Farook Jahoor

Effect of severe burn injury on substrate cycling by glucose and fatty acids

Increases in metabolic rate and core temperature are common responses to severe injury. We have investigated the hypothesis that these responses are due to increases in substrate cycling. A substrate cycle exists when opposing, nonequilibrium reactions catalyzed by different enzymes are operating simultaneously. At least one of the reactions must involve the hydrolysis of ATP. Thus, a substrate cycle both liberates heat and increases energy expenditure, yet there is not net conversion of substrate to product. In studies in volunteers (n = 18) and in patients with severe burns who were in a hypermetabolic state (n = 18), we used stable-isotope tracers to quantify substrate cycling in the pathways of glycolysis and gluconeogenesis and a cycle involving the simultaneous breakdown and synthesis of stored triglyceride (triglyceride-fatty acid cycle). The total rates of triglyceride-fatty acid and glycolytic-gluconeogenic cycling were elevated in the patients by 450 and 250 percent, respectively (P less than 0.01). An infusion of propranolol in the patients greatly reduced triglyceride-fatty acid cycling but did not affect gluconeogenic-glycolytic cycling. We conclude that increased substrate cycling contributes to the increased thermogenesis and energy expenditure following severe burns and that the increased triglyceride-fatty acid cycling is due to beta-adrenergic stimulation.

Lactic acidosis during sepsis is related to increased pyruvate production, not deficits in tissue oxygen availability.

OBJECTIVE The purpose of this study was to quantitate the derangements in intermediary carbohydrate metabolism and oxygen use in severely septic patients in comparison with healthy volunteers. SUMMARY BACKGROUND DATA It commonly has been assumed that the development of lactic acidosis during sepsis results from a deficit in tissue oxygen availability. Dichloroacetate (DCA), which is known to increase pyruvate oxidation but only when tissue oxygen is available, provides a means to assess the role of hypoxia in lactate production. METHODS Stable isotope tracer methodology and indirect calorimetry was used to determine the rates of intermediary carbohydrate metabolism and oxygen use in five severely septic patients with lactic acidosis and six healthy volunteers before and after administration of DCA. RESULTS Oxygen consumption and the rates of glucose and pyruvate production and oxidation were substantially greater (p < 0.05) in the septic patient compared with healthy volunteers. Administration of DCA resulted in a further increase in oxygen consumption and the percentage of glucose and pyruvate directed toward oxidation. Dichloroacetate also decreased glucose and pyruvate production, with a corresponding decrease in plasma lactate concentration. CONCLUSIONS These findings clearly indicate that the accumulation of lactate during sepsis is not the result of limitations in tissue oxygenation, but is a sequelae to the markedly increased rate of pyruvate production. Furthermore, the substantially higher rate of pyruvate oxidation in the septic patients refutes the notion of a sepsis-induced impairment in pyruvate dehydrogenase activity.

Intestinal Glutamate Metabolism

Although it is well known that the intestinal tract has a high metabolic rate, the substrates that are used to generate the necessary energy remain poorly established, especially in fed animals. Under fed conditions, the quantification of substrate used by the gut is complicated by the fact that potential oxidative precursors are supplied from both the diet and the arterial circulation. To circumvent this problem, and to approach the question of the compounds used to generate ATP in the gut, we combined measurements of portal nutrient balance with enteral and intravenous infusions of [U-(13)C]substrates. We studied rapidly growing piglets that were consuming diets based on whole-milk proteins. The results revealed that 95% of the dietary glutamate presented to the mucosa was metabolized in first pass and that of this, 50% was metabolized to CO(2). Dietary glucose was oxidized to a very limited extent, and arterial glutamine supplied no >15% of the CO(2) production by the portal-drained viscera. Glutamate was the single largest contributor to intestinal energy generation. The results also suggested that dietary glutamate appeared to be a specific precursor for the biosynthesis of glutathione, arginine and proline by the small intestinal mucosa. These studies imply that dietary glutamate has an important functional role in the gut. Furthermore, these functions are apparently different from those of arterial glutamine, the substrate that has received the most attention.

Dynamics of the protein metabolic response to burn injury.

