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M Brownlee

Publications and source records attributed to M Brownlee.

At least 109 records · Page 6Linked to original sources
PubMed1986

Nonenzymatic glycosylation: role in the pathogenesis of diabetic complications.

Prolonged hyperglycemia is the primary metabolic abnormality responsible for the development of irreversible tissue damage in chronic diabetes. However, patients with similar levels of chronic hyperglycemia can differ markedly in their susceptibility to diabetic complications. Among the mechanisms by which hyperglycemia may lead to tissue damage, nonenzymatic glycosylation involves excessive chemical attachment of glucose to proteins without the involvement of enzymes. The early Amadori products, resembling hemoglobin A1c, slowly give rise to complex irreversible glycosylation adducts. Only these post-Amadori products accumulate in diabetic tissues over long periods. However, early nonenzymatic glycosylation or Amadori product formation can alter such physiological processes as enzyme activity or binding of regulatory molecules. Advanced glycosylation end products can covalently trap extravasated serum proteins to the extravascular matrix, and thus may contribute to capillary closure in the retina and glomerulus, and to arterial narrowing in the coronary, cerebral, and peripheral circulation. Although a macrophage receptor system may antagonize this glycosylation-mediated accumulation of proteins by recognizing and ingesting those proteins with advanced glycosylation end products, excessive formation of those proteins in diabetes may saturate the capacity of the macrophage removal system.

Blood Glucose↗
PubMed1985

Carbodiimide-mediated O-sulfation of hydroxy-amino acids and peptides: a reaction suitable for radiolabeling.

Carbodiimide-mediated sulfation of hydroxy-amino acids, peptides, and proteins can be accomplished in dry dimethylformamide by incubation in a 20-50 molar excess of sulfuric acid and various concentrations of dicyclohexyl carbodiimide [(1-ethyl-(3-dimethylaminopropyl)carbodiimide or 1-cyclohexyl-3-(2-morpholoethyl)carbodiimide p-toluene sulfonate)] at 4 degrees C for 2-4 h. Under these conditions, hydroxy-amino acids are quantitatively converted into O-sulfates, while cysteine yields the S-sulfonate. Other amino acids, including tryptophan, do not react and are recovered quantitatively. Porcine sodium insulin yields a product that can be separated into six bands by nondenaturing polyacrylamide gel electrophoresis. Radiolabeling of peptides by this method can be carried out with a high degree of efficiency if the added [35S]sulfuric acid is used carrier free with an acid excess provided by trifluoromethyl sulfonic acid. Under these conditions, over 60% of [35S]sulfuric acid was incorporated into insulin and bovine serum albumin. This method may prove useful in the radiolabeling of other peptides and proteins.

Amino Acids↗
PubMed1985

Protein glycosylation and the pathogenesis of atherosclerosis.

This review summarizes the progress of research in nonenzymatic glycosylation that is of potential relevance to atherosclerosis and relates this knowledge to the accelerated large-vessel disease observed in diabetics through a hypothetical model based on current concepts of atherogenesis. Critical new information has recently been obtained about complex glycosylation adducts, which form very slowly through a series of further reactions and rearrangements from the initial Amadori product. These adducts, called advanced glycosylation end products (AGE), are not reversible like the Amadori product. Thus, they continue to accumulate indefinitely on long-lived molecules such as collagen and nucleic acids. AGE covalently trap soluble plasma proteins, act as signals for macrophage recognition and uptake, and induce mutations in double-stranded plasmid DNA. Covalent trapping of low-density lipoproteins by AGE on collagen may promote excessive lipid accumulation in the arterial walls of diabetics, whereas trapping of von Willebrand factor by AGE may increase platelet adhesion and aggregation, leading to smooth muscle cell proliferation in the arterial intima. Recognition and uptake of AGE-protein derivatives by scavenging macrophages may further contribute to the process of atherogenesis by stimulating the release of such macrophage secretory products as macrophage-derived growth factor. Accumulation of AGE on smooth muscle cell DNA may also enhance proliferation of arterial smooth muscle cells by increasing the rate of mutations that affect growth control.

Arteriosclerosis↗
PubMed1985

High-affinity-receptor-mediated uptake and degradation of glucose-modified proteins: a potential mechanism for the removal of senescent macromolecules.

