogenase complex by porcine and biosynthetic human insulin in cultured human fibroblasts

Cultured human fibroblasts represent an appropriate model for studying both insulin receptor interaction and hormone responsiveness. We have investigated the properties of the pyruvate dehydrogenase multi-enzyme complex (PDC) and have studied the effects of various concentrations of porcine and biosynthetic human insulin (BHI) on the activity of the enzyme. Under optimal conditions of the assay, both BHI and porcine insulin activated PDC in a dose-dependent fashion in which full activation of the enzyme was achieved with 10(-8) M insulin. The half-maximal concentration for porcine and human insulin was similar, occurring at the level of 5 X 10(-9) M for activation of the PDC of human fibroblasts. We conclude that the PDC of cultured human fibroblasts is activated by both human and porcine insulin at a comparable physiologic concentration. Human fibroblasts may therefore serve as a useful model to study insulin action in isolated human tissue.

A randomized study of phosphate therapy in the treatment of diabetic ketoacidosis

The use of phosphate therapy in the management of diabetic ketoacidosis (DKA) has been controversial, particularly with respect to the effect of phosphate intermediates on tissue oxygenation. In a prospective randomized study we evaluated the effect of phosphate (8.5 mmol/h or approximately 6 g phosphate/24 h) (experimental group) vs. no phosphate therapy (control group) in 30 DKA patients, 15 in each group. Various determinations including erythrocyte 2,3-diphosphoglycerate (2,3-DPG), oxyhemoglobin dissociation (p50), serum phosphate, calcium, lactate, pyruvate, electrolytes, and response time to reach predetermined values for glucose, bicarbonate, and pH were measured at frequent intervals during the first 24 h of therapy and daily for 5 days after metabolic control. Initial electrolytes, glucose, pH, erythrocyte 2,3-DPG, lactate, and p50 were not different in either group. Whereas the experimental group had a greater level of 2,3-DPG than the control group by 48 h, the difference was not statistically significant. Recovery indices, including hours to reach glucose of 250 mg/dl, bicarbonate greater than 15 meq/liter, pH greater than 7.3, and mental alertness, were not different in the two groups nor were the p50 or lactate measurements. The experimental group exhibited significantly lower plasma ionized calcium values during therapy. We conclude that phosphate therapy may accelerate regeneration of erythrocyte 2,3-DPG but in the relatively small number of patients studied it had no demonstrable influence on tissue oxygenation or clinical response to low dose insulin therapy of DKA. Furthermore, the exaggeration of hypocalcemia seen in phosphate-treated patients may be reason for caution in the use of such therapy.

Early detection of degraded A14-125I-insulin in human fibroblasts by the use of high performance liquid chromatography

We studied the metabolism of A14-125I-insulin in intact human fibroblasts using high performance liquid chromatography (HPLC) to detect and separate its early degradation products. The high resolving power of HPLC enabled us to separate what has been considered "intact insulin" by Sephadex G-50 chromatography or TCA precipitability into two additional peaks that had decreased biochemical properties with respect to immunoprecipitability and receptor binding but not decreased TCA precipitability. We conclude that human fibroblast is capable of metabolizing insulin within 2 min at 37 degrees C into intermediate molecules that can be detected by HPLC but not by TCA precipitability or molecular sieve chromatography.

A rapid means of separating A14-125I-insulin from heterogeneously labeled insulin molecules for biologic studies

We have used two methods for the preparation of a highly homogeneous insulin with high specific activity. After iodination with chloramine T, the labeled peptides were retained on a disposable Sep Pak cartridge and subsequently eluted. The eluted labeled insulins were further purified by either DEAE cellulose or high performance liquid chromatography (HPLC) to separate A14-125I- from A19-125I-insulin. Both methods of chromatography were effective, but HPLC offered the advantage of better resolution in less time and higher yields of A14-125I-insulin, which is suitable for biologic studies in various target tissues.

