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At least 1,063 records · Page 59Linked to original sources
PubMed1978

Comparison of double- and single-isotope enzymatic derivative methods for measuring catecholamines in human plasma.

We directly compared the reliability of a single-isotope enzymatic derivative technique for measurement of plasma catecholamines with that of the well-established double-isotope method. A significant (p less than 0.001) correlation was observed between measurements (n = 52) in the two assays, both for norepinephrine (r = 0.97) and epinephrine (r = 0.80). Means and coefficients of variation for the two analytes in a pooled specimen of plasma, measured repeatedly during six months, were virtually identical by each assay method. Basal plasma catecholamine concentrations in two different groups of apparently healthy subjects were also similar by each method. Dopamine concentrations in plasma were consistently below the limits measurable by either technique. The single-isotope assay requires half the assay time and 1/200th the sample as the double-isotope method. We conclude that this assay is just as reliable as the double-isotope technique and gives virtually identical values for norepinephrine and epinephrine concentrations in the physiological range.

Adult↗
PubMed1984

Peroxidase-catalyzed N-demethylation reactions. Substrate deuterium isotope effects.

Deuterium isotope effects on the kinetic parameters for the hydroperoxide-supported N-demethylation of N,N-dimethylaniline catalyzed by chloroperoxidase and horseradish peroxidase were determined using N,N-di-(trideuteromethyl)aniline. The isotope effect on the Vmax for the chloroperoxidase-catalyzed demethylation reaction supported by ethyl hydroperoxide was 1.42 +/- 0.31. The isotope effects on the Vmax for the horseradish peroxidase-catalyzed reaction supported by ethyl hydroperoxide and hydrogen peroxide were 1.99 +/- 0.39 and 4.09 +/- 0.27, respectively. Isotope effects ranging from 1.76 to 5.10 were observed on the Vmax/Km for the hydroperoxide substrate (i.e. the second order rate constant for the reaction of the hydroperoxide with the peroxidase to form compound I) in both enzyme systems when the N-methyl groups of N,N-dimethylaniline were deuterated. These results are not predicted by the simple ping-pong kinetic model for peroxidase-catalyzed N-demethylation reactions. The data are most simply explained by a mechanism involving the transfer of deuterium (or hydrogen) from N,N-dimethylaniline to the enzyme during catalysis. The deuterium must subsequently be displaced from the enzyme by the hydroperoxide, causing the observed isotope effects.

Aniline Compounds↗
PubMed1983

Duodenogastric bile reflux as measured by an isotope technique and its correlation with endoscopic findings.

76 patients were examined for duodenogastric reflux by gastroscopy and by a non-invasive isotope method using Tc-99m diethyliminodiacetic acid. The bile acid concentration of endoscopically aspirated gastric juice was measured. The patients with endoscopically diagnosed bile in the stomach (51 patients) had duodenogastric reflux significantly (p less than 0.05) more often in the isotope examination than the patients without bile observed in the stomach (25 patients). The amount of bile as estimated endoscopically correlated well with the bile and acid concentration of gastric juice. However, 18 patients with endoscopic bile reflux had no reflux in the isotope examination and 9 patients without endoscopic bile reflux had a mild reflux isotopically. The reflux estimations made endoscopically tend to give comparable results in general, but in many patients the results entirely differ from those obtained with the isotope method. We suggest that this difference is mainly due to the invasive nature of the endoscopic examination.

Bile Acids and Salts↗
PubMed1980

[Isotopic kidney investigations in staghorn lithiasis in the adult (author's transl)].

