The effect of total starvation and very low energy diet in lean men on kinetics of whole body protein and five hepatic secretory proteins

It is unclear whether the rate of weight loss, independent of magnitude, affects whole body protein metabolism and the synthesis and plasma concentrations of specific hepatic secretory proteins. We examined 1) whether lean men losing weight rapidly (starvation) show greater changes in whole body protein kinetics, synthesis, and circulating concentrations of selected hepatic secretory proteins than those losing the same amount of weight more slowly [very low energy diet (VLED)]; and 2) whether plasma concentrations and synthetic rates of these proteins are related. Whole body protein kinetics were measured using [1-(13)C]leucine in 11 lean men (6 starvation, 5 VLED). Fractional and absolute synthetic rates of HDL-apolipoprotein A1 (apoA1), retinol binding protein, transthyretin, alpha(1)-antitrypsin (alpha(1)-AT), and transferrin were measured using a prime-constant intravenous infusion of [(13)C(2)]glycine. Compared with VLED group, the starvation group showed greater increases (at a 5% weight loss) in whole body protein oxidation (P < 0.05); fractional synthetic rates of HDL-apoA1 (25.3 vs. -1.52%; P = 0.003) and retinol binding protein (30.6 vs. 7.1%; P = 0.007); absolute synthetic rates of HDL-apoA1 (7.1 vs. -3.8 mg.kg(-1).day(-1); P = 0.003) and alpha(1)-AT (17.8 vs. 3.6 mg.kg(-1).day(-1); P = 0.02); and plasma concentration of alpha(1)-AT (P = 0.025). Relationships between synthetic rates and plasma concentrations varied between the secreted proteins. It is concluded that synthetic rates of hepatic secreted proteins in lean men are more closely related to the rate than the magnitude of weight loss. Changes in concentration of these secreted proteins can occur independently of changes in synthetic rates, and vice versa.

Arginine flux and intravascular nitric oxide synthesis in severe childhood undernutrition

BACKGROUND

Although nutritionally dispensable amino acids are not essential in the diet, adequate synthesis is necessary for maintenance of good health. Whereas children with edematous severe childhood undernutrition (SCU) can maintain production rates of glycine and serine despite a slower body protein breakdown rate, it is unknown whether the same is true for the semidispensable amino acid arginine.

OBJECTIVE

We aimed to measure arginine flux and intravascular nitric oxide synthesis in children with SCU.

DESIGN

Arginine flux and the fractional and absolute synthesis rates of plasma nitrite plus nitrate were measured postabsorptively by using a 6-h infusion of [(15)N(2)]-arginine in 2 groups of children with edematous (n = 14) or nonedematous (n = 7) SCU when they were infected and malnourished (postadmission day approximately 3; clinical phase 1), when they were no longer infected (postadmission day approximately 15; clinical phase 2), and when they were recovered (postadmission day approximately 55; clinical phase 3).

RESULTS

Arginine flux was slower (P < 0.01) and plasma arginine concentrations were lower in the edematous group than in the nonedematous group at clinical phase 1. At clinical phase 2, flux doubled to a value that was not significantly different from the value at clinical phase 3. There were no significant differences in the plasma concentration or fractional or absolute synthesis rate of plasma nitrite plus nitrate between the groups at any clinical phase and among clinical phases within each group.

CONCLUSION

Whereas children with nonedematous SCU can maintain arginine flux at the same rate as when recovered, children with edematous SCU cannot. The slower arginine flux was not, however, associated with slower nitric oxide synthesis.

Effects of transgenic expression of HIV-1 Vpr on lipid and energy metabolism in mice

