Pressure dependent OH yields in the reactions of CH3CO and HOCH2CO with O2.

OH-formation in the reactions of CH3CO (R1) and HOCH2CO (R4) with O2 was studied in He, N2 and air (27 to 400 mbar) using OH-detection by laser induced fluorescence (LIF). 248 nm laser photolysis of COCl2 in the presence of CH3CHO or HOCH2CHO was used as source of the acyl radicals CH3CO and HOCH2CO. The LIF-system was calibrated in back-to-back experiments by the 248 nm laser photolysis of H2O2 as OH radical precursor. A straight-forward analytical expression was used to derive OH yields (α) for both reactions. A Stern-Volmer-analysis results in α1b(-1)(N2) = 1 + (9.4 ± 1.7) × 10(-18) cm(3) molecule(-1) × [M], α1b(-1)(He) = 1 + (3.6 ± 0.6) × 10(-18) cm(3) molecule(-1) × [M] and α4b(-1)(N2) = 1 + (1.85 ± 0.38) × 10(-18) cm(3) molecule(-1) × [M]. Our results for CH3CO are compared to the previous (divergent) literature values whilst that for HOCH2CO, for which no previous data were available, provide some insight into the factors controlling the yield of OH in these reactions.

Direct kinetic study of OH and O3 formation in the reaction of CH3C(O)O2 with HO2.

The reaction between HO2 and CH3C(O)O2 has three exothermic product channels, forming OH (R3a), peracetic acid (R3b), and acetic acid plus O3 (R3c). The branching ratios of the OH- and ozone-forming reaction channels were determined using a combination of laser-induced fluorescence (LIF, for time-resolved OH concentration measurement) and transient absorption spectroscopy (TAS, for time-resolved O3 concentration measurement) following pulsed laser generation of HO2 and CH3C(O)O2 from suitable precursors. TAS was also used to determine the initial concentration of the reactant peroxy radicals. The data were evaluated by numerical simulation using kinetic models of the measured concentration profiles; a Monte Carlo approach was used to estimate the uncertainties of the rate constants (k3) and branching ratios (α) thus obtained. The reaction channel forming OH (R3a) was found to be the most important with α3a = 0.61 ± 0.09 and α3c = 0.16 ± 0.08. The overall rate coefficient of the title reaction was found to be k3 = (2.1 ± 0.4) × 10(-11) cm(3) molecule(-1) s(-1) for both HO2 and DO2. Use of DO2 resulted in an increase in α3a to 0.80 ± 0.14. Comparison with former studies shows that OH formation via (R3) has been underestimated significantly to date. Possible reasons for these discrepancies and atmospheric implications are discussed.

Photolysis of CH3COCH3 at 248 and 266 nm: pressure and temperature dependent overall quantum yields.

The overall quantum yield of photolysis of acetone (CH(3)C(O)CH(3)) at 248 and 266 nm was measured using the pulsed laser photolysis technique. The organic photo-fragment radicals CH(3) and CH(3)CO were detected indirectly as Br atoms using time-resolved resonance fluorescence following their reaction with Br(2). Quantum yields for acetone photolysis were derived relative to COCl(2) (at 248 nm) or Cl(2) (at 266 nm) in back-to-back experiments in which Cl atoms were scavenged by Br(2) to form Br. At 248 nm, experiments were carried out at pressures between 60 and 760 Torr of N(2) and at three temperatures: 224, 234 and 298 K. At this wavelength, the overall quantum yield was 0.98 +/- 0.10 and, within experimental uncertainty, was independent of pressure and temperature in the ranges covered. At 266 nm, experiments were restricted to 298 K, where the quantum yield was also close to unity, but with a weak dependence on bath gas pressure. These results confirm our previous room temperature, 266 nm dataset obtained using a different experimental approach in which the yield of CH(3) was measured directly.

Atmospheric oxidation capacity sustained by a tropical forest.