The authors report a study of 37 kidneys in 30 patients with staghorn calculi undergoing isotopic investigation of renal function by sequential scintigraphy and isotopic nephrogram by gamma-camera. 7 of the patients had bilateral staghorn calculi. Tracers used: Hippuran labelled with I131 and DPTA labelled with Technetium 99 m. 62% of kidneys poorly tolerated their staghorn calculus as shown by severe impairment of the isotopic nephrogram involving the uptake phase. Amongst the 20 cases of unilateral lithiasis, a nephrectomy was performed whenever the functional value was less than 20% normal (7 kidneys). Twelve stones were removed with conservation of the kidney. It should be noted that with renal function greater than 70% normal, the thickness of the renal parenchyma as assessed by IVU is never less by more than 1/2 cm that of the contralateral kidney. In the group, 11 patients underwent postoperative urography and scintigraphy with a mean follow up of 4 years. All extractions of calculi by pyelotomy with or without a small nephrotomy benefited initially impaired kidneys. By contrast, in 5 kidneys where a large nephrotomy was made, there was a gain in function in only one. 1 remained stationary and the others lost respectively 20, 30 and 100% of their function. Correlation was good with other methods of split renal function studies (IVU and renal arteriography). Of essential importance is the fact that the irradiation associated with these isotopic investigations is 20 to 50 times less than that of IVU. Isotopic investigation is thus a good method for the assessment of changes in renal function after surgery for staghorn calculus.

Adult↗
PubMed1977

Suitability of [11, 12-3H2]chenodeoxycholic acid and [11, 12-3H2]lithocholic acid for isotope dilution studies of bile acid metabolism in man.

Studies were carried out to assess the stability of the label in [11,12-(3)H(2)]chenodeoxycholic acid and [11,12-(3)H(2)]lithocholic acid during enterohepatic cycling and thus define the suitability of these tracers for isotope dilution studies of bile acids in man. To do this, [11,12-(3)H(2)]-chenodeoxycholic acid and [24-(14)C]chenodeoxycholic acid were administered simultaneously to six healthy adults. The (3)H/(14)C ratio of biliary chenodeoxycholic acid and the specific activity of biliary chenodeoxycholic acid were determined to permit calculation of pool size and turnover rate by the conventional isotope dilution procedure of Lindstedt (1957. Acta Physiol. Scand. 40: 1). Excretion of label in urine was measured as well as the (3)H/(14)C ratio of fecal bile acids. Similar studies were carried out with [11,12-(3)H(2)]lithocholic acid and [24-(14)C]lithocholic acid. With [11,12-(3)H(2)]chenodeoxycholic acid, 10-15% of the label was lost during the first 24 hours, after which the (3)H/(14)C ratio of biliary chenodeoxycholic acid remained constant. The loss of (3)H caused a consistent overestimate of pool size by 10-15%, but since the fractional turnover rate of the two isotopes was identical, the calculated rate of synthesis was 10-15% greater when determined using (3)H. The loss of (3)H was attributed to the presence of (3)H in positions other than 11 and 12 occurring during the labeling procedure when the unsaturated precursor was exposed to carrier-free tritium gas. The (3)H/(14)C ratio of feces was consistently below that of bile, suggesting bacterial removal of additional (3)H during distal intestinal passage. In contrast, [11,12-(3)H(2)]lithocholic acid lost (3)H continuously, so that by 2 days after administration, the (3)H/(14)C ratio had fallen to nearly half of that of the administered mixture. It is concluded that isotope dilution studies may be carried out with one preparation of [11,12-(3)H(2)]chenodeoxycholic acid with an error of 10-15%; possibly a more stable preparation can be prepared with a gentler tritiation procedure. One preparation of [11,12-(3)H(2)]lithocholic acid was unsatisfactory for isotope dilution studies in man.

Adult↗
PubMed1998

Oxygen isotope exchange between refractory inclusion in allende and solar nebula Gas

A calcium-aluminum-rich inclusion (CAI) from the Allende meteorite was analyzed and found to contain melilite crystals with extreme oxygen-isotope compositions ( approximately 5 percent oxygen-16 enrichment relative to terrestrial oxygen-16). Some of the melilite is also anomalously enriched in oxygen-16 compared with oxygen isotopes measured in other CAIs. The oxygen isotopic variation measured among the minerals (melilite, spinel, and fassaite) indicates that crystallization of the CAI started from oxygen-16-rich materials that were probably liquid droplets in the solar nebula, and oxygen isotope exchange with the surrounding oxygen-16-poor nebular gas progressed through the crystallization of the CAI. Additional oxygen isotope exchange also occurred during subsequent reheating events in the solar nebula.