HIV infection is associated with abnormal lipid metabolism, body fat redistribution, and altered energy expenditure. The pathogenesis of these complex abnormalities is unclear. Viral protein R (Vpr), an HIV-1 accessory protein, can regulate gene transcription mediated by the glucocorticoid receptor and peroxisome proliferator-activated receptor-gamma and affect mitochondrial function in vitro. To test the hypothesis that expression of Vpr in liver and adipocytes can alter lipid metabolism in vivo, we engineered mice to express Vpr under control of the phosphoenolpyruvate carboxykinase promoter in a tissue-specific and inducible manner and investigated the effects of dietary fat, indinavir, and dexamethasone on energy metabolism and body composition. The transgenic mice expressed Vpr mRNA in white and brown adipose tissues and liver and immunoaffinity capillary electrophoresis revealed that they had free Vpr protein in the plasma. Compared with wild-type (WT) animals, Vpr mice had lower plasma triglyceride levels after 6 wk (P < 0.05) but not after 10 wk of a high-fat diet and lower plasma cholesterol levels after 10 wk of high-fat diet (P < 0.05). Treatment with dexamethasone obviated group differences, whereas indinavir had no significant independent effect on lipids. In the fasted state, Vpr mice had a higher respiratory quotient than WT mice (P < 0.05). These data provide the first in vivo evidence that HIV-1 Vpr expressed at low levels in adipose tissues and liver can 1) circulate in the blood, 2) regulate lipid and fatty acid metabolism, and 3) alter fuel selection for oxidation in the fasted state.

Sulfur amino acid metabolism in children with severe childhood undernutrition: methionine kinetics

BACKGROUND

Children with edematous but not nonedematous severe childhood undernutrition (SCU) have lower plasma and erythrocyte-free concentrations of cysteine and methionine, which suggests a decreased availability of methionine for cysteine synthesis. We propose that methionine production and metabolism will be slower in children with edematous SCU than in those with nonedematous SCU.

OBJECTIVE

We aimed to measure methionine flux, its transmethylation and its transsulfuration, and homocysteine remethylation in children with SCU.

DESIGN

Methionine kinetics were measured in 2 groups of children with edematous (n = 11) and nonedematous (n = 11) SCU when they were infected and malnourished (clinical phase 1), when they were still severely malnourished but no longer infected (clinical phase 2), and when they had recovered (clinical phase 3).

RESULTS

At clinical phase 1, children with edematous SCU had rates of total methionine flux, flux from protein breakdown, and flux to protein synthesis that were slower than the rates of the nonedematous group. There were no significant differences in homocysteine remethylation or methionine transsulfuration and transmethylation between the groups at clinical phase 1.

CONCLUSION

These findings suggest that, in the acutely malnourished and infected state, children with edematous SCU have slower methionine production than do children with nonedematous SCU because of a slower rate of release from protein breakdown. This slower methionine production is not, however, associated with slower rates of methionine transsulfuration and transmethylation or homocysteine remethylation.

Sulfur amino acid metabolism in children with severe childhood undernutrition: cysteine kinetics

BACKGROUND

Children with edematous but not nonedematous severe childhood undernutrition (SCU) have lower plasma and erythrocyte-free concentrations of cysteine, the rate-limiting precursor of glutathione synthesis. We propose that these lower cysteine concentrations are due to reduced production secondary to slower de novo synthesis plus decreased release from protein breakdown.

OBJECTIVE

We aimed to measure cysteine production, de novo synthesis, and the rate of cysteine release from protein breakdown in children with SCU.

DESIGN

Cysteine flux, de novo synthesis, and release from protein breakdown were measured in 2 groups of children with edematous (n = 11) and nonedematous (n = 11) SCU when they were infected and malnourished (clinical phase 1), when they were still severely malnourished but no longer infected (clinical phase 2), and when they had recovered (clinical phase 3).

RESULTS

In clinical phase 1, cysteine production and its release from protein breakdown were slower in both groups of children than were the values in the recovered state. These kinetic variables were significantly slower, however, in the children with edematous SCU than in those with nonedematous SCU. De novo cysteine synthesis in clinical phase 1 was faster than the rate at recovery in the edematous SCU group, and there were no significant differences between the groups at any clinical phase.

CONCLUSION

These findings suggest that cysteine production is reduced in all children with SCU because of a decreased contribution from protein breakdown and not from decreased de novo synthesis. The magnitude of this reduction, however, is much greater in children with edematous SCU than in those with nonedematous SCU.

Glycine production in severe childhood undernutrition

BACKGROUND

Although nutritionally dispensable amino acids are not essential in the diet, from a biochemical standpoint, dispensable amino acids such as glycine are essential for life. This is especially true under unique circumstances, such as when the availability of labile nitrogen for dispensable amino acid synthesis is reduced, as in severe childhood undernutrition.