Insulin degradation by human skeletal muscle

Although previous studies from this and other laboratories have extensively characterized insulin degrading activity in animal tissues, little information has been available on insulin responsive human tissues. The present study describes the insulin degrading activity in skeletal muscle from normal human subjects. Fractionation of a sucrose homogenate of skeletal muscle demonstrated that 97% of the total neutral insulin degrading activity was in the 100 000 x g supernatant with no detectable glutathione-insulin transhydrogenase activity. The 100000 x g pellet contained 85% of the total acid protease activity and all the glutathione-insulin transhydrogenase activity. The soluble insulin degrading activity was purified 1400-fold by ammonium sulfate fractionation, molecular exclusion, ion-exchange and affinity chromatography. Enzymatic activity was determined by measuring an increase in trichloroacetic acid-soluble products of the 125I-labeled hormone substrates. The purified enzyme showed marked proteolytic specificity for insulin with a Km of 1.63 X 10(-7)M (+/-0.32) and was competitively inhibited by proinsulin and glucagon with Ki values of 2.1 X 10(-6)M and 4.0 X 10(-6)M, respectively. This insulin protease exhibited a pH optimum between 7 and 8, a molecular weight of 120000 and was capable of degrading glucagon. Inhibition studies demonstrated that a sulfhydryl group is essential for activity. Molecular exclusion chromatography of [125I]insulin degraded products revealed a time-dependent increase in degradation products with molecular weights intermediate between intact insulin and iodotyrosine. These studies demonstrate that the major enzymatic system responsible for insulin degrading activity is a soluble cysteine protease capable of rapidly metabolizing insulin under physiologic conditions.

Characterization and solubilization of the specific binding sites for d-alpha-tocopherol from human erythrocyte membranes

Previous work from our laboratory has demonstrated the presence of specific binding sites for d-alpha-tocopherol (vitamin E) in intact human erythrocytes [A. E. Kitabchi and J. Wimalasena, Biochim. biophys. Acta 684, 300 (1982)]. The binding was time, temperature and cell concentration dependent. To localize the binding sites, red blood cells were further fractionated; greater than 90% of the tocopherol binding sites were localized on membranes. The washed membrane fraction from normal human erythrocytes has specific binding sites for d-alpha-tocopherol with properties suggestive of protein receptors. Two binding sites with Ka values of 3.31 x 10(1)M-1 and 1.51 x 10(6)M-1 were demonstrated, and solubilized d-alpha-tocopherol binding site complexes were resolved to a major component with an Mr of 65,000 and a minor component with an Mr of 125,000.

Hypercortisolism and insulin resistance: comparative effects of prednisone, hydrocortisone, and dexamethasone on insulin binding of human erythrocytes

We have studied the effects of 3 days of hydrocortisone (30 mg every 8 h), dexamethasone (1 mg every 8 h), and prednisone (7.5 mg every 8 h) ingestion on glucose intolerance and insulin resistance in three groups of lean normal volunteers and compared these parameters to specific insulin binding in erythrocytes. All three glucocorticoids caused significant reduction of glucose tolerance as assessed by glucose and insulin areas under oral glucose tolerance text curves and insulin sensitivity in response to 0.1 U insulin/kg BW, iv. Although both hydrocortisone and dexamethasone caused significant reduction in insulin binding compared to that during the pretreatment period (11.0 +/- 0.7 vs. 9.0 +/- 0.5% (P less than 0.01) and 11.8 +/- 0.7 vs. 9.0 +/- 0.5% (P less than 0.05), respectively), prednisone ingestion did not significantly alter insulin binding (10.6 +/- 0.6 before vs. 9.4 +/- 0.5% after). Decreased insulin binding with hydrocortisone and dexamethasone was caused by decreased binding affinity rather than by a decreased number of receptors. In two subjects in whom receptor binding was measured daily for 3 days during prednisone (one subject) or hydrocortisone (one subject) ingestion, the inhibition of binding was highest on the first day, with subsequent reduction of this inhibited binding toward normal by the third day. We conclude that although hydrocortisone, dexamethasone, and prednisone all cause deterioration of glucose tolerance and decreased insulin sensitivity, only hydrocortisone and dexamethasone exhibit significant decreases in insulin binding to erythrocytes. This decreased binding is not due to changes in receptor numbers but to decreased affinity brought about by hyperinsulinemia. Prednisone ingestion caused significant insulin resistance with an insignificant decrease in insulin binding. We believe that other mechanisms, such as alteration of postreceptor events, may play a major role in the induction of insulin resistance in hypercortisolism in man.