Journal Article↗
PubMed1998

Determination of low isotopic enrichment of L-[1-13C]valine by gas chromatography/combustion/isotope ratio mass spectrometry: a robust method for measuring protein fractional synthetic rates in vivo.

A method was developed for measuring protein fractional synthetic rates using the N-methoxycarbonylmethyl ester (MCM) derivative of L-[1-13C]valine and on-line gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). The derivatization procedure can be performed rapidly and GC separation of valine from the other branched-chain amino acids, leucine and isoleucine, is easily obtained. A good linear relationship was observed between the increment of the 13C/12C isotope ratio in CO2 gas derived from the combustion of derivatized valine and the tracer mole ratio of L-[1-13C]valine to unlabelled valine. The limit of quantitation was at an L-[1-13C]valine tracer mole ratio of 0.0002. The method was used to measure the isotopic enrichment of L-[1-13C]valine in standard mixtures and in skeletal muscle of six growing piglets infused with L-[1-13C]valine (2 mg kg-1 h-1 for 6 h). After infusion of L-[1-13C]valine the mean tracer mole ratio in plasma of L-[1-13C]valine at the isotopic steady state was 0.0740 +/- 0.0056 (GC/MS, mean +/- SEM) and the mean tracer mole ratio of valine in muscle protein fraction at 6 h was 0.000236 +/- 0.000038 (GC/C/IRMS). The resulting mean protein fractional synthetic rate in piglet skeletal muscle was 0.052 +/- 0.007% h-1, which is in good agreement with literature data obtained with alternative, more elaborate techniques. By this method protein fractional synthetic rates can be measured at low isotopic enrichment levels using L-[1-13C]valine, the MCM derivative and on-line GC/C/IRMS.

Animals↗
PubMed1997

Osteopenia and stable isotope ratios in bone collagen of Nubian female mummies.

Stable carbon and nitrogen isotopes were analysed on bone collagen of 43 Sudanese Nubians from the X-Group period to test dietary hypotheses for the high frequency of osteopenia in this population. Stable carbon isotope ratios indicate that both normal and osteopenic individuals consumed the same mixed diet of C3 and C4 sources, which are assumed to have been constituted by the grain staples wheat/barley and sorghum/millet respectively. Females with osteopenia, however, have significantly elevated delta 15N values. The enrichment effect is greatest in the third and fifth decades of life, and is consistently patterned with microstructural and frequency differences previously reported by other researchers. It is suggested that delta 15N is reflecting differences in urea excretion and the renal processing and clearance of calcium and phosphorus. The study not only alerts us to the susceptibility of stable nitrogen isotopes to non-dietary (i.e. physiological) factors, but also identifies nitrogen isotope ratios as a possible new marker for osteopenia.

Adolescent↗
PubMed2000

Stable carbon and oxygen isotopic analysis of carbon monoxide in natural waters.

Techniques have been developed to allow on-line simultaneous analysis of concentration and stable isotopic compositions ((13)C and (18)O) of dissolved carbon monoxide (CO) in natural water, using continuous-flow isotope ratio mass spectrometry (CF-IRMS). The analytical system consisted sequentially of a He-sparging bottle of water, a gas dryer, CO(2)-trapping stage using both Ascarite trap and silica-gel packed gas chromatography (GC), on-line oxidation to CO(2) using the Schütze reagent, cryofocusing, GC purification using a capillary column and measurement by CF-IRMS. Each sample analysis takes about 40 minutes. The detection limit with delta(13)C standard deviation of 0.5 per thousand is 300 pmol and that with delta(18)O deviation of 1.0 per thousand is 750 pmol. Analytical blanks associated with these methods are 21+/-9 pmol. The procedures are evaluated through analyses of temporally varying concentration and isotopic compositions of CO in an artificial lake on the university campus. The delta(13)C and delta(18)O values of CO showed wide variation in accordance with diurnal variation of CO concentration, probably due to significant isotopic effects during photochemical production and microbial oxidation of CO in the aquatic environment. The delta(13)C and delta(18)O values of CO should be a useful tool in studies of the mechanism and pathways of CO production and consumption in natural waters.