OBJECTIVE

We aimed to measure glycine production in children with edematous and nonedematous severe childhood undernutrition.

DESIGN

Glycine flux and splanchnic glycine extraction were measured in 2 groups of children with edematous (n = 8) and nonedematous (n = 9) severe childhood undernutrition when they were infected and malnourished (clinical phase 1), when they were still severely malnourished but no longer infected (clinical phase 2), and when they were recovered (clinical phase 3).

RESULTS

Total and endogenous glycine flux and splanchnic glycine uptake did not differ significantly between the edematous and nonedematous groups during any clinical phase. In both groups of subjects, none of the glycine kinetic parameters changed significantly from clinical phase 1 through phases 2 and 3. Compared with the value at clinical phase 3, plasma glycine concentrations were not significantly lower during clinical phase 1 or 2 in either group.

CONCLUSIONS

These findings suggest that children with severe childhood undernutrition can increase their de novo glycine synthesis to compensate for the reduced contribution from chronic food deprivation. The maintenance of the plasma glycine concentration suggests that the rate of glycine production was sufficient to satisfy metabolic demands in these children when they were acutely undernourished and infected.

In vivo arginine production and intravascular nitric oxide synthesis in hypotensive sepsis

BACKGROUND

Arginine is important in the response to infections and is a precursor for the synthesis of the vasodilator nitric oxide (NO). Low plasma arginine is correlated with a worse prognosis in patients with sepsis, and increased NO has been implicated in the hypotension of sepsis. Data on in vivo arginine and NO kinetics are lacking in hypotensive septic adults.

OBJECTIVE

We aimed to measure in vivo arginine production and the intravascular NO synthesis rate in hypotensive septic patients.

DESIGN

Arginine flux and the fractional and absolute synthesis rates of plasma NO were measured in fasted healthy (n = 10) and hypotensive septic (n = 6) adults by using a 6-h constant infusion of [15N2-guanidino]arginine. Urinary excretion of the NO metabolites nitrite and nitrate (NOx) and plasma concentrations of NOx, arginine, and creatinine were also measured.

RESULTS

All patients had hyperdynamic septic shock and impaired renal function. Compared with the control subjects, the patients had slower arginine flux (99 +/- 8 compared with 50 +/- 7 micromol x kg(-1) x h(-1); P < 0.01), lower plasma arginine concentrations (75 +/- 8 compared with 40 +/- 11 micromol/L; P < 0.01), higher plasma NOx concentrations (30 +/- 4 compared with 65 +/- 1.8 micromol/L), and a slower fractional synthesis rate of NOx. There was no significant difference in the absolute synthesis rate of NOx between groups. In patients with sepsis, the plasma NOx concentration correlated with the glomerular filtration rate and plasma creatinine but not with mean arterial pressure.

CONCLUSIONS

Patients with septic shock have a shortage in the availability of arginine associated with a slower production. Impaired renal excretion of NOx is a contributor to the high plasma NOx in these patients.

In vivo rates of erythrocyte glutathione synthesis in adults with sickle cell disease

Despite reports of lower GSH concentration in sickle cell disease (SCD), the in vivo kinetic mechanism(s) responsible for GSH deficiency is unknown. To determine whether suppressed synthesis was responsible for the lower erythrocyte GSH concentration, we used a primed intermittent infusion of [(2)H(2)]glycine to measure erythrocyte GSH synthesis in vivo in 23 individuals with homozygous beta(s) SCD and 8 healthy controls. Erythrocyte cysteine concentration, the rate-limiting precursor for GSH synthesis, plasma markers of oxidant damage, and dietary intakes of energy and protein were also measured. Compared with values of controls, SCD subjects had significantly lower erythrocyte GSH (P < 0.04) and cysteine concentrations (P < 0.004) but significantly faster fractional rates of GSH synthesis (P < 0.02). The absolute rates of GSH synthesis in SCD subjects compared with control subjects was greater by approximately 57% (P = 0.062). However, the concentrations of markers of oxidative damage, plasma derivatives of reactive oxygen metabolites, plasma nitrotyrosine, urinary isoprostane-to-creatinine ratio, and GSH-to-GSSG ratio, as well as dietary intakes of energy, protein, and GSH precursor amino acids, were not different between SCD subjects and controls. The findings of this study suggest that the lower erythrocyte GSH of SCD patients is not due to suppressed synthesis or impaired regeneration but rather to increased consumption. In addition, the lower erythrocyte cysteine concentration plus the faster rate of GSH synthesis strongly suggest that the endogenous cysteine supply is not sufficient to meet all anabolic demands; hence, cysteine may be a conditionally essential amino acid in individuals with SCD.