Increased insulin binding to erythrocytes in diabetic ketoacidosis: normalization with insulin therapy

We studied insulin binding to erythrocytes in eight male type I diabetic patients in diabetic ketoacidosis (DKA) before and 24 h and 5 days after low dose insulin treatment. Mean specific [125I]insulin binding in DKA (mean +/- SEM, 13.3 +/- 0.6%) was significantly higher than values 24 h and 5 days after treatment (10.7 +/- 0.7% and 9.6 +/- 0.4%, respectively; P less than 0.001). These values after treatment were similar to those of 15 age-matched normal males (10.5 +/- 0.4%) and 6 controlled type I diabetic patients (10.1 +/- 0.7%). The affinity constant (Ka = 2.26 +/- 0.12 10(9) M-1) was significantly higher in DKA patients than in normal subjects (1.75 +/- 0.15 10(9) M-1) or type I controlled diabetics (1.55 +/- 0.15 10(9) M-1; P less than 0.01). Insulin binding correlated inversely with arterial pH (r = -0.80; P less than 0.001; n = 8). These results indicate an increase in specific [125I]insulin binding to erythrocytes in DKA, which is due to elevated receptor affinity (but not receptor numbers) and is reversible with treatment.

Demonstration of specific binding sites for 3H-RRR-alpha-tocopherol on human erythrocytes

Previous work from our laboratory demonstrated specific binding sites for 3H-RRR-alpha-tocopherol (3H-d alpha T) in membranes of rat adrenal cells. As tocopherol deficiency is associated with increased susceptibility of red blood cells to hemolysis, we investigated tocopherol binding sites in human RBCs. Erythrocytes were found to have specific binding sites for 3H-d alpha T that exhibited saturability and time and cell-concentration dependence as well as reversibility of binding. Kinetic studies of binding demonstrated two binding sites--one with high affinity (Ka of 2.6 x 10(7) M-1), low capacity (7,600 sites per cell) and the other with low affinity (1.2 x 10(6) M-1), high capacity (150,000 sites per cell). In order to localize the binding sites further, RBCs were fractionated and greater than 90% of the tocopherol binding was located in the membranes. Similar to the findings in intact RBCs, the membranes exhibited two binding sites with a respective Ka of 3.3 x 10(7) M-1 and 1.5 x 10(6) M-1. Specificity data for binding demonstrated 10% binding for RRR-gamma-tocopherol, but not other tocopherol analog exhibited competition for 3H-d alpha T binding sites. Instability data suggested a protein nature for these binding sites. Preliminary studies on Triton X-100 solubilized fractions resolved the binding sites to a major component with an Mr of 65,000 and a minor component with an Mr of 125,000. We conclude that human erythrocyte membranes contain specific binding sites for RRR-alpha-tocopherol. These sites may be of physiologic significance in the function of tocopherol on the red blood cell membrane.

Optimal insulin delivery in diabetic ketoacidosis (DKA) and hyperglycemic, hyperosmolar nonketotic coma (HHNC)

Both diabetic ketoacidosis (DKA) and hyperglycemic, hyperosmolar nonketotic coma (HHNC) are stressful metabolic occurrences brought about by the orchestration of numerous events. Adequate hydration and replacement of electrolytes, along with physiologic doses of insulin, are treatment objectives for both of these conditions. Additionally, the physician must search for the factors precipitating these events and frequently evaluate the patient's overall condition.

