In vivo research with stable isotopes in biochemistry, nutrition and clinical medicine: an overview

13C-phenylalanine breath test detects altered phenylalanine kinetics in schizophrenia patients

Phenylalanine is an essential amino acid required for the synthesis of catecholamines including dopamine. Altered levels of phenylalanine and its metabolites in blood and cerebrospinal fluid have been reported in schizophrenia patients. This study attempted to examine for the first time whether phenylalanine kinetics is altered in schizophrenia using L-[1-(13)C]phenylalanine breath test ((13)C-PBT). The subjects were 20 chronically medicated schizophrenia patients (DSM-IV) and the same number of age- and sex-matched controls. (13)C-phenylalanine (99 atom% (13)C; 100 mg) was administered orally and the breath (13)CO(2) /(12)CO(2) ratio was monitored for 120 min. The possible effect of antipsychotic medication (risperidone (RPD) or haloperidol (HPD) treatment for 21 days) on (13)C-PBT was examined in rats. Body weight (BW), age and diagnostic status were significant predictors of the area under the curve of the time course of Δ(13)CO(2) (‰) and the cumulative recovery rate (CRR) at 120 min. A repeated measures analysis of covariance controlled for age and BW revealed that the patterns of CRR change over time differed between the patients and controls and that Δ(13)CO(2) was lower in the patients than in the controls at all sampling time points during the 120 min test, with an overall significant difference between the two groups. Chronic administration of RPD or HPD had no significant effect on (13)C-PBT indices in rats. Our results suggest that (13)C-PBT is a novel laboratory test that can detect altered phenylalanine kinetics in chronic schizophrenia patients. Animal experiments suggest that the observed changes are unlikely to be attributable to antipsychotic medication.

N-carbamylglutamate enhancement of ureagenesis leads to discovery of a novel deleterious mutation in a newly defined enhancer of the NAGS gene and to effective therapy

N-acetylglutamate synthase (NAGS) catalyzes the conversion of glutamate and acetyl-CoA to NAG, the essential allosteric activator of carbamyl phosphate synthetase I, the first urea cycle enzyme in mammals. A 17-year-old female with recurrent hyperammonemia attacks, the cause of which remained undiagnosed for 8 years in spite of multiple molecular and biochemical investigations, showed markedly enhanced ureagenesis (measured by isotope incorporation) in response to N-carbamylglutamate (NCG). This led to sequencing of the regulatory regions of the NAGS gene and identification of a deleterious single-base substitution in the upstream enhancer. The homozygous mutation (c.-3064C>A), affecting a highly conserved nucleotide within the hepatic nuclear factor 1 (HNF-1) binding site, was not found in single nucleotide polymorphism databases and in a screen of 1,086 alleles from a diverse population. Functional assays demonstrated that this mutation decreases transcription and binding of HNF-1 to the NAGS gene, while a consensus HNF-1 binding sequence enhances binding to HNF-1 and increases transcription. Oral daily NCG therapy restored ureagenesis in this patient, normalizing her biochemical markers, and allowing discontinuation of alternate pathway therapy and normalization of her diet with no recurrence of hyperammonemia. Inc.

N-carbamylglutamate augments ureagenesis and reduces ammonia and glutamine in propionic acidemia

OBJECTIVES

The objective of this study was to determine whether N-carbamylglutamate (NCG) reduces plasma levels of ammonia and glutamine and increases the rate of ureagenesis in patients with propionic acidemia (PA).

METHODS

Identical 4-hour studies were performed before and immediately after a 3-day trial of oral NCG in 7 patients with PA. An oral bolus of [(13)C]sodium acetate was administered at the start of each study, and sequential blood samples were obtained to measure [(13)C]urea, ammonia, urea, and amino acids.

RESULTS

With longitudinal mixed-effects linear regression, peak [(13)C]urea increased after treatment with NCG (from 2.2 to 3.8 microM; P < .0005). There were concomitant decreases in mean plasma ammonia (59-43 microM; P < .018) and glutamine (552-331 microM; P < .0005).