Physiologic growth hormone replacement improves fasting lipid kinetics in patients with HIV lipodystrophy syndrome

BACKGROUND

HIV lipodystrophy syndrome (HLS) is characterized by accelerated lipolysis, inadequate fat oxidation, increased hepatic reesterification, and a high frequency of growth hormone deficiency (GHD). The effect of growth hormone (GH) replacement on these lipid kinetic abnormalities is unknown.

OBJECTIVE

We aimed to measure the effects of physiologic GH replacement on lipid kinetics in men with HLS and GHD.

DESIGN

Seven men with HLS and GHD were studied with the use of infusions of [13C1]palmitate, [2H5]glycerol, and [2H3]leucine to quantify total and net lipolysis, palmitate and free fatty acid (FFA) oxidation, and VLDL apolipoprotein B-100 synthesis before and after 6 mo of GH replacement (maximum: 5 microg x kg(-1) x d(-1)).

RESULTS

GH replacement decreased the rates of total lipolysis [FFA(total) rate of appearance (x +/- SE): from 4.80 +/- 1.24 to 3.32 +/- 0.76 mmol FFA x kg fat(-1) x h(-1); P < 0.05] and net lipolysis (FFA(net) rate of appearance: from 1.87 +/- 0.34 to 1.20 +/- 0.25 mmol FFA x kg fat(-1) x h(-1); P < 0.05). Fat oxidation decreased (from 0.28 +/- 0.02 to 0.20 +/- 0.02 mmol FFA x kg lean body mass(-1) x h(-1); P < 0.002), as did the rate of appearance of FFAs available for intrahepatic reesterification (from 0.50 +/- 0.13 to 0.29 +/- 0.09 mmol FFA x kg fat(-1) x h(-1); P < 0.03). Fractional and absolute synthetic rates of VLDL apolipoprotein B-100 were unaltered. These kinetic changes were associated with a decrease in the waist-to-hip ratio but no significant change in fasting plasma lipid concentrations. Fasting plasma glucose concentrations increased after treatment (from 5.2 +/- 0.2 to 5.8 +/- 0.3 mmol/L; P < 0.01).

CONCLUSIONS

Physiologic GH replacement has salutary effects on abnormal lipid kinetics in HLS. The effects are mediated by diminished lipolysis and hepatic reesterification rather than by increased fat oxidation.

Lipid kinetic differences between children with kwashiorkor and those with marasmus

BACKGROUND

It has been hypothesized that one factor associated with poor prognosis in kwashiorkor, but not in marasmus, is impaired lipid catabolism, which limits the supply of energy that is essential for survival when dietary intake is inadequate. However, this hypothesis has not been tested.

OBJECTIVE

The objective was to measure lipid kinetics in malnourished children with kwashiorkor or marasmus.

DESIGN

Glycerol concentration and flux (index of total lipolysis), palmitate concentration and flux (index of net lipolysis), and palmitate oxidation rate (index of fatty acid oxidation) were measured in 8 children (n = 5 boys and 3 girls) with kwashiorkor and 7 (n = 4 boys and 3 girls) with marasmus, aged 4-20 mo, in the postabsorptive state. The measurements were made approximately 3 d after admission, when the children were malnourished, and after the children attained normal weight-for-length, ie, at recovery.

RESULTS

The glycerol concentration was higher in the malnourished stage than at recovery for the marasmus and kwashiorkor groups combined. Glycerol flux tended to be lower (P = 0.067) and palmitate flux significantly lower (P < 0.05) in the kwashiorkor group than in the marasmus group. Palmitate oxidation was significantly lower in the malnourished stage than at recovery in the kwashiorkor group but not in the marasmus group. In the malnourished stage, palmitate oxidation was slower in the kwashiorkor group than in the marasmus group, but no significant differences between groups were observed at recovery.