Specific binding sites for D-alpha-tocopherol on human erythrocytes

Since vitamin E deficiency is associated with increased susceptibility of erythrocytes to hemolysis, we investigated the presence of tocopherol binding sites in human red blood cells. Erythrocytes were found to have specific binding sites for D-alpha-[3H]tocopherol with properties of receptors. Kinetic studies of binding demonstrated two binding sites: one with high affinity (equilibrium association constant Ka = 2.6 x 10(7) M-1), low capacity (7600 sites/cell) and the second with low affinity (Ka = 1.24 x 10(6) M-1), high capacity (150 000 sites/cell). These sites are at least partly protein in nature.

Effects of insulin on plasma lipoproteins in diabetic ketoacidosis: evidence for a change in high density lipoprotein composition during treatment

To determine the acute effects of insulin on lipoprotein metabolism, we have followed the plasma lipoprotein lipid and apolipoprotein levels during insulin therapy for the first 24 hr in 13 patients with diabetic ketoacidosis. Corrections were made for plasma volume changes during treatment. Before insulin treatment, mean plasma triglyceride and cholesterol levels were 574 mg/dl (range 53-2355) and 212 mg/dl (range 118-416), respectively. Insulin therapy resulted in rapid decreases in triglyceride-rich lipoproteins, chylomicrons, and very low density lipoproteins (VLDL), with most patients achieving plasma triglyceride levels below 150 mg/dl at 24 hr. Mean basal levels of intermediate density lipoproteins (IDL) and low density lipoproteins (LDL)-cholesterol were low (9.9 and 72 mg/dl, respectively) and were statistically invariant with therapy. Mean basal levels of high density lipoprotein (HDL) cholesterol were also low (26 mg/dl, range 5-48) and were invariant during the first 12 hr and increased significantly to 29 mg/dl by the 24th hr. Plasma apoprotein (apo) B levels were in the upper normal range (101 mg/dl) before treatment and decreased with therapy due to significant decreases in VLDL, but not IDL or LDL apoB. VLDL appeared to have a normal apoprotein composition which did not change with treatment. Mean apoA-I levels which were near normal in plasma and HDL before therapy, decreased significantly (16%) by 12 hr and subsequently increased towards basal levels between 12 and 24 hr. The ratio of apoA-I to cholesterol in HDL also fell significantly during the entire 24 hr. Density gradient ultracentrifugal analysis of the d > 1.006 g/ml fractions indicated a selective decrease in "lighter" density fractions of HDL-apoA-I during treatment. These results provide evidence that insulin may decrease the secretion of apoA-I into plasma or increase catabolism.-Weidman, S. W., J. B. Ragland, J. N. Fisher, Jr., A. E. Kitabchi, and S. M. Sabesin. Effects of insulin on plasma lipoproteins in diabetic ketoacidosis: evidence for a change in high density lipoprotein composition composition during treatment.

Reversible hyperinsulinuria in diabetic ketoacidosis in man

Urinary clearance and fractional urinary clearance of immunoreactive insulin (IRI) and beta 2-microglobulin (I beta 2M) were studied in patients with diabetic ketoacidosis (DKA) before, during, and after treatment. Our results indicate that in DKA in man a) there is an approximate 250-fold increase in urinary and fractional urinary clearance of IRI and a 600-fold increase in urinary and fractional urinary I beta 2M clearance, which suggests that the hyperinsulinuria is secondary to a nonspecific defect in tubular luminal uptake of low-molecular-weight proteins, although decreased IRI degradation cannot be excluded; b) because increased IRI clearance is not changed by the pharmacologic plasma IRI levels achieved, the residual tubular absorptive capacity is not saturable; c) I beta 2M clearance but not IRI clearance is significantly improved by the time metabolic control is attained, suggesting separate tubular transport systems; d) a small, therapeutically insignificant fraction of the infused insulin is lost in the urine during therapy of DKA; and e) defective renal tubular luminal uptake (and possibly degradation) of IRI is reversible.