Proteins that have been modified by long-term exposure to glucose accumulate advanced glycosylation end products (AGE) as a function of protein age. In these studies, we have characterized the interaction of AGE-protein with mouse peritoneal macrophages, using AGE-modified bovine serum albumin (AGE-BSA, prepared by incubation with glucose) as a probe. AGE-BSA was specifically bound to cells at 4 degrees C and was taken up and degraded at 37 degrees C; these processes were concentration dependent and saturable. Competition experiments with AGE-BSA, BSA incubated with phosphate-buffered saline rather than glucose, and yeast mannan demonstrated that macrophages specifically recognize AGE on proteins by a receptor that is completely distinct from the mannose/fucose receptor. Scatchard analysis of AGE-BSA binding data indicated that there are approximately 1.06 X 10(5) receptors per macrophage, with an affinity constant of 1.75 X 10(-11) M. Specific binding of AGE-BSA to the macrophage receptor was competitively inhibited by BSA that had been chemically coupled to a synthetic analogue of the specific AGE, 2-(2-furoyl)-4(5)-(2-furanyl)-1H-imidazole (FFI-BSA). FFI-BSA was also taken up by macrophages in a concentration-dependent, saturable manner. Prior incubation of macrophages with AGE-BSA failed to influence the subsequent uptake and degradation of added AGE-BSA. Thus, the AGE receptor does not appear to be down-regulated by exposure to AGE-proteins. Results from these studies suggest that AGE could act in vivo as a specific signal for recognition and degradation of senescent macromolecules. Incomplete removal of AGE-proteins by macrophages may ultimately give rise to some of the physiologic changes that occur with normal aging.

Aging↗
PubMed1985

Recognition and uptake of human diabetic peripheral nerve myelin by macrophages.

Macrophages recognize and ingest several human proteins whose amino groups have been modified in vitro by specific chemical reagents. Because amino groups of human peripheral nerve myelin proteins become covalently modified in vivo by products of nonenzymatic glycosylation, we examined myelin/macrophage interactions using peripheral nerve myelin prepared from diabetic and nondiabetic patients. Intracellular accumulation of diabetic myelin increased with concentration in an apparently saturable fashion, reaching levels 3-4 times higher than those of age-matched nondiabetic samples. Low-temperature inhibition of cellular myelin accumulation further suggested that diabetic myelin uptake was associated with adsorptive endocytosis. Macrophage recognition and accumulation of nondiabetic myelin glycosylated in vitro increased with duration of sugar incubation, to a level nearly nine times that of the same sample incubated in buffer alone. Data from competition experiments with albumin and myelin glycosylated in vitro showed that recognition of human peripheral nerve myelin proteins by macrophages is specific for protein-bound products of nonenzymatic glycosylation. In vivo, such macrophage recognition of and interaction with nonenzymatic glycosylation products on diabetic peripheral nerve myelin could contribute to the pathogenesis of segmental demyelination.

Adult↗
PubMed1985

Nonenzymatic glycosylation products on collagen covalently trap low-density lipoprotein.

Advanced nonenzymatic glycosylation products capable of cross-linking proteins accumulate on collagen in vivo in proportion to time-averaged blood glucose concentration. In this report, we have evaluated the ability of advanced nonenzymatic glycosylation products formed on collagen in vitro to covalently bind low-density lipoprotein (LDL) in a manner similar to that which occurs in human atherosclerotic lesions. At constant LDL concentration, covalent trapping increased linearly with the extent of advanced glycosylation product formation, from 1.42 +/- 0.15 to 4.46 +/- 0.36 micrograms LDL protein/mg collagen. At a constant level of collagen advanced glycosylation product, LDL binding increased as a function of increasing LDL concentration. At an LDL-cholesterol level of 103 mg/dl, covalent trapping of LDL by nonenzymatic glycosylation products on collagen averaged 3.2 times as much as control (P less than 0.01). These data indicate that LDL is bound specifically by reactive products generated by nonenzymatic glycosylation of collagen, and suggest that excessive LDL trapping by hyperglycemia-induced advanced glycosylation endproducts may contribute to the accelerated development of atherosclerosis in patients with diabetes mellitus.

Animals↗
PubMed1984

Accumulation of diabetic rat peripheral nerve myelin by macrophages increases with the presence of advanced glycosylation endproducts.

