On-line analysis of the (13)CO(2) labeling of leaf isoprene suggests multiple subcellular origins of isoprene precursors.

Isoprene (2-methyl-1,3-butadiene) is the most abundant biogenic hydrocarbon released from vegetation, and there is continuing interest in understanding its biosynthesis from photosynthetic precursors in leaf chloroplasts. We used on-line proton-transfer-reaction mass spectrometry (PTR-MS) to observe the kinetics of (13)C-labeling of isoprene following exposure to (13)CO(2) and then the loss of (13)C after a return to normal (12)CO(2) in oak ( Quercus agrifolia Nee) and cottonwood (Populus deltoides Barr.) leaves. Assignments of labeled isoprene species were verified by gas chromatography-mass spectrometry. For the first time, it was possible to observe the half-lives of individually (13)C-labeled isoprene species during these transitions, and to trace some of the label to a C3 fragment that contained the two isoprene carbons derived from pyruvate via the deoxyxylulose-5-phosphate (DOXP) pathway. At steady state (under (13)CO(2)), approximately 80% of isoprene carbon was labeled, with fully labeled isoprene as the major species (approx. 60%). The source of the unlabeled C is suggested to be extrachloroplastic, but not from photorespiratory carbon. After a transfer to (12)CO(2), (13)C-labeling persisted in one isoprene carbon for several hours; this persistence was much more pronounced in (i) leaves inhibited by fosmidomycin, a specific inhibitor of the DOXP pathway, and (ii) in sun leaves which have higher ratios of soluble sugars to starch. From the mass 41-44 fragment data, and labeling predicted from the DOXP pathway in chloroplasts, precursors may arise from cytosolic pyruvate/phospho enolpyruvate equivalents transported into the chloroplast; this idea was supported by an indirect measure of pyruvate labeling. Other sources of cytosolic isoprene precursors (i.e. dimethylallyl diphosphate or pentose phosphate) could not be excluded. The data obtained shed light on the half-lives of photosynthetic metabolites, exchanges of carbon between cellular pools, and suggest multiple origins of isoprene precursors in leaves.

On-line analysis of reactive VOCs from urban lawn mowing.

We measured the release of volatile organic compounds (VOCs) resulting from lawn mowing during continuous ambient air measurements in July and August 1998 in the outskirts of Innsbruck, Austria. These measurements were made with a proton-transfer-reaction mass spectrometry system, which allowed simultaneous, on-line monitoring of VOCs in the pptv range. We observed the emission of C6 wound compounds, including (Z)-3-hexenal, (E)-2-hexenal, hexenol plus hexanal, and acetaldehyde immediately following lawn mowing, and a rise in background levels of C6 wound compounds that lasted for several hours. Peak levels of biogenic VOCs following mowing were in the same concentration range (20-60 ppbv) as those originating from combustion engines of lawn mowers, and integrated biogenic emissions were much greater in the drying grass clippings. Additional emissions of acetone and other VOCs resulted from rainfall on these clippings. Since the estimated atmospheric chemical reactivity of VOCs resulting from lawn mowing is of the same order of magnitude as unburned hydrocarbons released during the mowing by gasoline-powered lawn mowers, these biogenic VOCs should be considered in urban air-quality control strategies.

Human breath isoprene and its relation to blood cholesterol levels: new measurements and modeling.

Numerous publications have described measurements of breath isoprene in humans, and there has been a hope that breath isoprene analyses could be a noninvasive diagnostic tool to assess blood cholesterol levels or cholesterol synthesis rate. However, significant analytic problems in breath isoprene analysis and variability in isoprene levels with age, exercise, diet, etc., have limited the usefulness of these measurements. Here, we have applied proton transfer reaction-mass spectrometry to this problem, allowing on-line detection of breath isoprene. We show that breath isoprene concentration increases within a few seconds after exercise is started as a result of a rapid increase in heart rate and then reaches a lower steady state when breath rate stabilizes. Additional experiments demonstrated that increases in heart rate associated with standing after reclining or sleeping are associated with increased breath isoprene concentrations. An isoprene gas-exchange model was developed and shows excellent fit to breath isoprene levels measured during exercise. In a preliminary experiment, we demonstrated that atorvastatin therapy leads to a decrease in serum cholesterol and low-density-lipoprotein levels and a parallel decrease in breath isoprene levels. This work suggests that there is constant endogenous production of isoprene during the day and night and reaffirms the possibility that breath isoprene can be a noninvasive marker of cholesterologenesis if care is taken to measure breath isoprene under standard conditions at constant heart rate.

Analysis of volatile organic compounds: possible applications in metabolic disorders and cancer screening.