Glucose intolerance and insulin resistance in aplastic anemia treated with oxymetholone

Because of a suspected association between the drug oxymetholone and abnormal glucose metabolism, we determined immunoreactive insulin (IRI) and plasma glucose during oral glucose tolerance testing in seven patients with aplastic anemia, six of whom received oxymetholone therapy. All patients receiving oxymetholone therapy had abnormal glucose and/or IRI values. This finding was independent of GH, cortisol, and glucagon. In one patient, glucose and IRI levels were normal before oxymetholone but became abnormally elevated after the use of this drug. Furthermore, normal glucose and IRI values were present in the single patient not receiving oxymetholone. Thus, a positive relationship was demonstrated between oxymetholone administration and the presence of glucose intolerance and insulin resistance.

Correlation between plasma and urine glucose in diabetes

To determine whether semiquantitative glucose measurements of spot urine specimens accurately reflect prevailing plasma glucose levels, we compared reported levels from 400 second-voided urines to simultaneous plasma determinations from 246 adult diabetics. Quantitative urine levels and plasma glucose levels correlated. However, when semiquantitative urinary determinations were compared to plasma glucose stratified into 0 to 149, 150 to 199, and greater than 200 mg/dL, 75% of the urine samples associated with plasma levels from 150 to 199 mg/dL were negative by Diastix, and 16.5% of samples negative by Diastix were in the 200+ mg/dL plasma range. Only 9% of samples from 0 to 149 mg/dL showed any positive Diastix readings. Because of the low sensitivity of semiquantitative methods, we fell that, except for detection of marked hyperglycemia, spot urine glucose determinations are inadequate as the sole means of clinical assessment for management of diabetic patients. Home glucose monitoring may be a better alternative for follow-up of these patients.

Maturity-onset diabetes of the young: an illustrative case for control of diabetes and hormonal normalization with dietary management

The efficacy of diet therapy used for one patient with the newly classified maturity-onset diabetes of the young (MODY) is reported. Using the conventional American Diabetes Association's food exchange system with appropriate caloric restriction, the patient's weight was reduced from 25% to 6% above ideal body weight in the 14-wk study. Results of three meal tolerance tests compared with a group of lean, normal subjects revealed a steady improvement from baseline and postprandial hyperglycemia, with concomitant normalization of the counterregulatory hormones as well as hemoglobin A1, free fatty acids, and subsequent euglycemia. This case demonstrates the effectiveness of dietary manipulation as the major therapy in MODY with obesity.

Decreased insulin binding of human erythrocytes after dexamethasone or prednisone ingestion

We have investigated changes in insulin binding in erythrocytes in response to overnight ingestion of 1 mg dexamethasone or 10 mg of prednisone in two groups of eight lean, healthy subjects. Dexamethasone administration reduced insulin binding from 9.6 to 6.8% (P < 0.001) with concomitant increase in basal plasma insulin from 10.5 to 14.1 microU/ml (P < 0.05). Prednisone ingestion reduced insulin binding from 9.9 to 7.9% (P < 0.01), but the increase in basal insulin from 16.9 to 20.6 microU/ml was not significantly different. The decrease in insulin binding with both dexamethasone and prednisone was associated with decreased affinity of erythrocyte for insulin at low occupancy and the increase in the dose of unlabeled insulin resulted in 50% inhibition of specific binding without changes in the number of receptors. The earliest decrease in insulin binding was noted within 2 h after ingestion of 1 mg of dexamethasone. These data suggest that acute alteration of insulin receptor function could occur in erythrocytes by small amounts of dexamethasone or prednisone through a mechanism consistent with a decrease in receptor affinity rather than a decrease in the number of receptors.

beta-Endorphin and beta-lipotropin plasma levels in hirsute women: correlation with body weight

Based on the findings of elevated circulating beta-endorphin (beta-END) levels in genetically obese (fa/fa) fats and the reversal of the overeating of genetically obese (ob/ob) mice by naloxone, circulating beta-END and beta-lipotropin (beta-LPH) levels were measured in 8 hirsute, hyperandrogenic, oligo-amenorrheic women of varying weights. Circulating beta-END and beta-LPH levels were significantly elevated (p less than .001) above the levels in nonobese control subjects and were positively correlated with body weight (p less than .025). Based on these data and indirect evidence in the literature, we propose a role may exist for beta-END and/or beta-LPH in human obesity and in adrenal androgen secretion.