Comparison of Metabolic Response to Colonic Fermentation in Lean Youth vs Youth With Obesity

Importance

Pediatric obesity is a growing health care burden. Understanding how the metabolic phenotype of youth with obesity may modify the effect of intestinal fermentation on human metabolism is key to designing early intervention.

Objective

To assess whether adiposity and insulin resistance in youth may be associated with colonic fermentation of dietary fibers and its production of acetate, gut-derived hormone secretion, and adipose tissue lipolysis.

Design, Setting, and Participants

Cross-sectional study of youths aged 15 to 22 years with body mass index in the 25th to 75th percentile or higher than the 85th percentile for age and sex throughout the New Haven County community in Connecticut. Recruitment, studies, and data collection occurred from June 2018 to September 2021. Youths were assigned to a lean, obese insulin sensitive (OIS), or obese insulin resistant (OIR) group. Data were analyzed from April 2022 to September 2022.

Exposure

Participants consumed 20 g of lactulose during a continuous 10-hour sodium d3-acetate intravenous infusion to measure the rate of appearance of acetate in plasma.

Main Outcomes and Measures

Plasma was obtained hourly to measure acetate turnover, peptide tyrosine tyrosine (PYY), ghrelin, active glucagon-like peptide 1 (GLP-1), and free fatty acids (FFA).

Results

A total of 44 youths participated in the study (median [IQR] age, 17.5 [16.0-19.3] years; 25 [56.8%] were female; 23 [52.3%] were White). Consequent to lactulose ingestion, there was a reduction of plasma FFA, an improvement of adipose tissue insulin sensitivity index, an increase in colonic acetate synthesis, and an anorexigenic response characterized by an increased plasma concentration of PYY and active GLP-1 and a reduction of ghrelin in the subgroups. Compared with the lean and OIS groups, the OIR group showed a less marked median (IQR) rate of acetate appearance (OIR: 2.00 [-0.86 to 2.69] μmol × kg-1 × min-1; lean: 5.69 [3.04 to 9.77] μmol × kg-1 × min-1; lean vs OIR P = .004; OIS: 2.63 [1.22 to 4.52] μmol × kg-1 × min-1; OIS vs OIR P = .09), a blunted median (IQR) improvement of adipose insulin sensitivity index (OIR: 0.043 [ 0.006 to 0.155]; lean: 0.277 [0.220 to 0.446]; lean vs OIR P = .002; OIS: 0.340 [0.048 to 0.491]; OIS vs OIR P = .08), and a reduced median (IQR) PYY response (OIR: 25.4 [14.8 to 36.4] pg/mL; lean: 51.3 [31.6 to 83.3] pg/mL; lean vs OIR P = .002; OIS: 54.3 [39.3 to 77.2] pg/mL; OIS vs OIR P = .011).

Conclusions and Relevance

In this cross-sectional study, lean, OIS, and OIR youth demonstrated different associations between colonic fermentation of indigestible dietary carbohydrates and the metabolic response, with OIR youth showing minimal metabolic modifications as compared with the other 2 groups.

Trial Registration

ClinicalTrials.gov Identifier: NCT03454828.

Safety and pharmacodynamics of an engineered E. coli Nissle for the treatment of phenylketonuria: a first-in-human phase 1/2a study

Phenylketonuria (PKU) is a rare disease caused by biallelic mutations in the PAH gene that result in an inability to convert phenylalanine (Phe) to tyrosine, elevated blood Phe levels and severe neurological complications if untreated. Most patients are unable to adhere to the protein-restricted diet, and thus do not achieve target blood Phe levels. We engineered a strain of E. coli Nissle 1917, designated SYNB1618, through insertion of the genes encoding phenylalanine ammonia lyase and L-amino acid deaminase into the genome, which allow for bacterial consumption of Phe within the gastrointestinal tract. SYNB1618 was studied in a phase 1/2a randomized, placebo-controlled, double-blind, multi-centre, in-patient study ( NCT03516487 ) in adult healthy volunteers (n = 56) and patients with PKU and blood Phe level ≥600 mmol l^-1 (n = 14). Participants were randomized to receive a single dose of SYNB1618 or placebo (part 1) or up to three times per day for up to 7 days (part 2). The primary outcome of this study was safety and tolerability, and the secondary outcome was microbial kinetics. A D5-Phe tracer (15 mg kg^-1) was used to study exploratory pharmacodynamic effects. SYNB1618 was safe and well tolerated with a maximum tolerated dose of 2 × 10¹¹ colony-forming units. Adverse events were mostly gastrointestinal and of mild to moderate severity. All participants cleared the bacteria within 4 days of the last dose. Dose-responsive increases in strain-specific Phe metabolites in plasma (trans-cinnamic acid) and urine (hippuric acid) were observed, providing a proof of mechanism for the potential to use engineered bacteria in the treatment of rare metabolic disorders.

An engineered E. coli Nissle improves hyperammonemia and survival in mice and shows dose-dependent exposure in healthy humans

The intestine is a major source of systemic ammonia (NH₃); thus, capturing part of gut NH₃ may mitigate disease symptoms in conditions of hyperammonemia such as urea cycle disorders and hepatic encephalopathy. As an approach to the lowering of blood ammonia arising from the intestine, we engineered the orally delivered probiotic Escherichia coli Nissle 1917 to create strain SYNB1020 that converts NH₃ to l-arginine (l-arg). We up-regulated arginine biosynthesis in SYNB1020 by deleting a negative regulator of l-arg biosynthesis and inserting a feedback-resistant l-arg biosynthetic enzyme. SYNB1020 produced l-arg and consumed NH₃ in an in vitro system. SYNB1020 reduced systemic hyperammonemia, improved survival in ornithine transcarbamylase-deficient spf^ash mice, and decreased hyperammonemia in the thioacetamide-induced liver injury mouse model. A phase 1 clinical study was conducted including 52 male and female healthy adult volunteers. SYNB1020 was well tolerated at daily doses of up to 1.5 × 10¹² colony-forming units administered for up to 14 days. A statistically significant dose-dependent increase in urinary nitrate, plasma ¹⁵N-nitrate (highest dose versus placebo, P = 0.0015), and urinary ¹⁵N-nitrate was demonstrated, indicating in vivo SYNB1020 activity. SYNB1020 concentrations reached steady state by the second day of dosing, and excreted cells were alive and metabolically active as evidenced by fecal arginine production in response to added ammonium chloride. SYNB1020 was no longer detectable in feces 2 weeks after the last dose. These results support further clinical development of SYNB1020 for hyperammonemia disorders including urea cycle disorders and hepatic encephalopathy.

Whole Body Protein Turnover and Net Protein Balance After Pediatric Thoracic Surgery: A Noninvasive Single-Dose 15 N Glycine Stable Isotope Protocol With End-Product Enrichment

BACKGROUND

We used the 15 N glycine urinary end-product enrichment technique to quantify whole body protein turnover following thoracic surgery.

MATERIALS AND METHODS

A single dose of 15 N glycine (2 mg/kg) was administered orally on postoperative day 1 to children (1-18 years) following thoracic surgery. 15 N enrichment of ammonia and urea was measured in mixed urine after 12 and 24 hours, respectively, and protein synthesis, breakdown, and net balance determined. Nitrogen balance (dietary intake minus urinary excretion) was calculated. Urinary 3-methylhistidine:creatinine ratio was measured as a marker of skeletal muscle protein breakdown.

RESULTS

We enrolled 19 subjects-median (interquartile range): age, 13.8 years (12.2-15.1); weight, 49.2 kg (38.4-60.8)-who underwent thoracotomy (n = 12) or thoracoscopic (n = 7) surgery. Protein synthesis and breakdown by 15 N enrichment were 7.1 (5.5-9) and 7.1 (5.6-9) g·kg-1 ·d-1 with ammonia (12 hours) as the end product, and 5.8 (3.8-6.7) and 6.7 (4.5-7.6) with urea (24 hours), respectively. Net protein balance by the 15 N glycine and urinary urea nitrogen methods were -0.34 (-0.47, -0.3) and -0.48 (-0.65, -0.28) g·kg-1 ·d-1 , respectively (rs = 0.828, P < .001). Postoperative change in 3-methylhistidine:creatinine ratio did not correlate significantly with protein breakdown or balance.

CONCLUSION

The single-dose oral administration of 15 N glycine stable isotope with measurement of urinary end-product enrichment is a feasible and noninvasive method to investigate whole body protein turnover in children. After major surgery, children manifest increased protein turnover and net negative balance due to increased protein breakdown.

Role of Gut Microbiota and Short Chain Fatty Acids in Modulating Energy Harvest and Fat Partitioning in Youth

OBJECTIVE

We aimed at determining the relationship of the gut microbiota and short chain fatty acids with obesity and fat partitioning and at testing potential differences in the ability of gut microbiota to ferment equal amounts of carbohydrates (CHO) between lean and obese youth.

RESEARCH DESIGN AND METHODS

We analyzed the gut microbiota of 84 youth in whom body fat distribution was measured by fast-magnetic resonance imaging, de novo lipogenesis (DNL) quantitated using deuterated water, and the capability of gut flora to ferment CHO was assessed by ¹³C-fructose treatment in vitro.

