SEARCH · PubMed Health

Results for “Isotopes”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 recordsLinked to original sources

PubMed2002

Stable carbon and oxygen isotopic analysis of atmospheric carbon monoxide using continuous-flow isotope ratio MS by isotope ratio monitoring of CO.

We have developed a rapid and simple measurement system for both content and stable isotopic compositions (13C and 18O) of atmospheric CO, using continuous-flow isotope ratio mass spectrometry by simultaneously monitoring the CO+ ion currents at masses 28, 29, and 30. The analytical system consisted sequentially of a sample trapping port (liquid nitrogen temperature silica gel and molecular sieve 5A), a gas dryer, a CO purification column (molecular sieve 5A), a cryofocusing unit, and a final purification column using a GC capillary. Analytical precision of 0.2 per thousand for 13C and 0.4 per thousand for 18O can be realized for samples that contain as little as 300 pmol of CO within 40 min for one sample analysis. Analytical blanks associated with the method are less than 1 pmol. The extent of analytical error in delta13C due to mass-independent fractionation of oxygen in natural CO is estimated to be less than 0.3 per thousand. Based on this system, we report herein a kinetic isotopic effect during CO consumption in soil.

Air Pollutants, Occupational↗

PubMed1994

Isotope effects on the mechanism of calcineurin catalysis: kinetic solvent isotope and isotope exchange studies.

The reaction scheme of calcineurin was examined with kinetic and physical approaches. Proton inventory studies of the calcineurin-catalyzed hydrolysis of para-nitrophenyl phosphate were done to probe the role of proton transfer in the mechanism. Control experiments determined that the solvent did not cause the irreversible inactivation of the enzyme and had no effect on the dependence on metal ion or calmodulin. A solvent isotope effect was observed on the Vmax/Km term, but not the Vmax term. The isotope effect was modest with a value of 1.35. Proton inventory data could be fit by multiple parameter sets. The parameter sets yielded fractionation factors of 0.73 for a one-proton transfer or 0.85 for a two-proton transfer. These values compare to the value of 0.69 for reactions involving a water molecule or hydroxide coordinated to metal ion. A chemical mechanism consistent with the proton inventory data and other information about calcineurin catalysis is presented. The simplest model for catalysis involves a single proton transfer from water coordinated to metal that is reasoned to occur during association of the substrate with calcineurin. Questions about the reaction intermediate were also addressed. Attempts to monitor a phosphate-water exchange reaction with 31P nuclear magnetic resonance spectroscopy were unsuccessful. Failure to observe an exchange reaction suggests that no phosphoryl enzyme is formed during the progress of the reaction. Together these data are explained by a model in which cleavage of the phosphate ester bond is catalyzed by a water (hydroxide) molecule coordinated to a divalent metal ion without the formation of a covalent intermediate.

Animals↗

PubMed2004

Turnover of carbon isotopes in tail hair and breath CO2 of horses fed an isotopically varied diet.

Temporal stable isotope records derived from animal tissues are increasingly studied to determine dietary and climatic histories. Despite this, the turnover times governing rates of isotope equilibration in specific tissues following a dietary isotope change are poorly known. The dietary isotope changes recorded in the hair and blood bicarbonate of two adult horses in this study are found to be successfully described by a model having three exponential isotope pools. For horse tail hair, the carbon isotope response observed following a dietary change from a C3 to a C4 grass was consistent with a pool having a very fast turnover rate ( t1/2 approximately 0.5 days) that made up approximately 41% of the isotope signal, a pool with an intermediate turnover rate ( t1/2 approximately 4 days) that comprised approximately 15% of the isotope signal, and a pool with very slow turnover rate ( t1/2 approximately 140 days) that made up approximately 44% of the total isotope signal. The carbon isotope signature of horse blood bicarbonate, in contrast, had a different isotopic composition, with approximately 67% of the isotope signal coming from a fast turnover pool ( t1/2 0.2 days), approximately 17% from a pool with an intermediate turnover rate ( t1/2 approximately 3 days) and approximately 16% from a pool with a slow turnover rate ( t1/2 approximately 50 days). The constituent isotope pools probably correspond to one exogenous and two endogenous sources. The exogenous source equates to our fast turnover pool, and the pools with intermediate and slow turnover rates are thought to derive from the turnover of metabolically active tissues and relatively inactive tissues within the body, respectively. It seems that a greater proportion of the amino acids available for hair synthesis come from endogenous sources compared to the compounds undergoing cellular catabolism in the body. Consequently, the isotope composition of blood bicarbonate appears to be much more responsive to dietary isotope changes, whereas the amino acids in the blood exhibit considerable isotopic inertia.