Terrestrial vegetation, especially tropical rain forest, releases vast quantities of volatile organic compounds (VOCs) to the atmosphere, which are removed by oxidation reactions and deposition of reaction products. The oxidation is mainly initiated by hydroxyl radicals (OH), primarily formed through the photodissociation of ozone. Previously it was thought that, in unpolluted air, biogenic VOCs deplete OH and reduce the atmospheric oxidation capacity. Conversely, in polluted air VOC oxidation leads to noxious oxidant build-up by the catalytic action of nitrogen oxides (NO(x) = NO + NO2). Here we report aircraft measurements of atmospheric trace gases performed over the pristine Amazon forest. Our data reveal unexpectedly high OH concentrations. We propose that natural VOC oxidation, notably of isoprene, recycles OH efficiently in low-NO(x) air through reactions of organic peroxy radicals. Computations with an atmospheric chemistry model and the results of laboratory experiments suggest that an OH recycling efficiency of 40-80 per cent in isoprene oxidation may be able to explain the high OH levels we observed in the field. Although further laboratory studies are necessary to explore the chemical mechanism responsible for OH recycling in more detail, our results demonstrate that the biosphere maintains a remarkable balance with the atmospheric environment.

Absorption cross section and photolysis of OIO.

Pulsed laser photolysis combined with transient absorption spectroscopy and resonance fluorescence was used to examine the photolysis of OIO at a number of wavelengths corresponding to absorption bands in its visible spectrum between approximately 530 and 570 nm. Photolysis at 532 nm was found to result in substantial depopulation of the absorbing ground state, enabling an estimate for the absorption cross section of OIO at 610.2 nm of (6 +/- 2) x 10(-18) cm2 molecule(-1) to be obtained. No evidence was found for I atom formation following photolysis of OIO at 532, 562.3, 567.9 and 573.8 nm, enabling an upper limit to the I atom quantum yield of < 0.05 (560-580 nm) and < 0.24 (532 nm) to be established.

Kinetics of the reactions of HO with methanol (210-351 K) and with ethanol (216-368 K).

Absolute rate coefficients for the reaction of hydroxyl radicals (HO) with methanol, HO + CH3OH --> products (R1), and with ethanol, HO + C2H5OH --> products (R2) were measured over a range of temperatures using pulsed laser photolytic generation of HO coupled to its time resolved detection by pulsed laser induced fluorescence. The accuracy of the rate constants obtained was enhanced by on-line optical absorption measurements of the alcohol concentration. The temperature dependence of the rate coefficients is given by: k1(210-351 K) = 6.67 x 10(-18) T2 exp(140/T) cm3 molecule(-1) s(-1) with a rate coefficient at room temperature of (9.3 +/- 0.7) x 10(-13) cm3 molecule(-1) s(-1). For k2 we obtained: k2(216-368 K) = 4.0 x 10(-12) exp(-42/T) and a room temperature rate coefficient of (3.35 +/- 0.17) x 10(-12) cm3 molecule(-1) s(-1). The total error (at 95% confidence) associated with the rate coefficients derived from the expressions describing the temperature dependence is estimated as 7% at all temperatures. The present results, which extend the database on these reactions to cover temperatures relevant for the upper troposphere, are compared to previously published measurements, and values of k1 and k2 are recommended for atmospheric modelling.

CD28 and the tyrosine kinase lck stimulate mitogen-activated protein kinase activity in T cells via inhibition of the small G protein Rap1.

Proliferation of T cells via activation of the T-cell receptor (TCR) requires concurrent engagement of accessory costimulatory molecules to achieve full activation. The best-studied costimulatory molecule, CD28, achieves these effects, in part, by augmenting signals from the TCR to the mitogen-activated protein (MAP) kinase cascade. We show here that TCR-mediated stimulation of MAP kinase extracellular-signal-regulated kinases (ERKs) is limited by activation of the Ras antagonist Rap1. CD28 increases ERK signaling by blocking Rap1 action. CD28 inhibits Rap1 activation because it selectively stimulates an extrinsic Rap1 GTPase activity. The ability of CD28 to stimulate Rap1 GTPase activity was dependent on the tyrosine kinase Lck. Our results suggest that CD28-mediated Rap1 GTPase-activating protein activation can help explain the augmentation of ERKs during CD28 costimulation.

Somatostatin activation of mitogen-activated protein kinase via somatostatin receptor 1 (SSTR1).

Hormones and growth factors regulate cell growth via the mitogen-activated protein (MAP) kinase cascade. Here we examine the actions of the hormone somatostatin on the MAP kinase cascade through one of its two major receptor subtypes, the somatostatin receptor 1 (SSTR1) stably expressed in CHO-K1 cells. Somatostatin antagonizes the proliferative effects of fibroblast growth factor in CHO-SSTR1 cells via the SSTR1 receptor. However, in these cells, somatostatin robustly activates MAP kinase (also called extracellular signal regulated kinase; ERK) and augments fibroblast growth factor-stimulated ERK activity. We show that the activation of ERK via SSTR1 is pertussis toxin sensitive and requires the small G protein Ras, phosphatidylinositol 3-kinase, the serine/threonine kinase Raf-1, and the protein tyrosine phosphatase SHP-2. The activation of ERK by SSTR1 increased the expression of the cyclin-dependent protein kinase inhibitor p21(cip1/WAF1). Previous studies have suggested that somatostatin-stimulated protein tyrosine phosphatase activity mediates the growth effects of somatostatin. Our data suggest that SHP-2 stimulation by SSTR1 may mediate some of these effects through the activation of the MAP kinase cascade and the expression of p21(cip1/WAF1).