CONCLUSIONS

Children with kwashiorkor break down fat and oxidize fatty acids less efficiently than do children with marasmus; this factor may explain the better survival rate in marasmus.

Protein kinetic differences between children with edematous and nonedematous severe childhood undernutrition in the fed and postabsorptive states

BACKGROUND

Pathogenic factors that cause a child to develop the edematous instead of the nonedematous form of severe childhood undernutrition (SCU) during food deprivation are not clear. It was hypothesized that, in edematous but not nonedematous SCU, impaired protein breakdown leading to inadequate amino acids for maintenance of important organ systems was a factor.

OBJECTIVE

We measured protein kinetics in children with edematous and nonedematous SCU.

DESIGN

Endogenous leucine flux, an index of whole-body protein breakdown rate, was determined in 4 groups of children with edematous or nonedematous SCU in the malnourished and recovered states. Two groups were studied in the postabsorptive state, and 2 groups were studied in the fed state.

RESULTS

In the postabsorptive state, leucine flux was slower (P < 0.01) in the edematous group than in the nonedematous group in the malnourished state, but in the recovered state, it was faster (P < 0.05) in the children who previously had edematous SCU. When compared with the malnourished state value, leucine flux at recovery doubled in the group that previously had edematous SCU, but it did not change in the other group. In the fed state, leucine flux was slower (P < 0.01) in the edematous group than in the nonedematous group in the malnourished state but not in the recovered state. In the recovered state, enteral leucine extraction by splanchnic tissues trended higher in the group that previously had edematous SCU than in the nonedematous group.

CONCLUSION

These findings indicate different protein breakdown responses to food deprivation between children with edematous and nonedematous SCU and inherent differences in protein metabolism when they have recovered.

Whole-body and hindlimb protein breakdown are differentially altered by feeding in neonatal piglets

The high rate of muscle protein accretion in neonates is sustained by the marked increase in muscle protein synthesis in response to feeding. Little is known about the role of proteolysis in the regulation of protein accretion in response to feeding during the neonatal period. To determine the feeding-induced response of protein breakdown at the whole-body level and in the hindlimb of neonates, 10- and 28-d-old piglets that had been food deprived overnight were infused (7 h) with [1-13C]phenylalanine and [ring-2H4]tyrosine during an initial food deprivation period (3 h), followed by a feeding period (4 h). During feeding, endogenous flux of phenylalanine decreased (P < 0.01) in both the whole body and the hindlimb. Feeding reduced (P < 0.01) whole-body proteolysis but increased hindlimb proteolysis (P = 0.04), suggesting that tissues other than the hindlimb are involved in the reduction in whole-body proteolysis during feeding. Overnight food deprivation resulted in a net mobilization of phenylalanine from whole-body proteins (P < 0.01) but not hindlimb proteins. These responses were unaffected by age. The results suggest that the hindlimb requires a continuous supply of free amino acids to sustain the high rate of muscle protein turnover in neonates and that adaptive mechanisms provide free amino acids to sustain skeletal muscle protein accretion in early postnatal life when the amino acid supply is limited.

Severely dysregulated disposal of postprandial triacylglycerols exacerbates hypertriacylglycerolemia in HIV lipodystrophy syndrome

BACKGROUND

The pathogenesis of hypertriacylglycerolemia, a characteristic feature of HIV lipodystrophy syndrome (HLS), is incompletely understood. One mechanism is accelerated lipolysis in the fasted state, but the severity of the hypertriacylglycerolemia suggests that additional underlying abnormalities may exist in the disposal of dietary fat.

OBJECTIVE

Our objective was to investigate abnormalities in dietary fat disposal in the pathogenesis of hypertriacylglycerolemia in HLS.

DESIGN

We studied 6 nondiabetic men with HLS and 6 men without HIV matched for age and body mass index as control subjects for 8 h after consumption of an isocaloric meal containing 2 g labeled [(13)C(3)]tripalmitin. Chylomicron-triacylglycerol disposal was estimated from labeled [(13)C(1)]palmitate in the plasma chylomicron fraction, and [(13)C(1)]palmitate oxidation was estimated from the (13)CO(2) enrichment in the breath and CO(2) production, over 8 h after the meal.