The protein metabolic response to burn injury was assessed in 17 children aged 7.1 +/- 1.1 years (mean +/- SEM) and a mean burn size of 65 +/- 7% total body surface area (TBSA) during the acute, flow, convalescent, and recovery phases. Stable isotopes of leucine, valine, lysine, and urea were infused in postabsorptive patients in order to measure protein kinetics. The absolute rate of protein breakdown was assessed from the plasma flux of the essential amino acids (EAA), and the rate of urea production (Ra urea) was used as an index of net protein catabolism. Compared to values obtained in recovered patients, the plasma fluxes of all three EAAs were significantly increased (P less than .05), indicating an increased protein breakdown, during the acute, flow, and convalescent phases of injury. Ra urea, however, was only significantly increased during the flow phase (P less than .01), suggesting that protein breakdown was adequately counteracted in the acute and convalescent phases by elevations in protein synthesis but not in the flow phase. The protein kinetic response did not correlate with changes in the metabolic rate since resting energy expenditure (REE) was significantly increased above predicted levels during the acute and flow phases (by 40% and 50%, respectively), and returned to normal in convalescence.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutathione synthesis is diminished in patients with uncontrolled diabetes and restored by dietary supplementation with cysteine and glycine

OBJECTIVE Sustained hyperglycemia is associated with low cellular levels of the antioxidant glutathione (GSH), which leads to tissue damage attributed to oxidative stress. We tested the hypothesis that diminished GSH in adult patients with uncontrolled type 2 diabetes is attributed to decreased synthesis and measured the effect of dietary supplementation with its precursors cysteine and glycine on GSH synthesis rate and oxidative stress. RESEARCH DESIGN AND METHODS We infused 12 diabetic patients and 12 nondiabetic control subjects with [2H2]-glycine to measure GSH synthesis. We also measured intracellular GSH concentrations, reactive oxygen metabolites, and lipid peroxides. Diabetic patients were restudied after 2 weeks of dietary supplementation with the GSH precursors cysteine and glycine. RESULTS Compared with control subjects, diabetic subjects had significantly higher fasting glucose (5.0 ± 0.1 vs. 10.7 ± 0.5 mmol/l; P < 0.001), lower erythrocyte concentrations of glycine (514.7 ± 33.1 vs. 403.2 ± 18.2 μmol/l; P < 0.01), and cysteine (25.2 ± 1.5 vs. 17.8 ± 1.5 μmol/l; P < 0.01); lower concentrations of GSH (6.75 ± 0.47 vs. 1.65 ± 0.16 μmol/g Hb; P < 0.001); diminished fractional (79.21 ± 5.75 vs. 44.86 ± 2.87%/day; P < 0.001) and absolute (5.26 ± 0.61 vs. 0.74 ± 0.10 μmol/g Hb/day; P < 0.001) GSH synthesis rates; and higher reactive oxygen metabolites (286 ± 10 vs. 403 ± 11 Carratelli units [UCarr]; P < 0.001) and lipid peroxides (2.6 ± 0.4 vs. 10.8 ± 1.2 pg/ml; P < 0.001). Following dietary supplementation in diabetic subjects, GSH synthesis and concentrations increased significantly and plasma oxidative stress and lipid peroxides decreased significantly. CONCLUSIONS Patients with uncontrolled type 2 diabetes have severely deficient synthesis of glutathione attributed to limited precursor availability. Dietary supplementation with GSH precursor amino acids can restore GSH synthesis and lower oxidative stress and oxidant damage in the face of persistent hyperglycemia.