RESULTS

A significant association was found between the Firmicutes to Bacteroidetes ratio, and the abundance of Bacteroidetes and Actinobacteria with body mass index, visceral and SC fat (all P < .05). Plasma acetate, propionate, and butyrate were associated with body mass index and visceral and SC fat (all P < .05) and with hepatic DNL (P = .01, P = .09, P = .04, respectively). Moreover, the rate of CHO fermentation from the gut flora was higher in obese than in lean subjects (P = .018).

CONCLUSIONS

These data demonstrate that obese youth show a different gut flora composition than lean and that short chain fatty acids are associated with body fat partitioning and DNL. Also, the gut microbiota of obese youth have a higher capability than the gut flora of lean to oxidize CHO.

Use of an oral stable isotope label to confirm variation in red blood cell mean age that influences HbA1c interpretation

HbA1c is commonly used to monitor glycemic control. However, there is growing evidence that the relationship between HbA1c and mean blood glucose (MBG) is influenced by variation in red blood cell (RBC) lifespan in hematologically normal individuals. Correction of HbA1c for mean RBC age (MRBC ) requires a noninvasive, accurate, and affordable method to measure RBC survival. In this study, we evaluated whether a stable isotope approach would satisfy these requirements. RBC lifespan and MRBC were determined in a group of nine hematologically normal diabetic and nondiabetic subjects using oral (15) N-glycine to label heme in an age cohort of RBC. The MRBC was 58.7 ± 9.1 (2SD) days and RBC lifespan was 106 ± 21 (2SD) days. This degree of variation (±15-20%) is consistent with previous studies using other techniques. In a subset of seven subjects, MRBC determined with the biotin label technique were available from approximately five years prior, and strongly correlated with the stable isotope values (R(2) = 0.79). This study suggests that the MRBC is stable over time but varies substantially among individuals, and supports the importance of its variation in HbA1c interpretation. The characteristics of the stable isotope method support its suitability for studies to directly evaluate the impact of variation in MRBC on the interpretation of HbA1c.

Palmitoleic acid is elevated in fatty liver disease and reflects hepatic lipogenesis

BACKGROUND

Biochemical evidence has linked the coordinate control of fatty acid (FA) synthesis with the activity of stearoyl-CoA desaturase-1 (SCD1). The ratio of 16:1n-7 to 16:0 [SCD1₁₆] in plasma triacylglycerol FA has been used as an index to reflect liver SCD1₁₆ activity and has been proposed as a biomarker of FA synthesis, although this use has not been validated by comparison with isotopically measured de novo lipogenesis (DNL(Meas)).

OBJECTIVE

We investigated plasma lipid 16:1n-7 and FA indexes of elongation and desaturation in relation to lipogenesis.

DESIGN

In this cross-sectional investigation of metabolism, 24 overweight adults, who were likely to have elevated DNL, consumed D2O for 10 d and had liver fat (LF) measured by magnetic resonance spectroscopy. Very-low-density lipoprotein (VLDL)-triacylglycerols and plasma free FA [nonesterified fatty acids (NEFAs)] were analyzed by using gas chromatography for the FA composition (molar percentage) and gas chromatography-mass spectrometry and gas chromatography-combustion isotope ratio mass spectrometry for deuterium enrichment.

RESULTS

In all subjects, VLDL-triacylglycerol 16:1n-7 was significantly (P < 0.01) related to DNL(Meas) (r = 0.56), liver fat (r = 0.53), and adipose insulin resistance (r = 0.56); similar positive relations were shown with the SCD1₁₆ index, and the pattern in NEFAs echoed that of VLDL-triacylglycerols. Compared with subjects with low LF (3.1 ± 2.7%; n = 11), subjects with high LF (18.4 ± 3.6%; n = 13) exhibited a 45% higher VLDL-triacylglycerol 16:1n-7 molar percentage (P < 0.01), 16% of subjects had lower 18:2n-6 (P = 0.01), and 27% of subjects had higher DNL as assessed by using a published DNL index (ratio of 16:0 to 18:2n-6; P = 0.03), which was isotopically confirmed by DNL(Meas) (increased 2.5-fold; P < 0.01). Compared with 16:0 in the diet, the low amount of dietary 16:1n-7 in VLDL-triacylglycerols corresponded to a stronger signal of elevated DNL.

CONCLUSION

The current data provide support for the use of the VLDL-triacylglycerol 16:1n-7 molar percentage as a biomarker for elevated liver fat when isotope use is not feasible; however, larger-scale confirmatory studies are needed.

The effect of repeated microwave irradiation on the dimensional stability of a specific acrylic denture resin

PURPOSE

To determine the dimensional stability of a poly(methyl methacrylate) (PMMA) acrylic resin when subjected to multiple sessions of repeated microwave irradiation at power settings of 700 and 420 W.

MATERIALS AND METHODS

Twenty standardized denture bases were fabricated using a PMMA resin. Points of measurement were marked on each denture base with a standardized template, and the distances between points were recorded using a digital microscope. The denture bases were randomly placed into two experimental groups of 10 bases each. Individual denture bases were placed into a glass beaker containing 200 ml of room temperature deionized water and then exposed to either 700 or 420 W of microwave radiation for 3 minutes. The denture bases were allowed to cool to room temperature, and measurements between points were recorded. This process was carried out for two microwave periods with measurements being completed after each period. The data were then analyzed for any significant changes in distances between points using a Student's t-test.

RESULTS

All denture bases experienced 1.0 to 2.0 mm or approximately 3% linear dimensional change after each period of microwaving. Results were significant with all t-tests having values of p < 0.05.

CONCLUSION

This report showed that the denture bases deformed significantly under experimental conditions at either 700 W for 3 minutes in 200 ml of water or 420 W for 3 minutes in 200 ml of water.

Quantitative liver function tests improve the prediction of clinical outcomes in chronic hepatitis C: results from the Hepatitis C Antiviral Long-term Treatment Against Cirrhosis Trial

Risk for future clinical outcomes is proportional to the severity of liver disease in patients with chronic hepatitis C virus (HCV). We measured disease severity by quantitative liver function tests (QLFTs) to determine cutoffs for QLFTs that identified patients who were at low and high risk for a clinical outcome. Two hundred and twenty-seven participants in the Hepatitis C Antiviral Long-term Treatment Against Cirrhosis (HALT-C) Trial underwent baseline QLFTs and were followed for a median of 5.5 years for clinical outcomes. QLFTs were repeated in 196 patients at month 24 and in 165 patients at month 48. Caffeine elimination rate (k(elim)), antipyrine (AP) clearance (Cl), MEGX concentration, methionine breath test (MBT), galactose elimination capacity (GEC), dual cholate (CA) clearances and shunt, perfused hepatic mass (PHM), and liver and spleen volumes (by single-photon emission computed tomography) were measured. Baseline QLFTs were significantly worse (P = 0.0017 to P < 0.0001) and spleen volumes were larger (P < 0.0001) in the 54 patients who subsequently experienced clinical outcomes. QLFT cutoffs that characterized patients as "low" and "high risk" for clinical outcome yielded hazard ratios ranging from 2.21 (95% confidence interval [CI]: 1.29-3.78) for GEC to 6.52 (95% CI: 3.63-11.71) for CA clearance after oral administration (Cl(oral)). QLFTs independently predicted outcome in models with Ishak fibrosis score, platelet count, and standard laboratory tests. In serial studies, patients with high-risk results for CA Cl(oral) or PHM had a nearly 15-fold increase in risk for clinical outcome. Less than 5% of patients with "low risk" QLFTs experienced a clinical outcome.

CONCLUSION

QLFTs independently predict risk for future clinical outcomes. By improving risk assessment, QLFTs could enhance the noninvasive monitoring, counseling, and management of patients with chronic HCV.

A tight coupling between β₂Y97 and β₂F200 of the GABA(A) receptor mediates GABA binding

The GABA(A) receptor is an oligopentameric chloride channel that is activated via conformation changes induced upon the binding of the endogenous ligand, GABA, to the extracellular inter-subunit interfaces. Although dozens of amino acid residues at the α/β interface have been implicated in ligand binding, the structural elements that mediate ligand binding and receptor activation are not yet fully described. In this study, double-mutant cycle analysis was employed to test for possible interactions between several arginines (α₁R67, α₁R120, α₁R132, and β₂R207) and two aromatic residues (β₂Y97 and β₂F200) that are present in the ligand-binding pocket and are known to influence GABA affinity. Our results show that neither α₁R67 nor α₁R120 is functionally coupled to either of the aromatics, whereas a moderate coupling exists between α₁R132 and both aromatic residues. Significant functional coupling between β₂R207 and both β₂Y97 and β₂F200 was found. Furthermore, we identified an even stronger coupling between the two aromatics, β₂Y97 and β₂F200, and for the first time provided direct evidence for the involvement of β₂Y97 and β₂F200 in GABA binding. As these residues are tightly linked, and mutation of either has similar, severe effects on GABA binding and receptor kinetics, we believe they form a single functional unit that may directly coordinate GABA.