RESULTS

HLS patients had significantly elevated concentrations of fasting plasma triacylglycerols in both chylomicron (x + SE: 100.3 +/- 49.5 compared with 29.2 +/- 2.2 mg/dL; P < 0.01) and VLDL (82.4 +/- 39.0 compared with 10.8 +/- 2.8 mg/dL; P < 0.01) fractions. Chylomicron-triacylglycerol-derived [(13)C(1)]palmitate disposal was markedly lower in the HLS patients (3.09 +/- 0.41 compared with 6.42 +/- 0.18 mmol [(13)C(1)]palmitate/8 h; P < 0.001) in the 8-h postmeal period. Further, HLS patients had lowered storage of chylomicron-triacylglycerols (0.74 +/- 0.38 compared with 5.05 +/- 0.16 mmol; P < 0.0001) and elevated plasma [(13)C(1)]palmitate concentrations (2.01 +/- 0.27 compared with 1.18 +/- 0.16 mmol; P < 0.05) 8 h after the meal.

CONCLUSIONS

Patients with HLS have key defects that markedly impair postprandial disposal and storage of chylomicron-triacylglycerols. These defects contribute significantly to hypertriacylglycerolemia in HLS.

Relation between liver fat content and the rate of VLDL apolipoprotein B-100 synthesis in children with protein-energy malnutrition

BACKGROUND

Fatty infiltration of the liver is associated with an increased morbidity and mortality in children with severe protein-energy malnutrition (PEM), but its pathogenesis remains unclear. Although impaired synthesis of VLDL apolipoprotein B-100 (VLDL-apo B-100) is generally accepted as the pathogenetic mechanism, the rate of it synthesis has not been measured in children with PEM.

OBJECTIVE

The objective of the study was to ascertain the relation between the degree of hepatic steatosis and the rate of VLDL-apo B-100 synthesis in children with PEM.

DESIGN

The fractional and absolute rates of VLDL-apo B-100 synthesis were measured with a prime-constant intravenous infusion of [2H3]leucine in 13 severely malnourished children (8 boys and 5 girls) aged 7-18 mo. Hepatic fat content was estimated by computerized tomography scanning by using the ratio of liver to spleen (L:S) attenuation. The ratio is inversely related to hepatic fat content such that the lower the L:S, the greater the amount of fat in the liver.

RESULTS

There were significant inverse relations between L:S attenuation and VLDL-apo B-100 concentration (P < 0.02), the absolute rate of VLDL-apo B-100 synthesis (P < 0.02), and plasma triacylglycerol (P < 0.02) and serum cholesterol (P < 0.05) concentrations.

CONCLUSIONS

These results suggest that children with PEM synthesize VLDL-apo B-100 at a faster rate as the degree of hepatic fat infiltration increases. Thus, fatty infiltration of the liver in PEM is not due to a reduction in the synthesis of VLDL-apo B-100.

Assessment of cysteine synthesis in very low-birth weight neonates using a [13C6]glucose tracer

BACKGROUND/PURPOSE

Cysteine is an amino acid necessary for the synthesis of all proteins, the antioxidant glutathione, and the neuromodulator taurine. Whether cysteine is an essential amino acid for premature neonates remains controversial. Using a [13C6]glucose precursor in very-low-birth weight (VLBW) premature neonates, we measured the 13C content of cysteine in hepatically derived apolipoprotein (apo) B-100 and in the plasma to determine whether cysteine synthesis occurs and to relate minimum synthetic capacity to neonatal maturity.

METHODS

Twelve VLBW premature neonates (birth weight, 907 +/- 274 [SD] g; gestational age, 26.8 +/- 2.4 weeks) were studied on day of life 7.8 +/- 4.2 while on total parenteral nutrition (TPN) for 5.6 +/- 4.5 days. A 4-hour intravenous infusion of [13C6]glucose was administered. Blood samples were obtained immediately before and at the end of the infusion. Isotopic enrichment of cysteine was determined by gas chromatography/mass spectrometry. Analysis of variance, Student's t test, and linear regression were used for comparisons.