Amino Acids↗

PubMed2001

Natural intramolecular isotope measurements in physiology: elements of the case for an effort toward high-precision position-specific isotope analysis.

Chemical information available in organisms can be categorized into three major domains, macromolecular, small molecules, and isotope ratios. Information about physiological state is commonly obtained by qualitative and quantitative analysis in the macromolecular and small molecule domains. Genomics and proteomics are emerging approaches to analysis of macromolecules, and both areas yield definitive information on present physiological state. There is relatively little record of past physiological states of the individual available in these domains. Natural isotopic variability, particularly on an intramolecular level, is likely to retain more physiological history. Because of ubiquitous isotopic fractionation, every stereochemically unique position in every molecule has an isotope ratio that reflects the processes of synthesis and degradation. This fact highlights a vast amount of organismal chemical information that is essentially unstudied. Isotope measurements can be classified according to the chemical complexity of the analyte into bulk, compound-specific, and position-specific or intramolecular levels. Recent advances in analysis of isotope ratios are transforming natural science, and particularly answering questions about ecosystems using bulk methods; however, they have had relatively little impact on physiology. This may be because the vast complexities of physiological questions demand very selective information available in position-specific isotope analysis (PSIA). The relatively few high-precision PSIA studies, based on isotope ratio mass spectrometry (IRMS), have revealed intramolecular isotope ratio differences in pivotal physiological compounds including amino acids, glucose, glycerol, acetate, fatty acids, and purines. The majority of these analyses have been accomplished by laborious offline methods; however, recent advances in instrumentation presage rapid PSIA that will be necessary to attack real physiological problems. Gas-phase pyrolysis has been shown to be an effective method to determine (13)C/(12)C at high precision for molecular fragments, and technologies to extend C-based PSIA to N and other organic elements are emerging. Two related efforts are warranted, (a) development of rapid, convenient, and sensitive methods for high-precision PSIA, a necessary precursor to (b) a concerted investigation into the relationship of metabolic state to intramolecular isotope ratio. Inherent in this latter goal is the need to identify long-lived molecules in long-lived cells that retain a record of early isotopic conditions, as has been shown for post-mortem human neuronal DNA. Using known metabolic precursor-product relationships between intramolecular positions, future studies of physiological isotope fractionation should reveal the relationship of diet and environment to observed isotope ratio. This science of isotope physiology, or simply isotopics, should add an important tool for elucidation of early factors that effect later health, probably the most difficult class of biomedical issues.

Acetates↗

PubMed2006

Mass discrimination during MC-ICPMS isotopic ratio measurements: investigation by means of synthetic isotopic mixtures (IRMM-007 series) and application to the calibration of natural-like zinc materials (including IRMM-3702 and IRMM-651).