Three arginines in the GABAA receptor binding pocket have distinct roles in the formation and stability of agonist- versus antagonist-bound complexes

Binding of the agonist GABA to the GABA(A) receptor causes channel gating, whereas competitive antagonists that bind at the same site do not. The details of ligand binding are not well understood, including which residues interact directly with ligands, maintain the structure of the binding pocket, or transduce the action of binding into opening of the ion channel gate. Recent work suggests that the amine group of the GABA molecule may form a cation-π bond with residues in a highly conserved "aromatic box" within the binding pocket. Although interactions with the carboxyl group of GABA remain unknown, three positively charged arginines (α(1)Arg67, α(1)Arg132, and β(2)Arg207) just outside of the aromatic box are likely candidates. To explore their roles in ligand binding, we individually mutated these arginines to alanine and measured the effects on microscopic ligand binding/unbinding rates and channel gating. The mutations α(1)R67A or β(2)R207A slowed agonist binding and sped unbinding with little effect on gating, demonstrating that these arginines are critical for both formation and stability of the agonist-bound complex. In addition, α(1)R67A sped binding of the antagonist 2-(3-carboxypropyl)-3-amino-6-(4 methoxyphenyl)pyridazinium bromide (SR-95531), indicating that this arginine poses a barrier to formation of the antagonist-bound complex. In contrast, β(2)R207A and α(1)R132A sped antagonist unbinding, indicating that these arginines stabilize the antagonist-bound state. α(1)R132A also conferred a new long-lived open state, indicating that this arginine influences the channel gate. Thus, each of these arginines plays a unique role in determining interactions with agonists versus antagonists and with the channel gate.

A new 13C breath test to detect vitamin B12 deficiency: a prevalent and poorly diagnosed health problem

Vitamin B12 deficiency is emerging as a growing public health problem. The most commonly used diagnostic tests are limited in accuracy, sensitivity, and are non-specific for B12 deficiency. The aim of this study was to develop a simple B12 breath test (BBT) to more accurately evaluate vitamin B12 status as an alternative to the most common diagnostic test, serum B12 levels. The breath test is based on the metabolism of sodium 1-(13)C-propionate to (13)CO(2) which requires B12 as a cofactor. We initially compared the BBT to current B12 diagnostic methods in 58 subjects. Subjects also received a second BBT 1-3 days after initial testing to evaluate reproducibility of results. Propionate dosage, fasting times, and collection periods were compared, respectively. The dose of sodium 1-(13)C-propionate (10-50 mg) gave equivalent results while an 8 h fast was essential. Statistical analysis revealed that breath collection times could be reduced to just a baseline and 10 and 20 min following propionate dosing. We also measured the incidence of B12 deficiency with the BBT in 119 patients with chronic pancreatitis, Crohn's disease, small intestinal bacterial overgrowth, and subjects over 65 years of age. The BBT results agreed with previous publications showing a higher incidence of B12 deficiency in these patients. The BBT may provide clinicians with a non-invasive, accurate, reliable, and reproducible diagnostic test to detect vitamin B12 deficiency.

Human polymorphisms in the glutathione transferase zeta 1/maleylacetoacetate isomerase gene influence the toxicokinetics of dichloroacetate

Dichloroacetate (DCA), a chemical relevant to environmental science and allopathic medicine, is dehalogenated by the bifunctional enzyme glutathione transferase zeta (GSTz1)/maleylacetoacetate isomerase (MAAI), the penultimate enzyme in the phenylalanine/tyrosine catabolic pathway. The authors postulated that polymorphisms in GSTz1/MAAI modify the toxicokinetics of DCA. GSTz1/MAAI haplotype significantly affected the kinetics and biotransformation of 1,2-¹³C-DCA when it was administered at either environmentally (µg/kg/d) or clinically (mg/kg/d) relevant doses. GSTz1/MAAI haplotype also influenced the urinary accumulation of potentially toxic tyrosine metabolites. Atomic modeling revealed that GSTz1/MAAI variants associated with the slowest rates of DCA metabolism induced structural changes in the enzyme homodimer, predicting protein instability or abnormal protein-protein interactions. Knowledge of the GSTz1/MAAI haplotype can be used prospectively to identify individuals at potential risk of DCA's adverse side effects from environmental or clinical exposure or who may exhibit aberrant amino acid metabolism in response to dietary protein.

A state-dependent salt-bridge interaction exists across the β/α intersubunit interface of the GABAA receptor

The GABA(A) receptor is a multisubunit protein that transduces the binding of a neurotransmitter at an intersubunit interface into the opening of a central ion channel. The structural components that mediate the steps involved in this action are poorly defined. A large amount of work has focused on clarifying the specific functions and interactions of residues believed to surround the GABA binding pocket. Here, we explored two charged residues (β(2)Asp163 and α(1)Arg120), which have been suggested by homology models to participate in a salt-bridge interaction. When mutated to alanine, both single mutants, as well as the double mutant, increase EC(50-GABA), decrease the GABA binding rate, and accelerate deactivation and GABA unbinding rates. Double-mutant cycle analysis demonstrates that the effects of each alanine mutation on the GABA binding rate were additive and independent. In contrast, a significant coupling energy was found during an analysis of deactivation time constants. Using kinetic modeling, we further demonstrated that the GABA unbinding rates, in particular, are strongly coupled. These data suggest that β(2)Asp163 and α(1)Arg120 form a state-dependent salt bridge, interacting when GABA is bound to the receptor but not when the receptor is in the unbound state.

Low-wind/high particulate matter episodes in the Calexico/Mexicali region

The U.S. Environmental Protection Agency (EPA) currently classifies Imperial County, CA, as a nonattainment area for PM10 (particulate matter [PM] < or = 10 microm in diameter), and this region suffers from high rates of chronic bronchitis and childhood asthma. Although high annual and daily average PM levels can have negative health and economic effects, recent studies have identified an association between adverse health effects and short-term PM spikes of tens of micrograms per cubic meter. This study identified PM episodes in Calexico/Mexicali that involve PM concentration spikes with concentrations up to 10 times greater than those reported to cause adverse health effects. These episodes appear to be relatively common during the winter months, are associated with wind speeds below 2 m/sec and stable boundary level heights below 500 m, and can comprise a large portion of the 24-hr PM levels. The organic composition of the PM10 samples collected during the low-wind/ high-PM episodes differed from that collected at other times. However, a preliminary source attribution identified only one significant difference between the source classes: agricultural burning accounted for 6.7% of organic-fraction PM10 for low-wind/high-PM episodes versus 0.25% at other times. This preliminary source attribution also revealed that motor vehicles were the most important relative contributor to organic PM10.

An epilepsy-related region in the GABA(A) receptor mediates long-distance effects on GABA and benzodiazepine binding sites

The GABA(A) receptor mutation gamma(2)R43Q causes absence epilepsy in humans. Homology modeling suggests that gamma(2)Arg43, gamma(2)Glu178, and beta(2)Arg117 participate in a salt-bridge network linking the gamma(2) and beta(2) subunits. Here we show that several mutations at these locations exert similar long-distance effects on other intersubunit interfaces involved in GABA and benzodiazepine binding. These mutations alter GABA-evoked receptor kinetics by slowing deactivation, enhancing desensitization, or both. Kinetic modeling and nonstationary noise analysis for gamma(2)R43Q reveal that these effects are due to slowed GABA unbinding and slowed recovery from desensitization. Both gamma(2)R43Q and beta(2)R117K also speed diazepam dissociation from the receptor's benzodiazepine binding interface, as assayed by the rate of decay of diazepam-induced potentiation of GABA-evoked currents. These data demonstrate that gamma(2)Arg43 and beta(2)Arg117 similarly regulate the stability of both the GABA and benzodiazepine binding sites at the distant beta/alpha and alpha/gamma intersubunit interfaces, respectively. A simple explanation for these results is that gamma(2)Arg43 and beta(2)Arg117 participate in interactions between the gamma(2) and beta(2) subunits, disruptions of which alter the neighboring intersubunit binding sites in a similar fashion. In addition, gamma(2)Arg43 and gamma(2)Glu178 regulate desensitization, probably mediated within the transmembrane domains near the pore. Therefore, mutations at the gamma/beta intersubunit interface have specific long-distance effects that are propagated widely throughout the GABA(A) receptor protein.

Quantitative tests of liver function measure hepatic improvement after sustained virological response: results from the HALT-C trial

BACKGROUND

The impact of virologic response on hepatic function has not been previously defined.

AIM

To determine the relationships of quantitative liver function tests (QLFTs) with virological responses to peginterferon (PEG) +/- ribavirin (RBV) in patients with chronic hepatitis C and to use serial QLFTs to define the spectrum of hepatic improvement after sustained virological response (SVR).