Carbon Isotopes↗
PubMed1993

Age differences in stable carbon and nitrogen isotope ratios in a population of prehistoric maize horticulturists.

Stable carbon isotope ratios in prehistoric human bone collagen have been used extensively to document the introduction and intensification of maize horticulture in notheastern North America. Most previous studies are based on small samples of adults who are assumed to characterize the diet of the population. In this study, all 29 individuals buried within an Ontario Iroquoian village site dated A.D. 1530-1580 were analysed for stable isotopes of carbon and nitrogen. Age distribution of the sample ranges from preterm to elderly. Significant negative correlations between age and delta 13C, and age and delta 15N values were found. High delta 13C values in infants and young children (delta 13C = -6.8 to -12.3) suggest a weaning diet high in maize. High delta 15N values in infants relative to adults suggest a trophic level effect during breast-feeding which has been reported in a modern sample by Tuross et al. (Am. J. Phys. Anthropol., 1993). In addition to the isotopic evidence for extremely high carbohydrate (maize) intake, the MacPherson sample includes two juveniles aged 3-4 years, exhibiting circular caries. No other cases of this condition are known in the extensively studied southern Ontario skeletal collections. Together the evidence from dentition and stable carbon isotopes indicates a very high carbohydrate diet in subadults. Circular caries result from developmental stress during enamel formation with subsequent caries formation in areas of thinner enamel. These findings are relevant to studies of infant and early childhood morbidity and mortality among prehistoric maize horticulturists.

Adult↗
PubMed2002

Correcting mass isotopomer distributions for naturally occurring isotopes.

In one method of metabolic flux analysis, simulated mass spectrometry data is fitted to measured mass distributions of metabolites that are isolated from cultures with defined feeding of (13)C-labeled substrates. Doing so, simulated mass distributions must be corrected for the presence of naturally occurring isotopes. A method that was recently introduced for this purpose consists of consecutive correction steps for each isotope of each element in the considered compound. Here we show that all isotopes of each individual element must, however, be corrected in one single step. Furthermore, it is shown that the source of information with respect to isotopic compositions of the elements needs to be chosen with care.

Biomarkers↗
PubMed1980

A stable isotope dilution assay for disopyramide and its [13C, 15N]labelled analogue in biological fluids.

The mass spectra of disopyramide phosphate and two stable isotopically labelled analogues have been obtained using electron impact and chemical ionization. The low isotopic purity of [13C, 15N)disopyramide phosphate was shown to be due to the low isotopic purity of the 15N label. A stable isotope dilution assay for disopyramide and [13C, 15N]disopyramide in biological fluids has been developed using [2H14]disopyramide phosphate as the internal standard. This assay will be used to analyse samples obtained after the co-administration of disopyramide phosphate intravenously and [13C, 15N]disopyramide phosphate orally to several animal species.

Animals↗
PubMed1993

Improved measurement of stable isotope ratios in gas chromatography/mass spectrometry using the microwave-powered chemical reaction interface for mass spectrometry.