A long known way of anchoring isotope ratio values to the SI system is by means of gravimetrically prepared isotopic mixtures. Thermal ionization mass spectrometry (TIMS) is the traditionally associated measurement technique, but multi-collector double focusing inductively coupled plasma (MC-ICP)-MS now appears to be an attractive alternative. This absolute calibration strategy necessitates that mass discrimination effects remain invariant in time and across the range of isotope ratios measured. It is not the case with MC-ICPMS and the present work illustrates, in the case of Zn isotopic measurements carried out using locally produced synthetic Zn isotope mixtures (IRMM-007 series), how this calibration strategy must be adjusted. First, variation in mass discrimination effects across the measurement sequence is propagated as an uncertainty component. Second, linear proportionality during each individual measurement between normalized mass discrimination and the average mass of the isotope ratios is used to evaluate mass discrimination for the ratios involving low abundance isotopes. Third, linear proportionality between mass discrimination and the logarithm of the isotope ratio values for n(67Zn)/n(64Zn) and n(68Zn)/n(64Zn) in the mixtures is used iteratively to evaluate mass discrimination for the same ratios in the isotopically enriched materials. Fourth, ratios in natural-like materials (including IRMM-3702 and IRMM-651) are calibrated by external bracketing using the isotopic mixtures. The relative expanded uncertainty (k = 2) estimated for n(68Zn)/n(64Zn) and n(67Zn)/n(64Zn) ratio values in the synthetic isotopic mixtures and the natural-like zinc samples was in the range of 0.034 to 0.048%. The uncertainty on the weighing (0.01%, k = 1) was the largest contributor to these budgets. The agreement between these results and those obtained with a single detector TIMS and with another MC-ICPMS further validated this work. The absolute isotope ratio values found for IRMM-3702-material also proposed as "delta 0" for delta-scale isotopic measurements-are n(66Zn)/n(64Zn) = 0.56397 (30), n(67Zn)/n(64Zn) = 0.082166 (35), n(68Zn)/n(64Zn) = 0.37519 (16), and n(70Zn)/n(64Zn) = 0.012418 (23). The derived Zn atomic weight value Ar(Zn) = 65.37777 (22) differs significantly from the current IUPAC value by Chang et al. [1]. Remeasurement, with isotopic mixtures from the IRMM-007 series, of the Zn isotope ratios in the same Chang et al. [1] material have revealed large systematic differences (1.35 (27)% per atomic mass unit) that suggest unrecognized measurement biases in their results.

Calibration↗

PubMedDate not supplied

[The isotope effect in the glycine dehydrogenase reaction is the cause of the intramolecular isotope inhomogeneity of glucose carbon of starch synthesized during photorespiration].

The isotope distribution of glucose-6-phosphate in the main pathways of its biosynthesis (in the processes of CO2 assimilation and photorespiration in the Calvin cycle and during resynthesis from the degradation products of lipids and proteins) was analyzed. For reconstructing the isotope distribution of glucoso-6-phosphate synthesized in the Calvin cycle during photorespiration, the functioning of the cycle with regard to its coupling with the glycolate chain, which together constitute the photorespiration chain, was considered. In the glycine dehydrogenase reaction of the glycolate cycle, there arises an isotope effect, which determines the distribution of isotopes in the glucose-6-phosphate and other photorespiration products. The isotope effect of the glycine dehydrogenase reaction increases at the expense of the exhaustion of glucose resources feeding the photorespiration chain. As a result, atoms C-3 and C-4 of glucose become enriched with the heavy isotope, and subsequent mixing of atoms and the specificity of interactions in the photorespiration chain lead to an isotope weighting of the other atoms and an uneven distribution of carbon isotopes in glucose-6-phosphate and other photorespiration products. A comparison of the glucose-6-phosphate isotope patterns in different pathways of the synthesis with the experimental data on the distribution of carbon isotopes in starch glucose of storing plant organs led to the conclusion that the starch resources are predominantly formed at the expense of glucose-6-phosphate of photorespiration. This is consistent with the earlier observed enhancement of photorespiration at the stage of plant maturation.

Beta vulgaris↗

PubMed2002

Impact of duration of infusion and choice of isotope label on isotope recycling in glucose homeostasis.