METHODS

Participants (n = 232) were enrolled in the Hepatitis C Antiviral Long-term Treatment against Cirrhosis (HALT-C) Trial, had failed prior therapy, had bridging fibrosis or cirrhosis and were retreated with PEG/RBV. All 232 patients had baseline QLFTs; 24 patients with SVR and 68 nonresponders had serial QLFTs. Lidocaine, [24-(13)C]cholate, galactose and (99m)Tc-sulfur colloid were administered intravenously; [2,2,4,2-(2)H]cholate, [1-(13)C]methionine, caffeine and antipyrine were administered orally. Clearances (Cl), breath (13)CO(2), monoethylglycylxylidide (MEGX), perfused hepatic mass (PHM) and liver volume were measured.

RESULTS

Rates of SVR were 18-26% in patients with good function by QLFTs, but < or =6% in patients with poor function. Hepatic metabolism, measured by caffeine k(elim) (P = 0.02), antipyrine k(elim) (P = 0.05) and antipyrine Cl (P = 0.02) and the portal circulation, measured by cholate Cl(oral) (P = 0.0002) and cholate shunt (P = 0.0003) and PHM (P = 0.03) improved after SVR.

CONCLUSION

Hepatic dysfunction impairs the virological response to PEG/RBV. SVR improves hepatic metabolism, the portal circulation and PHM.

The spectrum of hepatic functional impairment in compensated chronic hepatitis C: results from the Hepatitis C Anti-viral Long-term Treatment against Cirrhosis Trial

BACKGROUND

The spectrum of functional impairment in patients with compensated chronic hepatitis C is incompletely defined.

AIM

To define hepatic impairment by quantitative tests (quantitative liver function tests) and correlate results with disease severity in patients with chronic hepatitis C.

METHODS

We studied 285 adult patients with chronic hepatitis C prior to treatment in the Hepatitis C Anti-viral Long-term Treatment against Cirrhosis Trial; 171 had Ishak fibrosis stages 2-4 (fibrosis) and 114 had stage 5 or 6 (cirrhosis). None had had clinical decompensation. A battery of 12 quantitative liver function test assessed the spectrum of hepatic microsomal, mitochondrial and cytosolic functions, and hepatic and portal blood flow.

RESULTS

Twenty-six to 63% of patients with fibrosis and 45-89% with cirrhosis had hepatic impairment by quantitative liver function test; patients with cirrhosis had the greatest impairment (P-value ranging from 0.15 to <0.0001). Cholate Cl(oral), cholate shunt and perfused hepatic mass correlated with cirrhosis, stage of fibrosis (r = -0.51, +0.49, -0.51), varices and variceal size (r = -0.39, +0.36, -0.41). PHM < 95 and cholate shunt >35% identified 91% of patients with medium- or large-sized varices.

CONCLUSIONS

Hepatic impairment is common in compensated patients with fibrosis or cirrhosis because of chronic hepatitis C. Cholate shunt, and cholate Cl(oral) and perfused hepatic mass, identify patients at risk for cirrhosis or varices.

Evaluation of postinjury hepatocyte function by central amino acid clearance

It has been demonstrated by other investigators that central plasma clearance of amino acids accurately predicts hepatocyte function in patients with liver disease and correlates with clinical outcome. This methodology has not heretofore been studied in the trauma patient. It is our hypothesis that central amino acid clearance in trauma patients is more reflective of hepatocyte function than traditional liver function tests. We examined the plasma amino acid clearance rates using L-[1-13C]phenylalanine. Clearance rates were compared to standard liver function tests (LFTs) and the sensitivity and predictability of the technique were determined. The study was conducted on uninjured control subjects and in seriously injured patients, both with and without significant liver injuries. Compared to baseline values in the control group, initial phenylalanine breath scores were reduced in the injured, but exceeded control levels at 7 days postinjury. These changes were statistically significant. There was no difference between those with and without liver trauma. LFTs showed inconsistent and conflicting results. Thus, central amino acid clearance measured by L-[1-13C]phenylalanine oxidation is depressed immediately following injury but reaches supranormal levels at 7 days postinjury. Compared to LFTs, amino acid clearance suggests initial hepatocyte suppression followed by hyperactivity and is a more accurate determinant of hepatocyte function.

Quantifying gastric Helicobacter pylori infection: a comparison of quantitative culture, urease breath testing, and histology

Although there are several methods to detect Helicobacter pylori infection, there is no simple validated test to quantify the density of infection, which is believed to play a major role in the pathogenesis of H. pylori-associated gastritis and peptic ulceration. The aim of this study was to evaluate and compare noninvasive and invasive tests for assessing the level of H. pylori infection so as to facilitate the development and clinical testing of new antibiotic treatments. Healthy volunteers (n=323) were screened for H. pylori infection by serology and, if positive (n=86), invited to undergo (13)C urea breath testing (UBT) (n=55). An increase of >2.4 parts per thousand (13)CO(2) at 15 min compared to baseline was considered a positive test. Total cumulative urease activity (mumol) at 60 min was also calculated. UBT-positive subjects underwent endoscopy and nine biopsies were obtained from defined sites for quantitative culture and histological grading using the modified Sydney System. A total of 19 subjects were studied, 4 of whom underwent repeat testing. All subjects were positive for H. pylori by serology, UBT, culture, and histology. The increase in (13)CO(2) at 30 min correlated with the total cumulative urease activity at 60 min (r (2)=0.92, P< 0.0001). Bacterial counts (log cfu/biopsy; mean+/-SD) were 3.9+/-0.5, 3.9+/-0.4, and 3.9+/-0.6 at the lesser curve antrum, greater curve antrum, and corpus, respectively. There was no significant correlation between UBT results and bacterial counts at any biopsy site. Nor was there any significant correlation between the histology grading and either the UBT or the bacterial counts at any site. This study indicates that there is little correlation among the three methods used to measure bacterial burden in H. pylori infection. Thus, decrements in bacterial numbers during single-agent therapy cannot be measured reliably by UBT and therefore cannot be used to evaluate the potential efficacy of novel agents to treat gastric H. pylori infection.

Comparison of the 13C-urea blood test to histology and rapid urease testing in the diagnosis of Helicobacter pylori infection in children

OBJECTIVE

Helicobacter pylori infection is commonly investigated in children with abdominal pain. The definitive means of diagnosing infection, histology, requires endoscopy and sedation, making it invasive and expensive. Our objective was to compare histology against a less invasive and safer method, the 13C-urea blood test.

PATIENTS AND METHODS

Forty children with abdominal pain undergoing upper endoscopy were randomized into either of 2 dosages of 13C-urea. Several biopsies were taken for histology and rapid urease testing. After endoscopy, each child ingested a randomly assigned dosage of either 75 mg or 125 mg 13C-urea, and blood was withdrawn 30 min later.

RESULTS

Irrespective of the dosage of 13C-urea, the 13C-urea blood test performed with high accuracy (89%) when compared against either histology or rapid urease testing. The sensitivity and specificity of the blood test was 83% and 91%, respectively. When the smaller dosage of 13C-urea was used, the accuracy of the blood test was 100% compared with histology. There were no adverse events related to using either dosage of 13C-urea.

CONCLUSIONS

The 13C-urea blood test may be comparable with histology in diagnosing H. pylori infection in children, and the smaller dosage of 13C-urea does not adversely affect blood test performance. The 13C-urea blood test is well tolerated in children.

Effect of triglyceride structure on fecal excretion of 13C-labeled triglycerides

OBJECTIVE

The aim of this work was to determine the effects of specific changes in the structure of (13)C-labeled triglyceride (TG*) on its fecal excretion relative to total stool fat excretion determined simultaneously in patients with reduced exocrine pancreatic function.

METHODS

A series of 47 studies were conducted in 26 young cystic fibrosis (CF) patients and 11 adult patients with chronic pancreatitis over a five year period. Each test consisted of ingesting a single high fat test meal containing both (13)C-labeled triglyceride (TG*) and dysprosium chloride (DyCl(3)) a nonabsorbable marker of intestinal transit; in most studies the food colorant brilliant blue (FD&C blue #1) was administered along with the DyCl(3). The TG*s tested were: P*P*P* = TRIPALMITIN-1,1,1-(13)C(3); SO*S = 2-OCTANOYL-1,3-DISTEARIN-2-octanoyl-1,2-(13)C(2); and P*LP* = 2-LAURYL-1,3-DIPALMITIN-dipalmitoyl-1,1,2,2-(13)C(4). Ingestion of the test meal was followed by collection of individual stools for at least 72 hours. Stools were analyzed for (13)C-Excess ((13)C*), total fat, and Dy.

RESULTS

Excretion of P*LP* showed a high degree of linear correlation with stool fat (r(2) = 0.924) over a wide-range of fecal fat values. Excretion of SO*S was also significantly correlated with stool fat, but its excretion was less than 10% at all levels of steatorrhea and the slope of the regression line relating TG* excretion to stool fat was some four to five times smaller than observed for P*LP*. Fecal excretion of P*P*P* was highly correlated with stool fat (r(2) = 0.941) in patients with moderate steatorrhea (<25 g fat/24 hours) and the slope of the regression line (3.20) was considerably greater than for P*LP*. Only results from those studies in which stool collections were complete (Dy excretion >90%) were utilized in the statistical comparisons (36 of 47 studies).