CONCLUSIONS

NCG augments ureagenesis and decreases plasma ammonia and glutamine in patients with PA. The drug may serve as an important therapeutic adjunct in the treatment of acute hyperammonemia in this disorder.

N-carbamylglutamate markedly enhances ureagenesis in N-acetylglutamate deficiency and propionic acidemia as measured by isotopic incorporation and blood biomarkers

N-acetylglutamate (NAG) is an endogenous essential cofactor for conversion of ammonia to urea in the liver. Deficiency of NAG causes hyperammonemia and occurs because of inherited deficiency of its producing enzyme, NAG synthase (NAGS), or interference with its function by short fatty acid derivatives. N-carbamylglutamate (NCG) can ameliorate hyperammonemia from NAGS deficiency and propionic and methylmalonic acidemia. We developed a stable isotope (13)C tracer method to measure ureagenesis and to evaluate the effect of NCG in humans. Seventeen healthy adults were investigated for the incorporation of (13)C label into urea. [(13)C]urea appeared in the blood within minutes, reaching maximum by 100 min, whereas breath (13)CO(2) reached a maximum by 60 min. A patient with NAGS deficiency showed very little urea labeling before treatment with NCG and normal labeling thereafter. Correspondingly, plasma levels of ammonia and glutamine decreased markedly and urea tripled after NCG treatment. Similarly, in a patient with propionic acidemia, NCG treatment resulted in a marked increase in urea labeling and decrease in glutamine, alanine, and glycine. These results provide a reliable method for measuring the effect of NCG on nitrogen metabolism and strongly suggest that NCG could be an effective treatment for inherited and secondary NAGS deficiency.

(13)C-phenylalanine breath test correlates with liver fibrosis in postoperative biliary atresia

BACKGROUND

Values derived from the (13)C-phenylalanine breath test (PBT) may serve as an index for liver fibrosis and clinically predictive readings for liver diseases in adults. In the present study the PBT was conducted in postoperative biliary atresia (BA) children to evaluate phenylalanine metabolism in the liver, and the results based on biochemical data, especially the index on liver fibrosis, were compared with PBT findings.

METHODS

Hepatofunctional evaluations were conducted in 10 postoperative BA children with moderate (group B; n = 4) and severe (group A; n = 6) liver dysfunction, and the PBT results were compared with those of 13 normal healthy children (group C). Subjects were orally given single-bolus (13)C-phenylalanine at 3.5 mg/kg (maximum dosing: 100 mg) in the morning. Time-related exhaled gas was periodically collected until 120 min after dosing. The (13)CO(2) levels were monitored with gas chromatography-mass spectrometry before and after administration, and the (13)C excretion rate, (13)C cumulative excretion and time of maximum (13)C excretion rate were monitored accordingly.

RESULTS

Total bile acid, hyaluronic acid, type IV collagen 7S, total bilirubin or albumin and the PBT findings were significantly correlated. The PBT findings in group A were significantly lower those of group B, indicating that phenylalanine metabolism was markedly attenuated in the former.

CONCLUSION

The PBT values correlated well with liver fibrosis in postoperative BA children. Because PBT is a non-invasive approach, results from this method may serve as a useful and reliable index for post-surgical monitoring of children operated on for liver fibrosis.

Synthetic 13C-dipeptide breath test for the rapid assessment of pancreatic exocrine insufficiency in rats

OBJECTIVE

(13)C-breath tests have been investigated in order to assess pancreatic exocrine function using various (13)C-compounds, but they have not been accepted for routine clinical use. One of the barriers to their acceptance is that these tests are time-consuming and require up to several hours for breath collection. The purpose of this study was to design a novel (13)C-compound that would make a rapid (13)C-breath test for assessing exocrine pancreatic function possible.