The purposes of this study were to quantify the impact of the duration of infusion and choice of stable isotope of glucose on measures of glucose rate of appearance (glucose R(a)) and to determine whether the differences observed were due to tracer recycling via the glycogen pool (direct pathway) or gluconeogenesis (indirect pathway). Six healthy adult volunteers were studied on four occasions in the postabsorptive state during infusions of [1-(13)C]glucose and [6,6-(2)H(2)]glucose: 2.5-h infusion of both (A), and 2.5-h infusion of one (B) and 14.5-h infusion of the other isotope (C), and 5-h infusion of [6,6-(2)H(2)]glucose and 2.5-h infusion of [1-(13)C]glucose (D). Infusion of both isotopes for 2.5 h resulted in similar glucose R(a) values. When compared with a 14.5-h infusion, the 2.5-h glucose tracer infusion overestimated glucose R(a) by 26-35%. Glucose (13)C recycled via the Cori cycle, resulting in slower decay from the plasma pool and longer half-life of [1-(13)C]glucose compared with [6,6-(2)H(2)]glucose. There was no detectable release of [(13)C]glucose or [(2)H(2)]glucose tracer into the plasma pool after administration of glucagon. These data demonstrate that glucose R(a) varies not as a result of isotope cycling but as a result of differences in duration of isotope infusion regardless of the isotope used. This is most likely due to incomplete isotope and substrate equilibration with the 2.5-h infusion. The potential error was reduced by nearly 80% using a 5-h infusion of [6,6-(2)H(2)]glucose. These studies demonstrate that the duration of isotope infusion has significantly greater impact on quantitation of glucose R(a) than does the selection of isotope.

Adult↗

PubMed2006

Interpretation of V/K isotope effects for enzymatic reactions exhibiting multiple isotopically sensitive steps.

Formulations of an enzyme mechanism where only a single step is presumed to be isotopically sensitive can be written in terms of forward and reverse commitments to catalysis. These commitments provide a natural and intuitive way of interpreting the observed isotope effects. Unfortunately, when multiple isotopically sensitive steps are present in the mechanism, including effects associated with pre-equilibria of the unbound substrate, the observed V/K kinetic isotope effect is expressed as a complicated expression of the intrinsic rate constants for each step, the interpretation of which is not always immediately obvious. We show here that V/K isotope effects from unbranched or rapid-equilibrium random Michaelis-Menten systems containing multiple isotopically sensitive steps can be written as a weighted average of the intrinsic isotope effects on each step, where this intrinsic isotope effect from each step is the product of the equilibrium isotope effect on the formation of the reacting intermediate for that step and the intrinsic kinetic effect on the forward rate constant for that step, and the weighting factors are simply the reciprocal sum of the forward and reverse commitments for each step i plus unity, 1/(C(fi)+C(ri)+1), equivalent to the sensitivity index [Ray, W.J., 1983.

Animals↗

PubMed2006

The prediction of isotopic patterns in phenylpropanoids from their precursors and the mechanism of the NIH-shift: basis of the isotopic characteristics of natural aromatic compounds.

The theoretical 2H-distribution in the aromatic ring of phenylpropanoids can be predicted from that of their precursors--erythrose-4-phosphate, phosphoenolpyruvate and NADPH--and by invoking the mechanism of the NIH-shift and implied deuterium isotope effects. For each position in the non-oxygenated ring, the predicted natural 2H-abundance is in excellent agreement with experimental data obtained from quantitative 2H NMR-measurements on natural compounds, especially concerning the relative 2H-abundances p > o > or = m. For the p-hydroxylated derivatives, the experimentally determined 2H-abundance sequence order m > o can also be deduced, assuming an anisotropic migration (intramolecular isotope effect) of the p-hydrogen atom to the two differently 2H-substituted m-positions during the NIH-shift (intramolecular hydrogen transfer) and an in vivo deuterium kinetic isotope effect of approximately 1.20 on the final hydrogen elimination from the proposed ketodiene intermediate. The predicted 2H-distribution pattern of methyl salicylate 10, a representative of an o-hydroxylated natural compound, is in excellent agreement with that reported from 2H NMR analyses. However, for salicyl alcohol, minor differences between the theoretical and experimentally determined pattern are found that cannot yet be satisfactorily explained. On the other hand, a very good agreement is found between the theoretical and experimental pattern of coumarin, provided a deuterium kinetic isotope effect of approximately 1.30 is assumed for the elimination of the H-atoms from the ketodiene intermediate. The secondary m-hydroxylation of p-coumaric acid in the biosynthesis of vanillin seems to proceed without large isotope effects. Parallel differences are also observed for the 18O-kinetic isotope effects on the corresponding monooxygenase-catalysed reactions. The results demonstrate convincingly that the mechanisms of these general reactions of the phenylpropanoid biosynthetic pathway are identical and follow general principles. Small observed differences between the 2H-patterns of individual natural aromatic compounds originating from the same hydroxylation type can therefore be assigned to differences of the patterns of the precursors, the extent and the orientation of the hydrogen migration, and the kinetic isotope effect on the final hydrogen elimination. The evidence for the existence of general systematic rules governing isotopic patterns in the shikimic acid pathway and its subsequent reactions is further supported by the recently reported 13C-distribution pattern of vanillin, which is also in agreement with that predicted from the precursors. Hence, it is apparent that the systematics of the isotope patterns of phenylpropanoids are in line with the generally accepted biosynthetic reactions in the shikimic acid pathway and that this knowledge can strengthen their value as an essential support for the distinction of natural and synthetic aromatic compounds.