We have previously shown that increased nonenzymatic glycosylation occurs in peripheral nervous tissue of diabetic humans and animals, primarily on the PO-protein of peripheral nerve myelin. The pathophysiologic mechanism by which this biochemical alteration leads to myelin breakdown and removal is not as yet understood. In the present study we show that advanced glycosylation end-product (AGE) adducts that form during long-term exposure of peripheral nerve myelin proteins to glucose in vitro and in vivo markedly alter the way in which myelin interacts with elicited macrophages. In this interaction, macrophages appear to specifically recognize AGEs on myelin, since AGE-BSA competes nearly as effectively as AGE-myelin, while neither unmodified BSA nor unmodified myelin compete. The failure of yeast mannan to interfere with macrophage recognition of AGE-myelin suggests that the mannose/fucose receptor does not mediate this process. Recognition of AGE-protein by macrophages is associated with endocytosis, as demonstrated by resistance of cell-associated radioactivity to removal by trypsin action, and by low temperature inhibition of ligand accumulation in the cellular fraction. 125I-labeled myelin that had been incubated in vitro with 50 mM glucose for 8 wk reached a steady state accumulation within thioglycolate-elicited macrophages that was five times greater than that of myelin incubated without glucose. Similarly, myelin isolated from rats having diabetes for 1.5-2.0 years duration had a steady state level that was 9 times greater than that of myelin from young rats, and 3.5 times greater than that of myelin from age-matched controls. In contrast, myelin isolated from rats having diabetes for 4-5 wk had the same degree of accumulation observed with myelin of age-matched normal rats. These data suggest that the amount of increased nonenzymatic glycosylation observed in the myelin of short-term diabetic rats had not yet resulted in the significant accumulation of AGE-myelin present both in vitro and in the long-term diabetic rats. The disappearance of acid-insoluble radioactivity from within the cells and the appearance of acid-soluble radioactivity released into the medium were very similar for the two groups, suggesting that the striking difference in accumulation seen between normal myelin and AGE-myelin is due primarily to increased uptake. Formation of irreversible AGE-adducts on myelin appears to promote the recognition and uptake of the modified myelin by macrophages. This interaction between AGE-myelin and macrophages may initiate or contribute to the segmental demyelination associated with diabetes and the normal aging of peripheral nerve.

Aging↗
PubMed1984

Association of insulin pump therapy with raised serum amyloid A in type I diabetes mellitus.

Serum concentrations of serum amyloid A protein, the high-density-lipoprotein-associated tissue amyloid A precursor, were determined in 29 diabetic patients receiving insulin by subcutaneous injection and in 50 receiving subcutaneous infusion pump therapy. Insulin delivered by continuous subcutaneous pumps stimulated serum amyloid A production to levels nearly six times those in normal subjects, nearly twice as much as insulin given by subcutaneous injection. 85% of patients with serum amyloid A levels greater than or equal to 10(4) ng/ml were being treated with insulin pump therapy. The relation between insulin aggregation and amyloid A in diabetes was evaluated in 1 patient; treatment with syringe-aggregated insulin resulted in a nearly 300% increase in serum amyloid A levels. The use of high-potency non-aggregating insulins in the pump treatment of type I diabetic patients may be necessary for optimum therapy.

Adolescent↗
PubMed1984

Inhibition of heparin-catalyzed human antithrombin III activity by nonenzymatic glycosylation. Possible role in fibrin deposition in diabetes.

The effect of nonenzymatic glycosylation on the biologic function of human antithrombin III was evaluated using a chromogenic thrombin substrate assay in the presence of catalytic amounts of heparin. Experimental conditions that increased the rate of nonenzymatic protein glycosylation were associated with decreases in the thrombin-inhibiting activity of antithrombin III. This glycosylation-induced inhibition of heparin-catalyzed antithrombin III activity was completely reversible by preassay incubation with excess sodium heparin. These observations provide a biochemical explanation for the heparin-reversible, accelerated fibrinogen disappearance rate induced by hyperglycemia in diabetic patients. Defective inhibition of the coagulation cascade induced by excessive nonenzymatic glycosylation of antithrombin III in vivo could contribute to accumulation of fibrin in various diabetic tissues.

Antithrombin III↗
PubMed1984

Nonenzymatic glycosylation and the pathogenesis of diabetic complications.

Glucose chemically attaches to proteins and nucleic acids without the aid of enzymes. Initially, chemically reversible Schiff base and Amadori product adducts form in proportion to glucose concentration. Equilibrium is reached after several weeks, however, and further accumulation of these early nonenzymatic glycosylation products does not continue beyond that time. Subsequent reactions of the Amadori product slowly give rise to nonequilibrium advanced glycosylation end-products which continue to accumulate indefinitely on longer-lived molecules. Excessive formation of both types of nonenzymatic glycosylation product appears to be the common biochemical link between chronic hyperglycemia and a number of pathophysiologic processes potentially involved in the development of long-term diabetic complications. The major biological effects of excessive nonenzymatic glycosylation include: inactivation of enzymes; inhibition of regulatory molecule binding; crosslinking of glycosylated proteins and trapping of soluble proteins by glycosylated extracellular matrix (both may progress in the absence of glucose); decreased susceptibility to proteolysis; abnormalities of nucleic acid function; altered macromolecular recognition and endocytosis; and increased immunogenicity.