MATERIAL AND METHODS

N-benzoyl-L-tyrosyl-1-(13)C-L-alanine was synthesized, and the characteristics of its cleavage in duodenal juice and in the duodenum of rats were examined. Thereafter, a (13)C-breath test was carried out in which N-benzoyl-L-tyrosyl-1-(13)C-L-alanine was given orally to pancreatic exocrine-insufficient and normal control rats.

RESULTS

N-benzoyl-L-tyrosyl-1-(13)C-L-alanine was readily cleaved and liberated 1-(13)C-L-alanine in the duodenal juice. Carboxypeptidase was a major contributor to the cleavage. When N-benzoyl-L-tyrosyl-1-(13)C-L-alanine was injected into the duodenum and orally administered to the rats, the (13)C atom% of CO(2) in breath increased rapidly. This indicated that N-benzoyl-L-tyrosyl-1-(13)C-L-alanine in the duodenum liberated (13)C-Ala on cleavage. (13)C-Ala is absorbed and metabolized to liberate (13)CO(2), which is exhaled. It was shown that the Delta(13)CO(2) ( per thousand) in the N-benzoyl-L-tyrosyl-1-(13)C-L-alanine breath test in the pancreatic exocrine-insufficient rats, in whom the absorption and metabolism of (13)C-Ala was unimpaired, was significantly lower than that in the control rats.

CONCLUSIONS

The rate of increase in the Delta(13)CO(2) ( per thousand) in the N-benzoyl-L-tyrosyl-1-(13)C-L-alanine breath test is expected to be proportional to the rate of N-benzoyl-L-tyrosyl-1-(13)C-L-alanine cleavage by pancreatic proteases in the duodenum. We propose the N-benzoyl-L-tyrosyl-1-(13)C-L-alanine breath test as a rapid test for assessing pancreatic exocrine function.

An Algorithm for the Deconvolution of Mass Spectrosopic Patterns in Isotope Labeling Studies. Evaluation for the Hydrogen−Deuterium Exchange Reaction in Ketones

An easy to use computerized algorithm for the determination of the amount of each labeled species differing in the number of incorporated isotope labels based on mass spectroscopic data is described and evaluated. Employing this algorithm, the microwave-assisted synthesis of various alpha-labeled deuterium ketones via hydrogen-deuterium exchange with deuterium oxide was optimized with respect to time, temperature, and degree of labeling. For thermally stable ketones the exchange of alpha-protons was achieved at 180 degrees C within 40-200 min. Compared to reflux conditions, the microwave-assisted protocol led to a reduction of the required reaction time from 75-94 h to 40-200 min. The alpha-labeled deuterium ketones were reduced by biocatalytic hydrogen transfer to the corresponding enantiopure chiral alcohols and the deconvolution algorithm validated by regression analysis of a mixture of labeled and unlabeled ketones/alcohols.

Collisionally assisted, highly selective laser isotope separation of carbon-13

We have further developed our recently reported two-laser technique for highly selective molecular isotope separation of carbon-13 [Boyarkin, Kowalczyk, and Rizzo, J. Chem. Phys. 118, 93 (2003)] with the objective of increasing the yield. An essential feature of this approach in its original conception is the significant increase of isotopic selectivity that occurs through collisions during the time between the overtone preexcitation laser pulse and the multiphoton dissociation pulse. We demonstrate here that under certain conditions, this collisional enhancement of the selectivity works equally well when the two pulses are overlapped in time, allowing the overall isotopic selectivity of the process to remain high while achieving a significant increase in the absolute dissociation yield. We also find that proper shaping of the CO2 laser dissociation pulse makes the fluence required for dissociation sufficiently low to allow irradiation of a large reaction volume by unfocused laser beams. Together, these factors may make this laser isotope separation scheme competitive with existing separation methods.