CONCLUSIONS

The observed highly significant linear correlation between P*LP* and stool fat over the entire range of steatorrhea suggests that P*LP* excretion may be a suitable surrogate for fecal fat in patients with reduced exocrine pancreatic function. Because fecal excretion of TG* administered as described can be accurately determined by sampling only two visually marked stools, development of a noninvasive test to replace the current 72-hour stool fat test using this approach is possible. Use of other engineered TG*s and/or labeled fatty acids, may provide a method for non-invasive in vivo assessment of the specific defect(s) leading to steatorrhea in other patient groups.

Kinetics and spontaneous open probability conferred by the epsilon subunit of the GABAA receptor

GABAA receptors mediate synaptic and extrasynaptic inhibition. Native receptors consist of alpha and beta subunits, which are required for function, and another "modulatory" subunit, for example, gamma, delta, or epsilon. Of these, the epsilon subunit has the most restricted distribution, confers resistance to neurosteroid and anesthetic modulation, and causes spontaneous channel opening. Little is known, however, about how epsilon affects receptor kinetics, which in turn shape responses to both ambient and synaptic GABA exposure. Here, we expressed human alpha2beta1, alpha2beta1gamma2, or alpha2beta1epsilon subunit combinations in human embryonic kidney 293 cells and used rapid solution exchange to study receptor kinetics in outside-out patches. The epsilon subunit greatly slowed deactivation and recovery after brief GABA pulses. During long, saturating GABA pulses, the rate of desensitization was slower for alpha2beta1epsilon and alpha2beta1gamma2 than for alpha2beta1. However, in alpha2beta1epsilon, the final extent of desensitization was large compared with that of alpha2beta1gamma2. Responses in alpha2beta1epsilon, but not the others, were often followed by an "overshoot" above the baseline, suggesting that a fraction of channels are spontaneously open and are transiently silenced by receptor activation and subsequent desensitization. The baseline current and associated noise were reduced by picrotoxin, revealing that epsilon-containing channels are open approximately 4% of the time in the absence of GABA. These results suggest that, if epsilon-containing receptors are expressed at synapses, the synaptic currents would be long-lasting but may rundown quickly under high-frequency activation. In addition, silencing of spontaneous openings by desensitization raises the possibility that tonic inhibition mediated by epsilon-containing receptors may be regulated by phasic inhibition.

Evaluation of the Ez-HBT Helicobacter blood test to establish Helicobacter pylori eradication

BACKGROUND

The urea blood test (Ez-HBT) has been shown to compare favourably with the urea breath test in the diagnosis of active Helicobacter pylori infection.

AIM

To examine the performance characteristics of the Ez-HBT Helicobacter blood test in establishing success or failure of therapy in H. pylori-infected adults using the 13C urea breath test as the reference method.

METHODS

13C urea breath test and Ez-HBT Helicobacter blood test were performed 4-6 weeks after completion of treatment in H. pylori positive subjects. Basal urea breath samples were collected; basal Ez-HBT Helicobacter blood test samples were not. Ez-HBT Helicobacter blood test results were reported as positive, negative, or indeterminate.

RESULTS

Seventy patients generated 126 measurable sets of urea breath and blood tests. The H. pylori cure rate was 93%. The sensitivity, specificity, and accuracy of the Ez-HBT Helicobacter blood test were 100%, 97%, and 97%, respectively. Six of eight false positive and indeterminate Ez-HBT Helicobacter blood test results could be attributed to incomplete fasting or a 13C enriched diet. After correcting for the non-fasting state, the positive predictive value of the Ez-HBT Helicobacter blood test improved from 56% to 86%.

CONCLUSION

The performance characteristics of the Ez-HBT Helicobacter blood test are comparable with that of 13C-urea breath test in establishing H. pylori eradication after therapy. Errors related to incomplete fasting can be mitigated by collection of a basal blood sample.

An arginine involved in GABA binding and unbinding but not gating of the GABA(A) receptor

GABA(A) receptor function can be conceptually divided into interactions between ligand and receptor (binding) and the opening and closing of the ligand-bound channel (gating). The relationship between binding, gating, and receptor structure remains unclear. Studies of mutations have identified many amino acid residues that contribute to the GABAbinding site. Most of these studies assayed changes in GABA dose-response curves, which are macroscopic measures that depend on the interplay of many processes and cannot resolve individual microscopic transitions. Understanding the microscopic basis of binding and gating is critical, because kinetic transition rates predict how receptors will behave at synapses. Furthermore, microscopic rates are directly related to the molecular interactions underlying receptor function. Here, we focused on a residue (beta2-R207) previously identified as lining the GABA-binding site that, when mutated to cysteine, greatly reduces apparent GABA affinity and was predicted to affect both binding and gating. To better understand the role of beta2-R207, we expressed alpha1beta2 and alpha1beta2-R207C receptors in human embryonic kidney 293 cells and studied receptor kinetics using fast solution applications. The mutation accelerated deactivation by 10-fold, without altering desensitization in the presence of saturating GABA. Maximum open probability and single-channel open times were also unaltered by the mutation, but the GABA-binding rate was reduced eightfold. Therefore, the effects of this mutation in a predicted binding site residue are solely attributable to changes in GABA-binding and unbinding kinetics, with no changes in channel gating. Because beta2-R207 stabilizes GABA in the binding pocket, it may directly contact the GABA molecule.

Phase and size distribution of polycyclic aromatic hydrocarbons in diesel and gasoline vehicle emissions

Emission measurements were obtained for a variety of military vehicles at Hill Air Force Base (Ogden, UT) in November 2000 as part of a Strategic Environmental Research and Development Program. Aircraft ground support equipment vehicles using gasoline, diesel, and JP8 fuels were tested using chassis dynamometers under predetermined load. The exhaust from the tested vehicle was passed to a dilution tunnel where it was diluted 30-40 times and collected using Micro-Orifice Uniform Deposit Impactor (MOUDI) fitted with aluminum substrates, an XAD-coated annular denuder, and a filter followed by a solid adsorbent. All MOUDI substrates were analyzed for mass and for organic and elemental (EC) carbon by the thermal/optical reflectance method and for polycyclic aromatic hydrocarbons (PAHs) by GC/MS. Black carbon was measured with a photoacoustic instrument. The denuder and filter/solid adsorbent samples were analyzed for semivolatile PAH. Overall, there is more mass and higher EC contribution when the vehicle is run under higher load in comparison with the low load. However, older vehicles generally show more mass and EC emissions than newer vehicles, and there is a shift toward smaller particle sizes for the low load, which is most pronounced for newer vehicles. The particle-associated semivolatile PAHs and nonvolatile four-through six-ring PAHs are present predominantly on the submicron particles collected on MOUDI stages 0.1-0.18, 0.18-0.32, and 0.32-0.56 microm. For the low-load runs, the distribution of PAHs seems to be shifted toward smaller size particles. The gas-particle phase distribution of semivolatile PAHs depends also on the engine loading. For idle, not only are the more volatile two- and three-ring PAHs, from naphthalene to dimethylphenanthrenes, retained on the denuder portion, but also less volatile four-ring PAHs, such as fluoranthene and pyrene, are retained by the denuder at the 80-90% range, which implies that they are present predominantly in the gas phase. In contrast, for engines under high loads, a much larger portion of three- and four-ring PAHs are partitioned to the particle phase.

Dysprosium chloride as a nonabsorbable gastrointestinal marker for studies of stable isotope-labeled triglyceride excretion in man

OBJECTIVE

The aim of this work was to determine if dysprosium chloride (DyCl(3)) is a suitable nonabsorbable marker for studies of labeled-triglyceride excretion in cystic fibrosis patients allowing excretion to be determined accurately after analysis of one or two stools.

METHODS

A series of 66 absorption studies were conducted in 36 young cystic fibrosis patients over a five year period. All tests consisted of ingesting a single test meal containing both (13)C-labeled triglyceride (TG*) and DyCl(3); in most studies the food colorant brilliant blue (FD&C blue #1) was administered along with the DyCl(3). Ingestion of the test meal was followed by collection of individual stools for 72 to 96 hours. Stools were analyzed for (13)C-Excess ((13)C*) and Dy.

RESULTS

Excretion of Dy in cystic fibrosis patients who exhibited a wide-range of steatorrhea was quantitative. Fractional excretion of Dy and (13)C* in individual stools showed a high linear correlation (r(2) = 0.969) with a slope and y-intercept close to unity and zero, respectively. As a result, estimates of TG* excretion based on analysis of only two stools (partial pool method, PPM) were not different from those based on the analysis of all stools or stool composites. This was true both when Dy content and when stool color due to ingested brilliant blue was used to determine which stools to analyze for the PPM.

CONCLUSIONS

Combining the use of Dy and brilliant blue permits reasonably accurate estimates of fecal TG* excretion after analysis of samples from two easily identified stools. This practical method can be used to address many important clinical and experimental questions regarding triglyceride digestion and absorption that may otherwise go unanswered.