Substrate oxidation by the portal drained viscera of fed piglets

Fully fed piglets (28 days old, 7-8 kg) bearing portal, arterial, and gastric catheters and a portal flow probe were infused with enteral [U-13C]glutamate ( n = 4), enteral [U-13C]glucose ( n = 4), intravenous [U-13C]glucose ( n = 4), or intravenous [U-13C]glutamine ( n = 3). A total of 94% of the enteral [U-13C]glutamate but only 6% of the enteral [U-13C]glucose was utilized in first pass by the portal-drained viscera (PDV). The PDV extracted 6.5% of the arterial flux of [U-13C]glucose and 20.4% of the arterial flux of [U-13C]glutamine. The production of13CO2(percentage of dose) by the PDV from enteral glucose (3%), arterial glucose (27%), enteral glutamate (52%), and arterial glutamine (70%) varied widely. The substrates contributed 15% (enteral glucose), 19% (arterial glutamine), 29% (arterial glucose), and 36% (enteral glutamate) of the total production of CO2 by the PDV. Enteral glucose accounted for 18% of the portal alanine and 31% of the portal lactate carbon outflow. We conclude that, in vivo, three-fourths of the energy needs of the PDV are satisfied by the oxidation of glucose, glutamate, and glutamine, and that dietary glutamate is the most important single contributor to mucosal oxidative energy generation.Fully fed piglets (28 days old, 7-8 kg) bearing portal, arterial, and gastric catheters and a portal flow probe were infused with enteral [U-(13)C]glutamate (n = 4), enteral [U-(13)C]glucose (n = 4), intravenous [U-(13)C]glucose (n = 4), or intravenous [U-(13)C]glutamine (n = 3). A total of 94% of the enteral [U-(13)C]glutamate but only 6% of the enteral [U- (13)C]glucose was utilized in first pass by the portal-drained viscera (PDV). The PDV extracted 6.5% of the arterial flux of [U-(13)C]glucose and 20.4% of the arterial flux of [U-(13)C]glutamine. The production of (13)CO(2) (percentage of dose) by the PDV from enteral glucose (3%), arterial glucose (27%), enteral glutamate (52%), and arterial glutamine (70%) varied widely. The substrates contributed 15% (enteral glucose), 19% (arterial glutamine), 29% (arterial glucose), and 36% (enteral glutamate) of the total production of CO(2) by the PDV. Enteral glucose accounted for 18% of the portal alanine and 31% of the portal lactate carbon outflow. We conclude that, in vivo, three-fourths of the energy needs of the PDV are satisfied by the oxidation of glucose, glutamate, and glutamine, and that dietary glutamate is the most important single contributor to mucosal oxidative energy generation.

Regulation of Lipolysis in Severely Burned Children

In this study, the rates of lipid mobilization and of lipolysis have been quantified in severely burned children. In all 12 patients studied, the basal rates were determined. In seven patients, the lipolytic responsiveness to an infusion of epinephrine (0.015 μg/kg/min) was tested, and in the other five patients, the response to beta-adrenergic blockade (propranolol, 1 mg/kg) was tested. The rate of appearance (Ra) of free fatty acids (FFA) was quantified by means of the infusion of 1-13C-palmitate to determine the rate of lipid mobilization, and Ra glycerol was determined using d5-glycerol to assess the rate of lipolysis more directly. In five patients, body composition was determined after recovery by means of H218O dilution. The basal rate of lipolysis was higher than normal in the burned children. In four of the seven patients infused with epinephrine, there was a pronounced increase in Ra glycerol. In all patients given beta-blockade, Ra glycerol decreased greatly. Changes in Ra FFA corresponded with the changes in Ra glycerol in each case. Total body fat was very low (approximately 2% body weight), reflecting the surgical removal of fat in the process of burn wound excision. From these data it is concluded that lipolytic responsiveness to catecholamines in severely burned children is variable, but not absent, despite chronically elevated levels of catecholamines. The total extent of lipolysis may be limited by the available fat mass in children treated with fascial excision. In such patients, the limitation in the ability to mobilize an adequate amount of FFA to fully meet energy requirements provides an important rationale for the clinical practice of providing nutritional support in hourly boluses, as opposed to infrequent meals, since any period of even a few hours in which nutrients are not being absorbed will result in an energy substrate deficiency and consequent increase in amino acid oxidation.

Role of insulin and glucagon in the response of glucose and alanine kinetics in burn-injured patients.

We investigated the roles of insulin and glucagon as mediators of changes in glucose and alanine kinetics during the hypermetabolic response to injury in 10 burn patients by infusing somatostatin with and without insulin replacement. Glucose and alanine kinetics were measured by primed-constant infusions of 6,6-d2-glucose and [3-13C]alanine. The basal rate of glucose production and alanine flux were significantly elevated in all patients. Lowering both hormones simultaneously caused an insignificant reduction in glucose production, but plasma glucose rose significantly (P less than 0.01), because of reduced clearance. Alanine flux and total plasma amino nitrogen increased significantly (P less than 0.05) above basal. Selectively lowering glucagon concentration decreased glucose production (P less than 0.05), and exogenous glucose was infused to maintain euglycemia. Alanine flux and total plasma amino nitrogen remained unchanged. In severely burned patients hyperglucagonemia stimulates increased glucose production, basal insulin suppression glucose production, stimulates basal glucose clearance, and is important for regulation of plasma amino acid concentrations, and the selective lowering of glucagon while maintaining basal insulin constant normalized glucose kinetics.