Blood Glucose↗
PubMed1983

Covalent attachment of soluble proteins by nonenzymatically glycosylated collagen. Role in the in situ formation of immune complexes.

The chronic tissue damage associated with long-term diabetes mellitus may arise in part from in situ immune complex formation by accumulated immunoglobulins and/or antigens bound to long-lived structural proteins that have undergone excessive nonenzymatic glycosylation. In this report, we have tested this hypothesis using nonenzymatically glycosylated collagen. Binding of both albumin and IgG averaged four times the amount bound to unmodified collagen. Both albumin and IgG (anti-BSA) bound to nonenzymatically glycosylated collagen retained their ability to form immune complexes in situ with free antibody and antigen.

Antigen-Antibody Complex↗
PubMed1983

Preparation of high-potency, non-aggregating insulins using a novel sulfation procedure.

The marked propensity of insulin to self-associate into large aggregates causes significant mechanical problems in insulin delivery devices and may also stimulate production of a tissue-amyloid A precursor in some patients. Although conventionally prepared sulfated insulin (SI) resists aggregation, clinical application has been limited by major insulin bioactivity losses that occur during synthesis. To eliminate this problem, insulin sulfation was carried out in the organic solvent dimethylformamide in the presence of condensing agents such as N,N'-dicyclohexyl carbodiimide (DCC) and a sulfate donor. With this new procedure, the degree of sulfation could be controlled over an eightfold range by varying the amount of condensing agent. The bioactivity of these new SI derivatives varied between 78% and 87% of unmodified insulin. Insulin aggregation, induced by passage through a syringe and needle, did not occur with derivatives having two or more sulfate moieties per insulin molecule. Diffusion velocity studies using "non-aggregated" insulin solutions demonstrated that aggregates were present in crystalline zinc and sodium porcine insulin. In contrast, SI having more than 0.5 mole sulfate per mole of insulin dialyzed as it were predominantly in the monomeric form. Results from the studies described in this report now provide the means for selectively designing and preparing specific high-potency, non-aggregating insulins, which may be necessary for optimal use of current and future insulin delivery devices.

Animals↗
PubMed1983

Glycosylated insulin complexed to Concanavalin A. Biochemical basis for a closed-loop insulin delivery system.

The oligosaccharides maltose, maltotriose, mannotriose, and mannotetrose have been chemically attached to insulin molecules. Incubation of oligosaccharide and insulin at different molar ratios, with or without addition of cyanoborohydride, showed a nearly linear increase in carbohydrate attachment over time. The intravenous t1/2 of 125I-labeled sugar-insulin derivatives was identical to that of unmodified insulin (3.0 min). Biologic activity of these derivatives, assessed in rats by use of a blood glucose depression assay, did not differ significantly from control. These glycosylated insulin molecules are reversibly bound to the glucose-binding lectin Concanavalin A (Con A). Such sugar-insulin/lectin complexes serve as an insulin reservoir from which sugar-insulin molecules are displaced by glucose. Release of sugar-insulin molecules is a function of the particular sugar-insulin and of the ambient glucose concentration. Glucose displacement of glycosylated insulin complexed to Con A is in direct proportion to the amount of glucose present in the surrounding fluid. At each glucose concentration, the relative binding affinity of the maltotriose derivative is less than that of the mannotriose derivative, while the relative binding affinity of both maltotriose and mannotriose are less than that of the mannotetrose derivative. Prolonged incubation at 37 degrees C causes sugar-insulin, like unmodified insulin, to spontaneously aggregate into high-mol-wt, nondiffusable complexes. This aggregation phenomenon was found to be markedly inhibited when glycosylated insulins were synthesized utilizing partially sulfated insulin. Results from the studies described in this report provide the biochemical basis for a closed-loop, glucose-controlled insulin delivery system, utilizing glycosylated insulin complexed to Con A.

Blood Glucose↗
PubMed1983

Excessive nonenzymatic glycosylation of peripheral and central nervous system myelin components in diabetic rats.