Evaluation of catalyzed and electrically heated filters for removal of particulate emissions from diesel-A- and JP-8-fueled engines

In-service diesel engines are a significant source of particulate matter (PM) emissions, and they have been subjected to increasingly strict emissions standards. Consequently, the wide-scale use of some type of particulate filter is expected. This study evaluated the effect of an Engelhard catalyzed soot filter (CSF) and a Rypos electrically heated soot filter on the emissions from in-service diesel engines in terms of PM mass, black carbon concentration, particle-bound polycyclic aromatic hydrocarbon concentration, and size distribution. Both filters capture PM. The CSF relies on the engine's exhaust to reach the catalyst regeneration temperature and oxidize soot, whereas the electrically heated filter contains a heating element to oxidize soot. The filters were installed on several military diesel engines. Particle concentrations and compositions were measured before and after installation of the filter and again after several months of operation. Generally, the CSF removed at least 90% of total PM, and the removal efficiency improved or remained constant after several months of operation. In contrast, the electrical filters removed 44-69% of PM mass. In addition to evaluating the soot filters, the sampling team also compared the results of several real-time particle measurement instruments to traditional filter measurements of total mass.

Characterization of exhaust particles from military vehicles fueled with diesel, gasoline, and JP-8

Diluted exhaust from selected military aircraft ground-support equipment (AGE) was analyzed for particulate mass, elemental carbon (EC) and organic carbon (OC), SO4(2-), and size distributions. The experiments occurred at idle and load conditions and utilized a chassis dynamometer. The selected AGE vehicles operated on gasoline, diesel, and JP-8. These military vehicles exhibited concentrations, size distributions, and emission factors in the same range as those reported for nonmilitary vehicles. The diesel and JP-8 emission rates for PM ranged from 0.092 to 1.1 g/kg fuel. The EC contributed less and the OC contributed more to the particulate mass than reported in recent studies of vehicle emissions. Overall, the particle size distribution varied significantly with engine condition, with the number of accumulation mode particles and the count median diameter (CMD) increasing as engine load increased. The SO4(2-) analyses showed that the distribution of SO4(2-) mass mirrored the distribution of particle mass.

Structural requirements for imidazobenzodiazepine binding to GABA(A) receptors

Several structural subclasses of ligands bind to the benzodiazepine (BZD) binding site of the GABA(A) receptor. Previous studies from this laboratory have suggested that imidazobenzodiazepines (i-BZDs, e.g., Ro 15-1788) require domains in the BZD binding site for high-affinity binding that are distinct from the requirements of classic BZDs (e.g., flunitrazepam). Here, we used systematic mutagenesis and the substituted cysteine accessibility method to map the recognition domain of i-BZDs near two residues implicated in BZD binding, gamma(2)A79 and gamma(2)T81. Both classic BZDs and i-BZDs protect cysteines substituted at gamma(2)A79 and gamma(2)T81 from covalent modification, suggesting that these ligands may occupy common volumetric spaces during binding. However, the binding of i-BZDs is more sensitive to mutations at gamma(2)A79 than classic BZDs or BZDs that lack a 3'-imidazo substituent (e.g., midazolam). The effect that gamma(2)A79 mutagenesis has on the binding affinities of a series of structurally rigid i-BZDs is related to the volume of the 3'-imidazo substituents. Furthermore, larger amino acid side chains introduced at gamma(2)A79 cause correspondingly larger decreases in the binding affinities of i-BZDs with bulky 3' substituents. These data are consistent with a model in which gamma(2)A79 lines a subsite within the BZD binding pocket that accommodates the 3' substituent of i-BZDs. In agreement with our experimental data, computer-assisted docking of Ro 15-4513 into a molecular model of the BZD binding site positions the 3'-imidazo substituent of Ro 15-4513 near gamma(2)A79.

Phenylalanine kinetics in healthy volunteers and liver cirrhotics: implications for the phenylalanine breath test

Expired 13CO2 recovery from an oral l-[1-13C]phenylalanine ([13C]Phe) dose has been used to quantify liver function. This parameter, however, does not depend solely on liver function but also on total CO2 production, Phe turnover, and initial tracer distribution. Therefore, we evaluated the impact of these factors on breath test values. Nine ethyl-toxic cirrhotic patients and nine control subjects received intravenously 2 mg/kg of [13C]Phe, and breath and blood samples were collected over 4 h. CO2 production was measured by indirect calorimetry. The exhaled 13CO2 enrichments were analyzed by isotope ratio mass spectrometry and the [13C]Phe and l-[1-13C]tyrosine enrichments by gas chromatography-mass spectrometry. The cumulative 13CO2 recovery was significantly lower in cirrhotic patients (7 vs. 12%; P < 0.01), in part due to lower total CO2 production rates. Phe turnover in cirrhotic patients was significantly lower (33 vs. 44 micro mol. kg(-1). h(-1); P < 0.05). When these extrahepatic factors were considered in the calculation of the Phe oxidation rate, the intergroup differences were even more pronounced (3 vs. 7 micro mol. kg(-1). h(-1)) than those for 13CO2 recovery data. Also, the Phe-to-Tyr conversion rate, another indicator of Phe oxidation, was significantly reduced (0.7 vs. 3.0 micro mol. kg(-1). h(-1)).

Altered kinetics and benzodiazepine sensitivity of a GABAA receptor subunit mutation [gamma 2(R43Q)] found in human epilepsy

The gamma-aminobutyric acid type A (GABA(A)) receptor mediates fast inhibitory synaptic transmission in the CNS. Dysfunction of the GABA(A) receptor would be expected to cause neuronal hyperexcitability, a phenomenon linked with epileptogenesis. We have investigated the functional consequences of an arginine-to-glutamine mutation at position 43 within the GABA(A) gamma(2)-subunit found in a family with childhood absence epilepsy and febrile seizures. Rapid-application experiments performed on receptors expressed in HEK-293 cells demonstrated that the mutation slows GABA(A) receptor deactivation and increases the rate of desensitization, resulting in an accumulation of desensitized receptors during repeated, short applications. In Xenopus laevis oocytes, two-electrode voltage-clamp analysis of steady-state currents obtained from alpha(1)beta(2)gamma(2) or alpha(1)beta(2)gamma(2)(R43Q) receptors did not reveal any differences in GABA sensitivity. However, differences in the benzodiazepine pharmacology of mutant receptors were apparent. Mutant receptors expressed in oocytes displayed reduced sensitivity to diazepam and flunitrazepam but not the imidazopyridine zolpidem. These results provide evidence of impaired GABA(A) receptor function that could decrease the efficacy of transmission at inhibitory synapses, possibly generating a hyperexcitable neuronal state in thalamocortical networks of epileptic patients possessing the mutant subunit.

Cytochrome P450 3A4 and P-glycoprotein mediate the interaction between an oral erythromycin breath test and rifampin

BACKGROUND

The intravenous (14)C-erythromycin breath test (ERMBT(IV)) does not measure aggregate liver and intestinal cytochrome P450 (CYP) 3A4 activity. Accordingly, we evaluated an oral stable-labeled ((13)C) formulation of the test (ERMBT(oral)) as an alternative CYP3A4 phenotyping probe.

METHODS

After an overnight fast, 14 young healthy volunteers (5 women and 9 men) received the ERMBT(IV) (0.07 micromol, 3 muCi), followed by the ERMBT(oral) (500 mg). The next morning, the CYP3A4 inhibitor troleandomycin (500 mg) was given, and both ERMBTs were repeated. After at least 24 hours, the CYP3A4 and P-glycoprotein inducer rifampin (600 mg; INN, rifampicin) was given daily for 7 days, and both ERMBTs were repeated 24 hours after the last dose of rifampin. Plasma samples were collected for 10 hours with each administration of the ERMBT(oral), and erythromycin levels were measured by liquid chromatography-mass spectrometry. Finally, the effect of troleandomycin on erythromycin transport was examined in Caco-2 cell monolayers.

RESULTS

Compared with baseline values, the median ERMBT(IV) and ERMBT(oral) results and erythromycin apparent oral clearance (CL/F) all significantly decreased, by at least 70%, with troleandomycin treatment (P =.001 for each comparison). With rifampin treatment, the median ERMBT(IV) result and CL/F increased 2-fold (P < or =.01), but the median ERMBT(oral) result was unchanged (P =.30). There were no rank-order correlations between the ERMBT(IV) and ERMBT(oral) results or between either ERMBT result and CL/F within each treatment group (P > or =.07). In addition, troleandomycin had no effect on erythromycin transport in Caco-2 cells (P > or =.20).

CONCLUSIONS

The ERMBT(oral) was influenced by processes in addition to intestinal and hepatic CYP3A4 activity and therefore did not provide a straightforward measure of aggregate CYP3A4 phenotype. The erythromycin-rifampin interaction cannot be attributed to CYP3A4 induction alone and probably also reflected intestinal P-glycoprotein induction.