The human breath contains a variety of endogenous volatile organic compounds (VOCs). The origin and pathophysiological importance of these VOCs is poorly investigated. Little is known about the interaction of VOCs from ambient air, such as those produced by plants and exhaust fumes, with the human organism. Gas chromatographic determination of VOC concentrations is tedious. Proton-transfer-mass spectroscopy (PTR-MS), a new technology for the online detection of VOC patterns, is a valuable alternative. We present two interesting molecular species, isoprene and ortho (o)-toluidine, as examples of endogenously produced VOCs. In a case study, breath isoprene reductions during lipid-lowering therapy (36%) were shown to correlate with cholesterol (32%) and LDL concentrations (35%) in blood (p < 0.001) over a period of 15 days. Therefore, isoprene concentrations in human breath (measured by PTR-MS) might serve as an additional parameter to complement invasive tests for controlling lipid-lowering therapy. Furthermore, it may be a useful parameter for lipid disorder screening. Mass-108, which presumably represents o-toluidine in our breath samples, was found in significantly higher concentrations in the breath of patients with different tumors (1.5 +/- 0.8 ppbv) than in age-matched controls (0.24 +/- 0.1 ppbv, p < 0.001). Inflammatory reactions do not seem to alter the pattern of mass-108. Therefore, it appears to be a currently underestimated carcinoma marker that deserves further investigation.

Online monitoring of air quality at the postanesthetic care unit by proton-transfer-reaction mass spectrometry.

The subthreshold exposure to trace anesthetic gases is not associated with considerable risk of adverse health effects. Online control of ambient air exchange at the postoperative workplace may help in supervising air quality and lead to cost reduction. A proton-transfer-reaction mass spectrometer system was used for online monitoring of volatile organic compounds, especially anesthetic gases. The mean exposure to sevoflurane and isoflurane at the urological postanesthesia care unit (PACU) was 15.9 and 9.5 parts per billion, respectively. Sevoflurane and isoflurane concentrations at the urological PACU showed a patient turnover-dependent burden during our investigation period. Because modern PACUs have a high ventilation capacity, the 24-h occupational burden by anesthetic gases at the PACU is relatively low. Monitoring and controlling of ambient air by automatic built-in alarm systems would be useful for quality control of the postoperative workplace. Moreover, energy costs of ventilation systems could be reduced by coupling ventilation capacity to the effective exposure.

Endogenous production of methanol after the consumption of fruit.

After the consumption of fruit, the concentration of methanol in the human body increases by as much as an order of magnitude. This is due to the degradation of natural pectin (which is esterified with methyl alcohol) in the human colon. In vivo tests performed by means of proton-transfer-reaction mass spectrometry show that consumed pectin in either a pure form (10 to 15 g) or a natural form (in 1 kg of apples) induces a significant increase of methanol in the breath (and by inference in the blood) of humans. The amount generated from pectin (0.4 to 1.4 g) is approximately equivalent to the total daily endogenous production (measured to be 0.3 to 0.6 g/day) or that obtained from 0.3 liters of 80-proof brandy (calculated to be 0.5 g). This dietary pectin may contribute to the development of nonalcoholic cirrhosis of the liver.

Detection of isoprene in expired air from human subjects using proton-transfer-reaction mass spectrometry.

A new analytical method using proton-transfer-reaction mass spectrometry (PTRMS) is described for the determination of trace constituents in human breath. PTRMS is sufficiently sensitive and specific that it does not require preconcentration or separation. At its present stage of development it is capable of detecting trace constituents present in air at the part-per-billion level. These capabilities are illustrated for isoprene, one of the most abundant endogenous hydrocarbons. Our results confirm recent observations of a diurnal level variation associated with sleep or wakefulness; a new finding is that young children have much lower levels of isoprene in breath than adults. To address the metabolic origin of human isoprene, we used PTRMS to analyze expired air for allylic C5 alcohols that have been proposed to be non-enzymatic precursors of isoprene. The lack of correlation between peak breath isoprene and these alcohols suggests that the hydrocarbon is formed by some other mechanism.

Methanol in human breath.

Using proton transfer reaction-mass spectrometry for trace gas analysis of the human breath, the concentrations of methanol and ethanol have been measured for various test persons consuming alcoholic beverages and various amounts of fruits, respectively. The methanol concentrations increased from a natural (physiological) level of approximately 0.4 ppm up to approximately 2 ppm a few hours after eating about 1/2 kg of fruits, and about the same concentration was reached after drinking of 100 ml brandy containing 24% volume of ethanol and 0.19% volume of methanol.