Erythrocyte glutathione deficiency in symptom-free HIV infection is associated with decreased synthesis rate

Although several studies have documented intra- and extracellular glutathione (GSH) deficiency in asymptomatic human immunodeficiency virus (HIV) infection, the mechanisms responsible for the altered GSH homeostasis remain unknown. To determine whether decreased synthesis contributes to this alteration of GSH homeostasis, a primed-constant infusion of [2H2]glycine was used to measure the fractional and absolute rates of synthesis of GSH in five healthy and five symptom-free HIV-infected subjects before and after supplementation for 1 wk with N-acetylcysteine. The erythrocyte GSH concentration of the HIV-infected group was lower (P < 0.01) than that of the control group (1.4 +/- 0.16 vs. 2.4 +/- 0.08 mmol/l). The smaller erythrocyte GSH pool of the HIV-infected group was associated with a significantly slower (P < 0.01) absolute synthesis rate of GSH (1.15 +/- 0.14 vs. 1.71 +/- 0.15 mmol. l-1. day-1) compared with controls. Cysteine supplementation elicited significant increases in both the absolute rate of synthesis and the concentration of erythrocyte GSH. These results suggest that the GSH deficiency of HIV infection is due in part to a reduced synthesis rate secondary to a shortage in cysteine availability.Although several studies have documented intra- and extracellular glutathione (GSH) deficiency in asymptomatic human immunodeficiency virus (HIV) infection, the mechanisms responsible for the altered GSH homeostasis remain unknown. To determine whether decreased synthesis contributes to this alteration of GSH homeostasis, a primed-constant infusion of [2H2]glycine was used to measure the fractional and absolute rates of synthesis of GSH in five healthy and five symptom-free HIV-infected subjects before and after supplementation for 1 wk with N-acetylcysteine. The erythrocyte GSH concentration of the HIV-infected group was lower ( P < 0.01) than that of the control group (1.4 ± 0.16 vs. 2.4 ± 0.08 mmol/l). The smaller erythrocyte GSH pool of the HIV-infected group was associated with a significantly slower ( P < 0.01) absolute synthesis rate of GSH (1.15 ± 0.14 vs. 1.71 ± 0.15 mmol ⋅ l-1 ⋅ day-1) compared with controls. Cysteine supplementation elicited significant increases in both the absolute rate of synthesis and the concentration of erythrocyte GSH. These results suggest that the GSH deficiency of HIV infection is due in part to a reduced synthesis rate secondary to a shortage in cysteine availability.

Arginine, citrulline and nitric oxide metabolism in sepsis.

Arginine has vasodilatory effects, via its conversion by NO synthase into NO, and immunomodulatory actions which play important roles in sepsis. Protein breakdown affects arginine availability and the release of asymmetric dimethylarginine, an inhibitor of NO synthase, may therefore affect NO synthesis in patients with sepsis. The objective of the present study was to investigate whole-body in vivo arginine and citrulline metabolism and NO synthesis rates, and their relationship to protein breakdown in patients with sepsis or septic shock and in healthy volunteers. Endogenous leucine flux, an index of whole-body protein breakdown rate, was measured in 13 critically ill patients with sepsis or septic shock and seven healthy controls using an intravenous infusion of [1-13C]leucine. Arginine flux, citrulline flux and the rate of conversion of arginine into citrulline (an index of NO synthesis) were measured with intravenous infusions of [15N2]guanidino-arginine and [5,5-2H2]citrulline. Plasma concentrations of nitrite plus nitrate, arginine, citrulline and asymmetric dimethylarginine were measured. Compared with controls, patients had a higher leucine flux and higher NO metabolites, but arginine flux, plasma asymmetric dimethylarginine concentration and the rate of NO synthesis were not different. Citrulline flux and plasma arginine and citrulline were lower in patients than in controls. Arginine production was positively correlated with the protein breakdown rate. Whole-body arginine production and NO synthesis were similar in patients with sepsis and septic shock and healthy controls. Despite increased proteolysis in sepsis, there is a decreased arginine plasma concentration, suggesting inadequate de novo synthesis secondary to decreased citrulline production.