Simultaneous measurement of gastric emptying with a simple muffin meal using [13C]octanoate breath test and scintigraphy in normal subjects and patients with dyspeptic symptoms

The objective of this study was to determine how the [13C]octanoate breath test (OBT) using a muffin meal correlates with gastric emptying scintigraphy (GES) in normal subjects and patients with dyspeptic symptoms. Ten normal subjects and 23 patients with dyspeptic symptoms underwent simultaneous GES and [13C]OBT. After an overnight fast, a muffin labeled with [99mTc]-sulfur colloid and [13C]octanoate was ingested along with water labeled with [111In]DTPA. Breath samples and scintigraphic images were obtained at baseline and at regular postprandial intervals over 6 hr. In the normal subjects, the mean GES 71/2 of solids and liquids were 64 +/- 17 and 55 +/- 27 minutes, respectively. The calculated OBT T1/2 using the 6-hr breath collection was 138 +/- 15 min and correlated with T1/2 for solids by GES (r = 0.664; P = 0.051), but did not correlate with T1/2 for liquids by GES (r = 0.13; P = 0.738). In dyspeptic patients, the T1/2 for GES was 87 +/- 53 min and 81 +/- 70 min for solids and liquids, respectively. The mean OBT T1/2 was 155 +/- 57 min and correlated with GES T1/2 for solids (r = 0.86; P < 0.001) and GES T1/2 for liquids (r = 0.73; P < 0.001). Delayed gastric emptying (GE) of the muffin meal was identified by scintigraphy in seven patients. The sensitivity and specificity for OBT identifying delayed GE were 86% and 94%. Use of the initial truncated 4-hr OBT results also revealed a significant correlation between OBT and GES T1/2 for solids (r = 0.86; P < 0.001) with sensitivity and specificity for detecting delayed GE of 86% and 94%, respectively. In addition, a linear regression model was able to reduce the number of collection points to four, while maintaining the same sensitivity and specificity. In conclusion, the OBT for GE, using an easily prepared muffin meal, significantly correlates with GES for solids. This muffin-based OBT is a sensitive and specific method to detect delayed GE in dyspeptic patients.

Protein mobility and GABA-induced conformational changes in GABA(A) receptor pore-lining M2 segment

Protein movements underlying ligand-gated ion channel activation are poorly understood. Here we used disulfide bond trapping to examine the proximity and mobility of cysteines substituted for aligned GABAA receptor alpha1 and beta1 M2 segment channel-lining residues in resting and activated receptors. With or without GABA, disulfide bonds formed at alpha1N275C/beta1E270C (20') and alpha1S272C/beta1H267C (17'), near the extracellular end, suggesting that this end is more mobile and/or flexible than the rest of the segment. Near the middle of M2, at alpha1T261C/beta1T256C (6'), a disulfide bond formed only in the presence of GABA and locked the channels open. Channel activation must involve an asymmetric rotation of two adjacent subunits toward each other. This would move aligned engineered cysteines on different subunits into proximity and allow disulfide bond formation without blocking conduction. Asymmetric rotation of M2 segments is probably a common gating mechanism in other ligand-gated ion channels.

Evidence for direct protein kinase-C mediated modulation of N-methyl-D-aspartate receptor current

Protein kinase-C (PKC) activation differentially affects currents from N-methyl-D-aspartate (NMDA) type glutamate receptors depending upon their subunit composition. Experiments using chimeras initially indicated that the cytoplasmic C-terminal tails of NR2B (responsive to PKC) and NR2C (unresponsive to PKC) subunits contain the amino acid residues responsible for the observed disparity of PKC effects. However, truncation and point mutation experiments have suggested that PKC action on NMDA receptors may be entirely indirect, working via the phosphorylation of associated proteins. Here we suggest that PKC does, in fact, affect NR2B/NR1-011 NMDA currents by direct phosphorylation of the NR2B tail at residues S1303 and S1323. Replacement of either of these residues with Ala severely reduces PKC potentiation. To verify that S1303 and S1323 are sites of direct phosphorylation by PKC, synthetic peptides from the regions surrounding these sites were used as substrates for in vitro assays with purified rat brain PKC. These results indicate that PKC can directly phosphorylate S1303 and S1323 in the NR2B C terminus, leading to enhanced currents through NMDA receptor channels. The direct action of PKC on certain NMDA receptor subtypes may be important in any physiological or pathological process where PKC and NR2B/NR1 receptors interact.

Structure and dynamics of the GABA binding pocket: A narrowing cleft that constricts during activation

Photo-affinity labeling and mutagenesis studies have identified several amino acids that may contribute to the ligand binding domains of ligand-gated ion channels. These types of studies, however, only generate a one-dimensional, static description of binding site structure. In this study, we used the substituted cysteine accessibility method not only to identify binding pocket residues but also to elicit information about binding site dynamics and structure. Residues surrounding the putative loop C ligand binding domain of the GABA(A) receptor (beta(2)V199 to beta(2)S209) were individually mutated to cysteine, and the mutant subunits were coexpressed with wild-type alpha(1) subunits in Xenopus oocytes. N-biotinylaminoethyl methanethiosulfonate (MTSEA-biotin) reacts with cysteines introduced at positions G203, S204, Y205, P206, R207, and S209. This accessibility pattern is not consistent with either an alpha-helix or beta-strand. Instead, G203-S209 seems to form a water-accessible extended coil, whereas V199-T202 appears to buried in the protein or membrane. Coapplication of either GABA or the competitive antagonist SR-95531 significantly slows MTSEA-biotin modification of cysteines introduced at positions S204, Y205, R207, and S209, demonstrating that these residues line and face into the GABA binding pocket. MTSEA-biotin reaction rates reveal a steep accessibility gradient from G203-S209 and suggests that the binding pocket is a deep narrowing cleft. Pentobarbital activation of the receptor significantly slows MTSEA-biotin modification of cysteines at S204, R207, and S209, suggesting that the binding site may constrict during gating.

Gastric emptying in trauma patients

BACKGROUND/AIM

The aim of this study was to obtain quantitative data on gastric emptying following trauma.

METHODS

In order to assess gastric emptying for early enteral feeding, we evaluated the absorption of an amino acid, L-[1-(13)C]phenylalanine, within 24 h of admission and 7 days later in 14 trauma patients (injury severity score 36 +/- 2). Following nasogastric administration of 100 mg L-[1-(13)C]phenylalanine, the plasma L-[1-(13)C]phenylalanine enrichment at 30 and 60 min and the expired (13)CO(2) for 1 h in the breath were used to measure the degree of gastric emptying.

RESULTS

The plasma L-[1-(13)C]phenylalanine enrichment concentration at 30 min was 0.53 +/- 0.23 mmol/l during the first study and 2.46 +/- 0. 62 mmol/l during the second study (p = 0.006, a fivefold increase). The L-[1-(13)C]phenylalanine plasma level in historic controls was 4. 57 +/- 1.48 mmol/l. The percent of the dose oxidized and expired as (13)CO(2) in 1 h was 0.51 +/- 0.17 during the first 24-hour study compared to the second study of 3.37 +/- 0.68 (p = 0.0008) 7 days later (an over sixfold increase). The percent of the dose oxidized in 1 h in 37 normal historic controls was 7.08 +/- 0.33.

CONCLUSION

These data indicate delayed gastric emptying with limited recovery in 1 week. We conclude that gastric feeding should not be employed, and the route for early nutritional intervention should be transpyloric for the trauma patient.

Identification of benzodiazepine binding site residues in the gamma2 subunit of the gamma-aminobutyric acid(A) receptor

gamma-Aminobutyric acid(A) receptor gamma-subunits are important for benzodiazepine (BZD) binding and modulation of the gamma-aminobutyric acid-mediated Cl(-) current. Previously, by using gamma2/alpha1 chimeric subunits, we identified two domains of the gamma2-subunit, Lys-41-Trp-82 and Arg-114-Asp-161, that are, in conjunction, necessary and sufficient for high-affinity BZD binding. In this study, we generated additional gamma2/alpha1 chimeric subunits and gamma2 point mutants to identify specific residues within the gamma2 Lys-41-Trp-82 region that contribute to BZD binding. Mutant gamma2 and gamma2/alpha1 chimeric subunits were expressed with wild-type alpha1 and beta2 subunits in HEK 293 cells, and the binding of several BZDs was measured. We present evidence that the gamma2 region Met-57-Ile-62 is important for flunitrazepam binding and that, in particular, gamma2 Met-57 and gamma2 Tyr-58 are essential determinants for conferring high-affinity binding. Furthermore, we identify an additional residue, gamma2 Ala-79, that not only is important for high-affinity binding by flunitrazepam (a strong positive modulator) but also plays a crucial role in the binding of the imidazobenzodiazepines Ro15-1788 (a zero modulator) and Ro15-4513 (a weak negative modulator) in the BZD binding pocket. Results from site-directed mutagenesis of gamma2 Ala-79 suggest that this residue may be part of a microdomain within the BZD binding site that is important for binding imidazobenzodiazepines. This separation of drug-specific microdomains for competitive BZD ligands lends insight into the structural determinants governing the divergent effects of these compounds.