RESULTS

The 13C isotope ratio of apo B-100-derived cysteine after the [13C6]glucose infusion was significantly higher than baseline (18.57 +/- 0.38 [SEM] vs 17.54 +/- 0.25 mol%, P < .05). The 13C isotope ratio of plasma cysteine was also significantly higher than baseline (17.36 +/- 0.25 vs 16.91 +/- 0.16 mol%, P < .05). When expressed as a product/precursor ratio, the mole percent above baseline of [13C]apo B-100 cysteine/[13C6]glucose correlated with birth weight (r = 0.74, P < .01).

CONCLUSIONS

Very low-birth weight neonates are capable of cysteine synthesis as evidenced by incorporation of 13C label into hepatically derived apo B-100 cysteine and plasma cysteine from a glucose precursor. The minimum capacity for intrahepatic cysteine synthesis appears to be directly proportional to the maturity of the neonate and may impact the capabilities of VLBW neonates to counteract oxidative stresses such as bronchopulmonary dysplasia and necrotizing enterocolitis.

Supplementation with aromatic amino acids improves leucine kinetics but not aromatic amino acid kinetics in infants with infection, severe malnutrition, and edema

We investigated whether supplementation with an aromatic amino acid (AAA) cocktail consisting of 0.5 mmol each of phenylalanine, tryptophan, and tyrosine compared with isonitrogenous amounts of alanine (Ala) would improve measures of protein kinetics in 14 (8 with AAA, 6 Ala) children with edematous malnutrition (aged 6-24 mo) during the infected acute malnourished state. Supplementation started immediately after the baseline experiment, 2 d postadmission and continued to the end of the acute phase of treatment. The second (postsupplementation) experiment was done approximately 12 d postadmission. We measured leucine kinetics, phenylalanine and tyrosine fluxes, using an i.g. 8-h prime continuous infusion of (2)H(3)-leucine, and an i.v. 6-h prime continuous infusion of (13)C-leucine, (2)H(2)-tyrosine, and (2)H(5)-phenylalanine in the fed state. Leucine flux tended to be faster (P = 0.06) in the AAA group compared with Ala group after supplementation (mean difference +/- SEM): 22.6 +/- 10.9 micromol/(kg . h). The rate of leucine appearance from protein breakdown [28.1 +/- 9.4 micromol/(kg . h)] and the nonoxidative disposal of leucine [i.e., leucine to protein synthesis; 35.4 +/- 12.9 micromol/(kg . h)] were faster (P < 0.02) in the AAA group than in the Ala group. There was no significant effect of supplementation on leucine splanchnic metabolism, phenylalanine, and tyrosine fluxes. These findings are consistent with the hypothesis that the blunting of the protein catabolic response to infection in children with edematous malnutrition syndrome is due to limited availability of aromatic amino acids.

Clinical consequences of urea cycle enzyme deficiencies and potential links to arginine and nitric oxide metabolism

Urea cycle disorders (UCD) are human conditions caused by the dysregulation of nitrogen transfer from ammonia nitrogen into urea. The biochemistry and the genetics of these disorders were well elucidated. Earlier diagnosis and improved treatments led to an emerging, longer-lived cohort of patients. The natural history of some of these disorders began to point to pathophysiological processes that may be unrelated to the primary cause of acute morbidity and mortality, i.e., hyperammonemia. Carbamyl phosphate synthetase I single nucleotide polymorphisms may be associated with altered vascular resistance that becomes clinically relevant when specific environmental stressors are present. Patients with argininosuccinic aciduria due to a deficiency of argininosuccinic acid lyase are uniquely prone to chronic hepatitis, potentially leading to cirrhosis. Moreover, our recent observations suggest that there may be an increased prevalence of essential hypertension. In contrast, hyperargininemia found in patients with arginase 1 deficiency is associated with pyramidal tract findings and spasticity, without significant hyperammonemia. An intriguing potential pathophysiological link is the dysregulation of intracellular arginine availability and its potential effect on nitric oxide (NO) metabolism. By combining detailed natural history studies with the development of tissue-specific null mouse models for urea cycle enzymes and measurement of nitrogen flux through the cycle to urea and NO in UCD patients, we may begin to dissect the contribution of different sources of arginine to NO production and the consequences on both rare genetic and common multifactorial diseases.