Benzaldehydes↗

PubMed1998

Isotope trapping and kinetic isotope effect studies of rat liver alpha-(2-->6)-sialyltransferase.

A mechanistic study of rat liver alpha-(2-->6) sialyltransferase (ST) is presented that includes isotope trapping experiments and kinetic isotope effects on V/K for the ST-catalyzed reaction of isotopically labeled CMP-N-acetylneuraminate and N-acetyllactosamine. The isotope trapping experiments confirmed that the kinetic mechanism is steady-state random, and further analysis indicated that for this sialyltransferase the experimentally observed isotope trapping ratio (product trapped/substrate released) was equivalent to the commitment to catalysis, Cf, the quantity required to correct the kinetic isotope effects. Cf was found to range from 1.0 (at 1.6 mM LacNAc) to 1.7 (at 100 mM LacNAc). After correction for Cf, the isotope effects were as follows: secondary beta-dideuterium, 1.04-1. 05; anomeric carbon primary 14C, 1.000 +/- 0.004; a small 3H binding effect of 1.016 +/- 0.007 at C9; and a carboxylate carbon secondary 14C isotope effect of 0.998 +/- 0.004. This pattern of KIEs is quite different than observed for solvolysis of CMP-NeuAc [Horenstein, B. A., and Bruner, M. (1996) J. Am. Chem. Soc. 118, 10371-10379]. Based on the results of ab-initio modeling of isotope effects, a hypothesis is presented which reconciles the unusual pattern of KIEs on the basis of binding interactions at the carboxylate carbon.

Animals↗

PubMedDate not supplied

Quantitative comparison of single-isotope and dual-isotope stress-rest single-photon emission computed tomographic imaging for reversibility of defects.

BACKGROUND: Dual-isotope rest/stress single-photon emission computed tomographic (SPECT) imaging is a time-saving imaging protocol. However, the stress radiotracer, technetium 99m-labeled sestamibi, and the rest radiotracer, thallium 201, have different physical properties and myocardial kinetics. In patients with abnormal resting myocardial perfusion, these differences may affect quantification of rest defect size and defect reversibility. The purpose of the study was to compare myocardial perfusion defect reversibility quantitatively by single-isotope (rest/stress sestamibi) and dual-isotope (rest thallium/stress sestamibi) SPECT. METHODS AND RESULTS: Thirty patients with prior myocardial infarction underwent rest/stress sestamibi SPECT imaging and rest thallium SPECT imaging. Defects were quantified according to circumferential count profiles with a normal sestamibi database. The images of a subgroup of 21 patients were processed with radiotracer-specific normal databases. Defect size and defect reversibility were compared quantitatively for single-isotope and dual-isotope SPECT. Rest sestamibi defect size was significantly larger than rest thallium defect size (19 +/- 15 vs 14 +/- 16; p = 0.007). Defect reversibility was larger with thallium than with sestamibi (10 +/- 9 vs 6 +/- 6; p = 0.002). With radiotracer-specific normal databases, mean rest sestamibi and thallium defect sizes in 21 patients were not different (23 +/- 19 vs 21 +/- 17; difference not significant). With radiotracer-specific normal databases, mean defect reversibility was not different with either sestamibi or thallium (6 +/- 6 vs 8 +/- 9; difference not significant), although correlation among individual patients was only fair (r2 = 0.48). CONCLUSION: In patients with prior myocardial infarction, stress-induced defect reversibility is quantitatively larger with dual-isotope imaging than with single-isotope imaging. Quantitative processing of dual-isotope images requires radiotracer-specific normal databases. Because of different characteristics of sestamibi and thallium, assessment of defect reversibility on dual-isotope images should be made with caution. Only relatively large defect reversibility can be assumed to represent true stress-induced myocardial ischemia.