Protein kinase C potentiation of currents from mouse zeta1/epsilon2 NMDA receptors expressed in Xenopus oocytes depends on f-actin/g-actin cycling

NMDA currents from Xenopus oocytes expressing recombinant zeta1/epsilon2 NMDA receptors can be potentiated by activation of protein kinase-C (PKC) and also demonstrate time-dependent rundown. In order to determine whether cytoskeletal proteins are involved in either of these phenomena, experiments were performed using the f-actin stabilizer phalloidin, the f-actin de-stabilizer cytochalasin-D, and the microtubule stabilizer taxol. Phalloidin treatment both prevented rundown and inhibited PKC-potentiation of whole-cell currents but did not affect baseline current amplitudes. Treatment with cytochalasin-D also prevented rundown and inhibited PKC-potentiation of whole-cell currents, but baseline currents from cytochalasin treated cells were only 50% as large as those from control cells. Taxol had no effect on either rundown or PKC potentiation of NMDA currents. The results indicate that both spontaneous rundown and PKC potentiation of currents from heterologously expressed zeta1/epsilon2 NMDA receptors depend on dynamic actin polymerization/depolymerization but do not involve changes in microtubules.

Cerebral asymmetries in processing strategies for letter and symbol trigrams

Several studies have shown that laterally presented consonant-vowel-consonant (CVC) strings produce both superior performance, and a more wholistic processing strategy in the right visual field/left hemisphere (RVF/LHEM), and a more sequential strategy in the inferior left visual field (LVF). To determine whether these strategies are applied to other types of trigrams subjects (n = 30) were asked to identify consonant and symbol trigrams briefly projected unilaterally to the LVF or RVF, or bilaterally (the same trigram in both fields--BVF). A second group of subjects (n = 30) first practiced pronouncing consonant trigrams and then viewed them tachistoscopically. Both tasks yield RVF advantages. Symbols are processed more wholistically in the LVF, more sequentially in the RVF and in an intermediate pattern when presented bilaterally. In contrast, subjects seem to chunk letters as bigrams, and do so equally well in all fields, and visual field differences in strategies emerge for consonants only when they are pronounced. Pronounceability of consonant trigrams, assessed with ratings and vocal reaction times, was predicted by orthographic regularity. Since the RHEM has limited phonetic skills, but it, like the LHEM, is privy to information on orthographic regularity, the error pattern on consonant strings indicates non-phonetic processing, whereas the RVF wholistic strategy for consonant-vowel-consonant strings appears to reflect phonetic processing.

Effect of protein kinase-C activation on the Mg(2+)-sensitivity of cloned NMDA receptors

The mechanisms responsible for protein kinase-c (PKC) mediated potentiation of NMDA receptors are poorly understood. One hypothesis is that PKC-activation reduces the receptor's characteristic voltage-dependent Mg(2+)-blockade. Experiments performed on Xenopus oocytes expressing cloned NMDA receptors demonstrated that PKC-activation induced no change in the sensitivity of zeta 1/epsilon 3 and zeta 1/epsilon 4 receptors to Mg(2+)-blockade and, even though PKC-activation did induce a small shift in Mg2+ sensitivity for the zeta 1/epsilon 1 and zeta 1/epsilon 2 receptors, the change seen was not large enough to account for an appreciable increase in NMDA receptor activity. Baseline Mg(2+)-sensitivities and levels of PKC-mediated potentiation were also quantified for each of the di-heteromeric NMDA receptors. The order of Mg(2+)-sensitivity is zeta 1/epsilon 1 (most sensitive) > zeta 1/epsilon 2 > zeta 1/epsilon 4 > zeta 1/epsilon 3 (least sensitive). PKC-activation caused a 2-fold increase in zeta 1/epsilon 1 currents, a 4-fold increase in zeta 1/epsilon 2 currents and no change in either zeta 1/epsilon 3 or zeta 1/epsilon 4 currents. These data suggest that PKC-potentiation of the cloned di-heteromeric NMDA receptors does not involve a reduction in Mg(2+)-blockade. The di-heteromeric receptors possess varied properties in regard to PKC-potentiation and Mg(2+)-blockade which have been quantified here.

Determination of the isotope enrichment of one or a mixture of two stable labelled tracers of the same compound using the complete isotopomer distribution of an ion fragment; theory and application to in vivo human tracer studies

Calculations of flux rates for stable isotope tracer studies are based upon enrichment values of an infused tracer. We propose the determination of enrichment values by gas chromatography/mass spectrometry, which is based on tracer mole fraction and mass spectrometer signals, normalized over the total signal of an ion fragment isotopomer distribution. The method accounts for overlap of the signals of one or two tracers and the tracee, high tracer mole fraction and incomplete labelling of the (infused) tracer. For the single and multiple tracer case a linear relationship between tracer mole fraction (from zero to one) and all normalized mass spectrometer signals is derived. This linearity over the entire range is demonstrated with a single (1-13C)glucose tracer and for mixtures of (1-13C)- and (3,3-2H2)tyrosine tracers. The linearity allows determination of the tracer mole fraction for two tracers, using multiple linear regression. The corresponding calibration can rely on measurements of the pure tracer and tracee compound, without weighing or check for chemical purity. This is compared with a calibration based on tracer/tracee mixtures. Estimates for the tracer mole fraction are slightly better if based on a calibration, using standard mixtures. In all cases the tracer mole fraction can be determined with high precision (coefficient of variation smaller than 5%) and high accuracy. For tyrosine it is demonstrated that the measurement of seven channels rather than three, for the main isotopomers, does not reduce the precision in the prediction of the tracer mole fraction. Equations are also derived to use the tracer mole fraction to estimate the endogenous production of the tracee under study conditions, assuming a steady state of the host metabolism.

Proline metabolism in adult male burned patients and healthy control subjects

Postabsorptive proline flux, oxidation, and endogenous biosynthesis were determined in five severely burned intensive-care-unit patients (mean age 27 y) and in six healthy, young-adult control subjects. Continuous primed, intravenous, 160-min, dual stable-isotope-tracer infusions of L-[1-13C]proline and L-[methyl-2H3]leucine were used in conjunction with measurement of plasma proline concentration and 24-h urinary hydroxyproline output. Burn patients, compared with normal individuals, demonstrated a doubling in proline and leucine flux (P less than 0.01 for both findings), a threefold enhancement of proline oxidation (P less than 0.05), a trend toward decreased proline synthesis, and a 37% reduction in plasma proline concentrations (P less than 0.05). Further, the injured group, unlike the control group, was in a distinct negative body proline balance, as proline oxidation greatly exceeded endogenous proline biosynthesis (P less than 0.01). These studies indicate that significant proline deficits may evolve during the postabsorptive period in severely burned patients and that an exogenous supply of proline might benefit the nitrogen economy of the traumatized patient.

Branched-chain amino acid interactions with reference to amino acid requirements in adult men: valine metabolism at different leucine intakes

We explored whether the oxidation of valine and by implication the physiological requirement for this amino acid are affected by changes in leucine intake over a physiological range. Six young adult men received, in random order, four L-amino acid-based diets for 5 d supplying either 20 or 10 mg valine.kg body wt-1.d-1, each in combination with 80 or 40 mg leucine.kg-1.d-1. On day 6 subjects were studied with an 8-h continuous intravenous infusion of [1-13C]valine (and [2H3]leucine) to determine valine oxidation in the fasted state (first 3 h) and fed state (last 5 h). Valine oxidation in the fasted state was similar among all diets but was lower (P less than 0.05) in the fed state for the 10 vs 20 mg valine.kg-1.d-1 intake. Leucine intake did not affect valine oxidation. Mean daily valine balance approximated +1.3 mg.kg-1.d-1 for the 20-mg intake and -1.6 mg.kg-1.d-1 for the 10-mg intake. These findings support our previously suggested mean valine requirement estimate of approximately 20 mg.kg-1.d-1.

Methionine kinetics and balance at the 1985 FAO/WHO/UNU intake requirement in adult men studied with L-[2H3-methyl-1-13C]methionine as a tracer

The upper range of the requirement for methionine plus cystine in healthy adults was proposed in 1985 by FAO/WHO/UNU to be 13 mg.kg body wt-1.d-1. To explore the validity of this estimate, five healthy, young adult men were given for 7 d a diet based on an L-amino acid mixture supplying 13 mg methionine.kg-1.d-1 (87 mumol.kg-1.d-1) without cystine. Constant intravenous infusions of L-[2H3-methyl-1-13C]methionine were given on days 5 and 7 while subjects were in the fed and postabsorptive states, respectively. Estimates were made of methionine oxidation, and daily methionine balance was derived from the intake-oxidation data. For the five subjects, methionine balances were -0.9, +0.7, +3.5, -3.1, and -3.8 mg kg-1.d-1, or -6, +5, +23, -21, and -26 mumol.kg-1.d-1. These findings lead to the conclusion that the upper range of the requirement for methionine plus cystine probably exceeds 13 mg.kg-1.d-1 in healthy young adults. The implications of this conclusion for establishing an appropriate amount of sulfur amino acids in an amino acid requirement pattern for adults is discussed.