The microwave-powered chemical reaction interface for mass spectrometry (CRIMS) has been successfully used for selective detection of analytes labeled with 13C, 15N and D following capillary gas chromatography separation with good analytical characteristics in biological applications. In this study we evaluated how an advanced data system coupled to a quadrupole mass analyzer could improve precision and sensitivity of stable isotope ratio measurements for 13C and 15N. The enrichments of 13C and 15N are determined by monitoring CO2 (m/z 44 and 45) and NO (m/z 30 and 31). These small molecules are produced from the analyte in the chemical reaction interface in the presence of SO2 as a reactant gas. Using caffeine and its 13C1, 15N2-labeled analog for these quantitative studies, we have found that the Vector 2 system improves overall precision (RSD = 0.6% for both carbon and nitrogen) and sensitivity of stable isotope measurements by at least a factor of two compared to the Vector 1 system, and by more than an order of magnitude compared to our older results. With the optimum system we are now able to measure an atom% enrichment of 0.0044 for 15N and 0.015 for 13C in caffeine in the presence of 300 ng of unlabeled material. This is more than half way between isotopic detection limits of conventional gas chromatography/mass spectrometry and the state-of-the-art, which is a gas chromatograph coupled to a chemical combustor and a dual-collector isotope ratio mass spectrometer.

Caffeine↗
PubMed2003

High-temperature elemental analysis and pyrolysis techniques for stable isotope analysis.

A universal method for pyrolysis and elemental analysis, suitable for the online determination of deuterium, carbon, nitrogen and oxygen isotopes for organic and inorganic substances, is presented. The samples are pyrolytically decomposed in a high-temperature pyrolysis (HTP) system, at a temperature exceeding 1400 degrees C, in the presence of reactive carbon. The method is suitable for the analysis of stable isotope ratios from hydrogen, carbon, nitrogen and oxygen. The instrumentation and experimental procedure are simple and cost-effective. The reproducibility of the delta values for D/H is better than 3 per thousand, and for (18)O, (13)C (organic) and (15)N (inorganic) it is approximately 0.2 per thousand. The HTP system is suitable for solid and liquid samples and can use an autosampler for the samples. Results are presented for the isotopic composition of international reference materials and selected laboratory reference materials, which demonstrate the precision and accuracy of the method. Possible problems in the measurement of nitrates and their solutions are particularly discussed. The analyses of oxygen isotopes in selected geological samples (carbonates, silicate, biotite) are demonstrated.

Aluminum Silicates↗
PubMed2004

N2O influence on isotopic measurements of atmospheric CO2.

In spite of extensive efforts, even the most experienced laboratories dealing with isotopic measurements of atmospheric CO2 still suffer from poor inter-laboratory consistency. One of the complicating factors of these isotope measurements is the presence of N2O, giving rise to mass overlap in the isotope ratio mass spectrometer (IRMS). The aim of the experiment reported here has been twofold: first, the re-establishment of the correction for 'mechanical' interference of N2O in the IRMS, along with its variability and drift, and the best way to quantitatively determine the correction factors. Second, an investigation into secondary effects, i.e. the influence of N2O admitted with the CO2 sample on the "cross contamination" between sample and (pure CO2) working gas. To make the suspected effects better detectable, isotopically enriched CO2 gas with different concentrations of N2O has been measured for the first time. No evidence of secondary effects was observed, from which we conclude that N2O is not a major player in the inter-laboratory consistency problems. Still, we also found that the determination of the 'mechanical' N2O correction needs to be very carefully determined for each individual IRMS, and should be periodically re-determined. We show that the determination of the correction should be performed using CO2/N2O mixtures with concentration ratios around that of the atmosphere, as the extrapolation from pure gas end member behaviour will give erroneous results due to non-linearities. For our IRMS, a VG SIRA series II, we find a correction of 0.23 per thousand for delta45CO2 and 0.30 per thousand for delta46CO2 of atmospheric samples, (with 0.85 per thousand mixing ratio). This implies that the relative ionisation efficiency (E) value associated with this machine is 0.75.

Atmosphere↗
PubMed2004

The effect of N2O on the isotopic composition of air-CO2 samples.