Dipyridamole↗

PubMed2007

The calibration of the intramolecular nitrogen isotope distribution in nitrous oxide measured by isotope ratio mass spectrometry.

Two alternative approaches for the calibration of the intramolecular nitrogen isotope distribution in nitrous oxide using isotope ratio mass spectrometry have yielded a difference in the 15N site preference (defined as the difference between the delta15N of the central and end position nitrogen in NNO) of tropospheric N2O of almost 30 per thousand. One approach is based on adding small amounts of labeled 15N2O to the N2O reference gas and tracking the subsequent changes in m/z 30, 31, 44, 45 and 46, and this yields a 15N site preference of 46.3 +/- 1.4 per thousand for tropospheric N2O. The other involves the synthesis of N2O by thermal decomposition of isotopically characterized ammonium nitrate and yields a 15N site preference of 18.7 +/- 2.2 per thousand for tropospheric N2O. Both approaches neglect to fully account for isotope effects associated with the formation of NO+ fragment ions from the different isotopic species of N2O in the ion source of a mass spectrometer. These effects vary with conditions in the ion source and make it impossible to reproduce a calibration based on the addition of isotopically enriched N2O on mass spectrometers with different ion source configurations. These effects have a much smaller impact on the comparison of a laboratory reference gas with N2O synthesized from isotopically characterized ammonium nitrate. This second approach was successfully replicated and leads us to advocate the acceptance of the site preference value 18.7 +/- 2.2 per thousand for tropospheric N2O as the provisional community standard until further independent calibrations are developed and validated. We present a technique for evaluating the isotope effects associated with fragment ion formation and revised equations for converting ion signal ratios into isotopomer ratios.

Air Pollutants↗

PubMed2002

On-line measurement of intramolecular carbon isotope distribution of acetic acid by continuous-flow isotope ratio mass spectrometry.

Molecular and intramolecular carbon isotope measurements of acetic acid present in natural environments have been performed by off-line procedures. The off-line method is complicated and time-consuming and requires micromolar to millimolar amounts of sample. This limits geochemical isotopic studies, especially at the intramolecular level, on acetic acid present in natural samples. Here, we examine an on-line measurement of intramolecular carbon isotope distribution of acetic acid using continuous-flow isotope ratio mass spectrometry (CF-IRMS) coupled with an on-line pyrolysis system. This is achieved by measurement of the respective carbon isotope ratios of CH4 and CO2 produced by on-line pyrolysis of acetic acid. Results for authentic standards of pure acetic acid demonstrated the practicality of this on-line method, although the carbon isotope ratio of the methyl group could not be determined directly. The precision of the carbon isotope measurements was 0.4 per thousand (1sigma). The carbon isotope distribution determined by the on-line method was identical to that determined by the conventional off-line method within analytical error. The advantages of the on-line method compared with the conventional off-line method are that it is less laborious, requires less analytical time (less than one hour per sample) and, most importantly, uses smaller sample sizes (ca. 10 nanomole). An application of this on-line method to natural geochemical samples will provide an insight into the geochemical cycle of acetic acid.

Acetic Acid↗

PubMed1992

The isotopic composition of lead in man and the environment in Finland 1966-1987: isotope ratios of lead as indicators of pollutant source.