Cardiovascular implications of HIV-associated dyslipidemic lipodystrophy

The emergence of a new metabolic syndrome in patients with HIV infection, termed "HIV-associated dyslipidemic lipodystrophy" (HADL), is characterized by central fat redistribution, severe dyslipidemia, and insulin resistance and predisposes to an increased risk of cardiovascular disease. The factors promoting the development of cardiovascular disease in this condition are not well understood and may involve contributions from antiretroviral drugs and components of the HIV virus, as well as inflammatory cytokines, leading to accelerated lipolysis, dyslipidemia, lipotoxic insulin resistance, and vascular inflammation. In this article, we review HADL in terms of metabolic, molecular, and cytokine derangements leading to cardiovascular disease.

Response of hepatic proteins to the lowering of habitual dietary protein to the recommended safe level of intake

The plasma concentrations of albumin, HDL apolipoprotein A1 (apoA1), retinol-binding protein (RBP), transthyretin (TTR), haptoglobulin, and fibrinogen were measured, and a stable isotope infusion protocol was used to determine the fractional and absolute synthesis rates of RBP, TTR, and fibrinogen in 12 young adults on three occasions during a reduction of their habitual protein intake from 1.13 to 0.75 g x kg(-1) x day(-1) for 10 days. This study was performed to determine whether healthy adults could maintain the rates of synthesis of selected nutrient transport and positive acute-phase proteins when consuming a protein intake of 0.75 g x kg(-1) x day(-1). During the lower protein intake, the plasma concentration of all the proteins, other than HDL-apoA1, remained unchanged. HDL-apoA1 concentration was significantly reduced (P < 0.05) after 3 days of the lower protein intake, but not at 10 days. The rates of synthesis of RBP and TTR declined significantly (P < 0.05), whereas the rate of synthesis of fibrinogen remained unchanged. The results indicate that, when normal adults consume the recommended safe level of protein, 0.75 g x kg(-1) x day(-1), there is a slower rate of turnover of nutrient transport proteins than on their habitual diet. Hence, healthy individuals consuming this amount of protein may be less able to mount an adequate metabolic response to a stressful stimulus.

Mechanism of dichloroacetate-induced hypolactatemia in humans with or without cirrhosis

Dichloroacetate (DCA) has been used as an experimental treatment for lactic acidosis because it lowers plasma lactic acid concentration. Three potential mechanisms could underlie the hypolactatemic action of DCA, but the dominant mechanism in vivo remains unclear. This study tested whether DCA-induced hypolactatemia occurs via decreased lactate production, increased lactate clearance, or decreased rate of glycolysis in healthy humans and in patients with end-stage cirrhosis. Cirrhosis is associated with decreased hepatic pyruvate dehydrogenase (PDH) content. Six healthy volunteers and 7 cirrhotic patients received a primed, constant infusion of 1-13C-pyruvate and 15N-alanine for 5 hours. DCA (35 mg/kg intravenously) was administered at 2 hours. Plasma isotopic enrichment was measured by gas chromatography/mass spectrometry (GC/MS), and exhaled CO2 enrichment by isotope ratio mass spectrometry. Pyruvate and alanine production rates (Ra) were determined by isotope dilution, and pyruvate oxidation calculated as 13CO2 production from 13C-pyruvate. Ra lactate was calculated as the difference between Ra pyruvate and its disposal by oxidation to CO2 and conversion to alanine. Baseline plasma lactate kinetics in cirrhotic patients did not differ from controls. DCA decreased lactate concentration in both groups by approximately 53%. DCA decreased glycolysis (Ra pyruvate) by 24%, increased the fraction of pyruvate oxidized to CO2 by 26%, and decreased pyruvate transamination to alanine by 25%. DCA also inhibited lactate production by 85%, but decreased plasma lactate clearance by 60% in both groups. DCA reduces plasma lactic acid concentration by inhibiting production, via stimulating pyruvate oxidation and inhibiting glycolysis, rather than increasing clearance. In addition, end-stage cirrhosis does not alter either the mechanism or the magnitude of the metabolic response to DCA.