The ionisation efficiencies of N2O vs. CO2 as well as their ratios were measured in detail introducing clean N2O and CO2 into the electron impact ion source of an isotope ratio mass spectrometer. Changes in the ionisation efficiency ratio (IER) were found for different electron energy settings and compared with the ratios of literature ionisation cross-section values for pure N2O and CO2. To establish the influence of mixtures of N2O and CO2 in a mass spectrometer, artificial air mixtures were prepared by mixing different amounts of N2O and CO2 from well-calibrated spike cylinders with CO2-free air. The mixing ratios varied from 8-512 ppb for N2O and from 328-744 ppm for CO2. With these mixtures the effects of varying N2O concentrations on apparent CO2 isotope ratios in air samples were determined. After applying a mass balance correction the delta13C results were consistent within small error margins. The data seemed almost independent from a particular choice for the IER of N2O vs. CO2 in the correction algorithm. For delta18O a small effect of the ionisation efficiency ratio of N2O vs. CO2 was found. Several sets of calculations were made varying the IER between 0.88 and 0.62. The dependence of delta18O was the smallest with an adopted IER of 0.68-0.72 in the mass balance correction equation for isotopic analysis of CO2 in air. For high-precision measurements of the CO2 stable isotope ratios in air samples a careful assessment of the mass spectrometer performance is necessary. Different ion sources, even different ion source settings, alter the IER of N2O vs. CO2 which is used in the N2O correction algorithm. Preferably, the specific mass spectrometric behaviour should be established with clean N2O/CO2 mixtures or with air mixtures covering a larger range of N2O concentrations.

Atmosphere↗
PubMed2007

Increasing the sensitivity of delta13C and delta15N abundance measurements by a high sensitivity elemental analyzer connected to an isotope ratio mass spectrometer.

A common elemental analyzer system connected to a temperature-controlled gas chromatography (GC) column and coupled to an isotope ratio mass spectrometer was improved to decrease the determination limit for a simultaneous stable isotope ratio measurement of nitrogen and carbon dioxide. The additional use of a special ashtray system to collect the combustion residuals permitted more time-efficient work. These modifications to the elemental analyzer allowed precise measurements to be made down to 1.5 microg nitrogen and 10 microg carbon for stable isotope analysis. Low system background values and an acceptable signal-to-noise ratio have made an additional blank correction for these low sample measurements unnecessary. We provide a precision of this stable isotope analysis for lowest amounts of 1.2-2 microg nitrogen with a standard deviation of +/-0.496 per thousand (n = 27) and for 8.2-15 microg carbon with a standard deviation of +/-0.257 per thousand (n = 31) across different sample runs under stipulated conditions. This application can be established in an automatic mode without cryofocusing procedures.

Carbon Isotopes↗
PubMed2002

Amino acid residue specific stable isotope labeling for quantitative proteomics.

Various stable isotope labeling (SIL) techniques have recently emerged to improve the efficiency and accuracy of protein quantitation by mass spectrometry (MS). We have developed a mass-tagging strategy to incorporate stable isotope tagged amino acids into cellular proteins in a residue-specific manner during cell growth. In this study, we further extend this residue-specific SIL approach to the accurate quantitation of protein abundances in different cell populations. For proteins whose expression levels are the same in cells grown in the normal and labeled media, the relative areas of the normal (light) and labeled (heavy) isotopic peaks are linearly correlated with the cells mixing ratios. This approach was first used to determine the effect of the zinc-responsive transcription factor Zap1 on the yeast proteome. Ten protein spots from a PAGE gel were chosen randomly and their differential protein expression levels in wild-type and zap1delta cells were readily determined by the isotopic ratio. Methionine synthase (Met6) was identified to be up-regulated more than four times in the zap1delta mutant strain whereas the expression level of other nine proteins remained unchanged. Further, we applied this strategy to study the cellular response to radiation in human skin fibroblast cells. Analyzing one protein band randomly selected from SDS-PAGE, the expression level of a novel protein was found to increase two-fold in response to radiation whereas the expression level of a control protein remained unchanged. This strategy is generally applicable using any particular type of amino acid as the labeling precursors for accurate quantitation of protein relative abundances.

Amino Acid Sequence↗