The amount of nonenzymatic glycosylation present in normal and diabetic rat peripheral nerve myelin, whole brain, brain myelin, and individual myelin protein components was determined using NaB3H4 reduction followed by either boronic acid affinity chromatography or SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Diabetic peripheral nerve myelin (PNS-M) showed a 5.2-fold increase over normal, indicating that myelin is the major peripheral nerve component undergoing excessive glycosylation in diabetes. SDS-PAGE of diabetic and normal PNS-M showed no differences in the pattern of protein bands or in the distribution of glycosylated adducts. However, in the diabetic, the amount of incorporated radioactivity was 3.74 times greater in the P0 protein and 2.8 times greater in the high-molecular-weight material that did not enter the gel. In whole brain, a 2.4-fold increase in the amount of nonenzymatic glycosylation was observed when diabetic was compared with normal, while diabetic brain myelin (CNS-M) was 3.8 times more glycosylated than normal brain myelin. SDS-PAGE of diabetic and normal CNS-M, like that of PNS-M, showed no differences in the pattern of protein bands or in the distribution of glycosylated adducts. The amount of incorporated radioactivity, however, was 3.18 times greater in the proteolipid region, 2.37 times greater for basic myelin protein, and 2.9 times greater for the high-molecular-weight proteins that did not enter the gel. This excessive nonenzymatic glycosylation of the main peripheral and central nervous system myelin components may contribute to the functional abnormalities of myelinated neurons associated with diabetes.

Animals↗
PubMed1983

Nonenzymatic glycosylation reduces the susceptibility of fibrin to degradation by plasmin.

The effect of nonenzymatic glycosylation on the susceptibility of fibrin to degradation by the specific fibrinolytic enzyme plasmin was evaluated using both a fibrin plate assay and a fluorogenic synthetic plasmin substrate assay. Data from both types of experiments demonstrate that nonenzymatic glycosylation reduces the susceptibility of fibrin to plasmin degradation. Acetylation and carbamylation have qualitatively similar effects, indicating that chemical modification of lysine amino groups is the underlying phenomenon responsible for the observed degradative defect produced by glucose. Experimental conditions that increased the rate of nonenzymatic protein glycosylation (higher monosaccharide concentration, glucose-6-phosphate) were associated with correspondingly greater degrees of resistance to degradation by plasmin. Such reduced degradation of nonenzymatically glycosylated proteins in vivo may contribute to the accumulation of fibrin and several other proteins observed in those tissues most frequently affected by the complications of diabetes.

Binding Sites↗
PubMed1982

Assessment of diabetic control by measurement of urinary glycopeptides.

The relationship between improvement in diabetic control and changes in levels of glycosylated urinary peptides was investigated. Eight poorly controlled Type 1 (insulin-dependent) diabetic patients were studied as optimal metabolic control was achieved. Mean daily blood glucose values and weekly haemoglobin A1 levels were determined simultaneously. Urinary glycosylated peptide levels fell 50% in 15 days, compared with 23 days for haemoglobin A1. Levels of glycosylated urinary peptides were sensitive to increased mean blood glucose concentrations of 9.72 mmol/l and increased linearly up to 20.0 mmol/l (r = 0.98) when compared with mean blood glucose levels obtained 8-9 days earlier. A similar correlation was found with haemoglobin A1 levels. Levels of glycosylated urinary peptides before and after optimal control were compared, and a decrease of 40% was observed (pre-control: 269 +/- 44 mumol/day, optimal control: 162 +/- 45 mumol/day, mean +/- SEM). The lag time between the fall in mean blood glucose level and the parallel fall in glycosylated urinary peptides was 8-9 days, suggesting that measurement of these compounds may become a useful clinical laboratory technique for monitoring short-term integrated glycaemia in diabetic patients.

Adolescent↗
PubMed1981

Nonenzymatic glycosylation of peripheral nerve protein in diabetes mellitus.

A new affinity chromatography system that selectively retains glycosylated amino acids has been utilized to determine the amount of nonenzymatic glycosylation present in peripheral nerve from diabetic and control rats and dogs. The mean value for glycosylated amino acids in diabetic rats was 2.8 times greater than the mean value in normal rats (P less than 0.001). In diabetic dogs, mean values were 2.15 times greater than normal values (P less than 0.05). Amino acid analysis of reduced, glycosylated amino acids previously isolated by affinity chromatography showed that glycosylated lysine and its hydrolysis rearrangement products were the major borohydride-reducible adduct present. In addition, another glycosylated product was noted to be present in major proportions. This radioactive product did not chromatograph with any of the available glycosylated amino acid standards. The finding that diabetes results in a nearly 3-fold increase of peripheral nerve glycosylation is consistent with a number of previous investigations in which glycosylation was measured in hemoglobin, serum albumin, and urinary amino acids and peptides from diabetics and normals. The results reported here provide evidence that increased nonenzymatic glycosylation is occurring in a tissue where physiological, morphological, and clinical degeneration characteristically develop as a result of diabetes mellitus.

Animals↗