The isotopic composition of lead was determined in samples collected between 1966 and 1987, mainly from the Helsinki area, in emission sources (gasoline, incinerator and lead smelter emissions, coal), air, in samples representing long-term deposition (lichen, soil, lake sediments), and in human tissue. Isotope ratios were determined by thermal ionization mass spectrometry after chemical separation of lead by anion exchange and cathodic electrodeposition. The origin of lead in man and the environment in the Helsinki area was evaluated by using the differences in the measured isotope ratios as an indicator. The mean of the ratio in gasoline (206Pb/207Pb 1.124 +/- 0.026) and the ratios in other emission sources in Helsinki (1.149-1.226) were significantly different. However, the wide range of isotope ratios in gasoline (1.063-1.173) reduced the accuracy when assessing the contribution of the different sources. Lead in air samples from Helsinki (1.123 +/- 0.013) could be attributed to gasoline, as could lead in soil near a highway (1.136 +/- 0.003). By contrast, isotope ratios measured in lichen (1.148 +/- 0.006) indicated considerable amounts of lead from sources with higher 206Pb abundances, evidently industrial sources. The isotope ratios in human liver, lung, and bone from individuals dying between 1976-79 (206Pb/207Pb ratio 1.142 +/- 0.015, 1.151 +/- 0.011, and 1.156 +/- 0.013, respectively) reflect the large lead emissions from the incinerators and lead smelters in the Helsinki area in the 1960s and 1970s. In lake sediment cores a correlation was found between the isotope ratios, lead concentration, and depth. The nonanthropogenic lead of high isotope ratios from bedrock was the major component at depths dated older than 100 years. At the surface of the sediment atmospheric lead prevailed, with ratios similar to those of gasoline, air samples and lichen. In the post-1900 layers, anthropogenic lead made up about 40-95% of the total sedimentary lead.

Bone and Bones↗

PubMed1981

pH variation of isotope effects in enzyme-catalyzed reactions. 2. Isotope-dependent step not pH dependent. Kinetic mechanism of alcohol dehydrogenase.

Theory is developed for th pH dependence of isotope effects in a mechanism where a pH-dependent step precedes the isotope-sensitive bond-breaking step, and the rate of the latter varies only slightly with the state of protonation of the acid-base catalytic group on the enzyme. In such a mechanism, the isotope effects fall to 1.0 in the forward direction and to the equilibrium isotope effect in the reverse direction at pH values where the pH-sensitive step becomes totally rate limiting in the reverse direction. This model accurately describes the kinetics of yeast alcohol dehydrogenase, where V/Kacetone and the isotope effects on V2-propanol and V/K2-propanol decrease above a pK of 8.8 (both isotope effects becoming 1.0 at pH 10). The model also fits the kinetics of liver alcohol dehydrogenase, where Vcyclohexanol and V/Kcyclohexanol decrease below pKs of 6.2 and 7.1, and above pKs of 9.5 and 10.3. pKi trifluoroethanol decreases below a pK of 7.2, and above pK of 10.1, while pKi isobutyramide drops above a pK of 10.0. Vcyclohexanone decreases above a pK of 8.4 while V/Kcyclohexanone decreases above pKs of 8.8 and 9.7. Isotope effects on V/Kcyclohexanol and V/Kcyclohexanone decrease above identical pKs of 9.4 to values of 1 and 0.88, respectively, at pH 11. Comparison of a value of 2.5 for D(V/Kcyclohexanol) with an average value of 5.53 for T(V/Kcyclohexanol) allowed circulation of 6.3 as the intrinsic deuterium isotope effect. These data suggest that E-DPN-alcohol undergoes a proton transfer to the enzyme to give an EH-DPN-alkoxide complex which can lose its proton at high pH to give E-DPN-alkoxide and that both of these alkoxide complexes undergo hydride transfer to give DPNH and ketone. the alkoxide intermediate is not free to dissociate until it is protonated, either because it is coordinated to Zn or because the enzyme is in a closed catalytic configuration.

Alcohol Oxidoreductases↗

PubMed1999

The retention and distribution by healthy young men of stable isotopes of selenium consumed as selenite, selenate or hydroponically-grown broccoli are dependent on the isotopic form.