Regiospecificity in deuterium labeling determined by mass spectrometry

Several mass spectrometry methods were explored to determine the regiospecificity of deuterium substitutions in hydrocarbon mixtures. The case investigated in this work was that of ethane mixtures obtained by catalytic HD exchange between either C(2)H(6) and D(2) or C(2)D(6) and H(2) over platinum surfaces. A total of ten isotopologs are possible, and were indeed detected in all cases. Deconvolution of low-resolution mass spectra was found sufficient to determine the composition of the gas mixtures in terms of the total number of deuterium substitutions, but not to identify symmetric versus asymmetric substitutions in the C(2)D(2)H(4), C(2)D(3)H(3), and C(2)D(4)H(2) products. High-resolution mass spectrometry allowed the separation of the intensities due to C(2)X(4)(+) fragments from those from molecular C(2)X(6)(+) signals (X = H or D), and with that for a more accurate determination of the composition of the mixtures. Relative probabilities were determined for the symmetric versus asymmetric removal of X(2) from C(2)X(6)(+) ions and for isotope scrambling in the mass spectrometer, and with that information fairly good cracking patterns were then calculated for the C(2)X(4)(+) fragments produced by each individual pure C(2)X(6) isotopologue. However, total deconvolution of all ten components in the ethane mixtures obtained by HD exchange catalysis was beyond the experimental accuracy of the measurements. Tandem mass spectrometry/collision-induced decomposition mass spectrometry (MS/CID-MS) proved more useful for this task. In particular, it was possible to determine the proportion of symmetric versus asymmetric double HD exchange in samples for which the total ethane-d(2) (in the case of C(2)H(6) + D(2)) or ethane-d(4) (with C(2)D(6) + H(2)) amounted to only approximately 3% on the ethane mix. A comparison with other analytical methods, NMR in particular, is provided.

Measurement of hepatic phenylalanine metabolism in children using the [13C]-phenylalanine breath test and gas chromatography–mass spectrometry

The essential amino acid, phenylalanine (PA), is known to be metabolized mainly in the liver of human adults. Because the liver is still in the developmental phase, the PA-related metabolic events in infants remain unsolved. In this study, evaluations of development in hepatic PA metabolism in 37 children and 16 adults were attempted using the (13)C -PA breath test (PBT). The subjects were categorized into four groups according to their ages in years and months: 2 years and 0 month to 3 years and 5 months (group I; n = 12); 3 years and 6 months to 4 years and 11 months (group II, n = 12); 5 years and 0 month to 6 years and 11 months (group III, n = 13); and healthy adults (group IV; n = 16). Changes in CO(2) level of exhaled gas at various time intervals after oral administration of (13)C -PA were monitored using gas chromatography-mass spectrometry to derive the (13)C excretion rate, cumulative excretion curve and time maximum [(13)C excretion rate (T(MAX)). In the present investigation involving children, significant increases of maximum(13)C excretion rate and cumulative excretion at 120 min after administration were established in group III. Furthermore, differences in PBT were not established between groups III and IV. The index for first-pass effect, T(MAX), did not change with time. From the above findings, the (13)C excretion rate increased with time although hepatic PA metabolism in infants remained underdeveloped, and children at the age of 5-7 years manifested PA metabolism similar to that of adults.

Mass spectrometry methods for metabolic and health assessment

Beginning in the mid 1960s, mass spectrometry was introduced in a few academic laboratories for the analysis of organic acids by gas chromatography-mass spectrometry. Since then, multiple-stage mass spectrometers have become available and many new applications have been developed. Major advantages of these new techniques include their ability to rapidly determine many different compounds in complex biological matrices with high sensitivity and in sample volumes of usually < 100 microL. A high sample throughput is further realized because extensive sample preparations are often not necessary. However, because the technical know-how is not yet widely available and significant experience is required for correct interpretation of results, these methods are being implemented slowly in routine clinical laboratories as opposed to research laboratories. Several of these new applications are considered with regard to clinical medicine.