Twenty-seven healthy young men were randomly assigned to diets that supplied low (32.6 microg/d) or high (226.5 microg/d) levels of selenium for a 105-d study. After consuming the diets for 85 d, subjects were fed a test meal that contained 74Se in the form of selenite or selenate and 82Se incorporated into hydroponically-raised broccoli. Urine, fecal and blood samples were collected daily. Isotope absorption was not different (P > 0.05) for selenate and Se in broccoli; Se absorption from selenite was highly variable and was not included in statistical analyses. Significantly more isotope was absorbed by subjects fed the high Se diet (P = 0. 015). Urinary isotope excretion was greater when selenate was fed than when broccoli was fed (P = 0.0001), and consequently more Se from broccoli (as compared to selenate) was retained (59.2 +/- 2.4 and 36.4 +/- 4.6% for Se in broccoli and selenate, respectively; P = 0.0001). Despite the higher retention, less isotope from broccoli than from selenate was present in the plasma. Plasma proteins separated by gel permeation chromatography showed that most of the isotopes were distributed between two medium molecular weight peaks. Less isotope was found in plasma proteins of subjects fed the high Se diet, but the form of Se had no effect on isotope distribution. These results show that dietary Se intake alters the retention of stable isotopes of Se and that humans retain and distribute Se from broccoli in a different manner than Se from inorganic salts.

Adult↗

PubMed1998

Comparison of isotopic and non-isotopic labelling for in situ hybridisation of various mRNA targets with cRNA probes.

In situ hybridisation methods to localise messenger ribonucleic acid (mRNA) targets in tissue sections or cell preparations using riboprobes can be successful with either isotopic or non-isotopic labelling. Investigators often wish to decide which labelling method provides the maximum specificity, sensitivity and resolution, with minimum nonspecific background. In this study we compared isotopic (35S) and non-isotopic (digoxigenin) labelling, using a variety of probes and paraffin-embedded tissues. The targets were human beta-actin and von Willebrand Factor mRNAs in archival human tissues; and mRNAs for two closely related trefoil factor family (TFF) peptides, TFF2 and TFF3, in rat duodenum. Patterns of localisation with both isotopic and non-isotopic probes were broadly similar for each target. The 35S labelling provided good contrast and sensitive detection under darkfield illumination, but the cellular or subcellular resolution of the target was less precise than that obtained with the digoxigenin-labelled probes in transmitted light. Digoxigenin labelling in individual cells was more clearly demonstrated, but occasionally the contrast of positive staining with background was poor. The sensitivity of each method appeared to be similar for these high-abundance targets, therefore the choice between isotopic and non-isotopic labels is dependent upon the aim of the study and the cellular resolution required.

Animals↗

PubMed2005

Interpretation of butyltin mass spectra using isotope pattern reconstruction for the accurate measurement of isotope ratios from molecular clusters.

The fragmentation patterns of butyltin compounds (mono-, di-, and tributyltin) in an electron impact ion source were studied using an isotope pattern reconstruction algorithm with emphasis on isotope ratio measurements from molecular clusters. For this purpose, standards of natural tin isotope abundance and a (119)Sn-enriched mixture of the three compounds were both ethylated and propylated using sodium tetraalkylborates. The corresponding mass spectra of the various tetraalkyltin compounds prepared were obtained by GC/MS after their extraction with hexane. The results showed that pure interference-free molecular clusters were obtained only for certain R(3)Sn(+) ions where no isobaric overlap with R(2)SnH(+) ions occurred (e.g. BuEt(2)Sn(+) overlaps with Bu(2)SnH(+)). These ions are ideal candidates for accurate Sn isotope ratio measurements, while isotope pattern perturbing interferences are observed for other molecular fragments down to Sn(.)(+). Isotope pattern reconstruction algorithm thus can be used as an analytical tool to ensure the absence of molecular interferences--a requirement for accurate isotope ratio measurements from molecular clusters. The relevance of these studies for the determination of butyltin compounds in environmental samples by isotope dilution GC/MS is also discussed.

Environmental Monitoring↗