Plasma elastin-derived peptide levels in normal adults, children, and emphysematous subjects. Physiologic and computed tomographic scan correlates.

Pulmonary emphysema is likely to be the result of elastic tissue digestion by unrestrained elastase activity in the lung. Elastin breakdown by elastases results in the release of soluble elastin fragments (EDP), which may be measured in plasma by an ELISA. Plasma EDP levels measured using an ELISA were determined in the following groups: disease-free children (n = 24), 0.162 +/- 0.082 ng/ml; disease-free adult nonsmokers (n = 114), 1.74 +/- 0.8 ng/ml; smokers (n = 68), 2.76 +/- 4.59 ng/ml; reformed smokers (n = 43), 1.91 +/- 1.14 ng/ml. Adults with established pulmonary emphysema (n = 50), as defined by bullous formation on the chest radiograph, had levels of 50.83 +/- 24.8 ng/ml, significantly higher than the disease-free groups at p < 0.01. Pulmonary emphysema can be reflected by pulmonary function tests, especially those that measure the pulmonary elastic properties, and by computed tomographic (CT) scan percent emphysema score. We therefore examined the relationship of plasma EDP to these other indicators of pulmonary emphysema in a separate group of 26 subjects using elastic recoil measurements (K), and a further group of 30 subjects with CT scan percent emphysema score. A significant correlation of p < 0.001 was shown for plasma EDP and K and a significant correlation of p < 0.01 was shown for plasma EDP and CT scan percent emphysema score, these correlations suggesting that plasma EDP levels are indicators of the loss of pulmonary distensibility and of mild to moderate pulmonary emphysema. These findings suggest that pulmonary emphysema is characterized by active elastin breakdown.

A rapid procedure for the measurement of elastase inhibitory function of plasma and bronchoalveolar lavage fluid.

We describe a method to measure human leukocyte elastase (HLE) inhibitory capacity and compared it with porcine pancreatic elastase (PPE) inhibitory capacity and with a turbidimetric method using a specific antibody to alpha-1 antitrypsin (AAT), all performed on a Cobas Bio centrifugal analyser. This assay used methoxysuccinyl-dialanine-proline-valine-p-nitroanilide as substrate in the presence of 0.01% Brij 35, an HLE enzyme activator. Samples containing commonly used anti-coagulants and serum could be used in the assay, except for those containing heparin which strongly inhibited HLE. This assay was used to determine the functional AAT concentrations in plasma from a number of normal volunteers and patient groups, and was compared to the immuno-turbidimetric AAT assay. No difference in the proportion of functional to immuno-turbidimetric AAT was noted between any of the groups studied except for the adult respiratory distress syndrome (ARDS), where this percentage was reduced (p less than 0.05). An increase in both immuno-turbidimetric and functional AAT was seen for children (both p less than 0.01), in emphysema (p less than 0.05 and p less than 0.01 respectively) and ARDS (both p less than 0.05) when compared to adult non-smokers. This assay was also used to determine the HLE inhibitory capacity of serum and bronchoalveolar lavage (BAL) fluid from normal volunteer smokers (n = 4) and non-smokers (n = 4), and in the serum and BAL fluid from patients with ARDS (n = 5). Serum AAT was 94% functional in non-smokers (91% with PPE functional assay) and 96% in smokers (97% with PPE assay).(ABSTRACT TRUNCATED AT 250 WORDS)

Heparin and heparan sulphate are inhibitors of human leucocyte elastase.

1. Heparin and heparan sulphate strongly inhibited human leucocyte elastase activity in an automated assay using the soluble substrate, n-succinyl-(L-alanine)3-p-nitroanilide (50% inhibition of 250 microliters of 10 micrograms of human leucocyte elastase/ml was obtained with 80 microliters of 2.8 micrograms of heparin/ml and 8 micrograms of heparan sulphate/ml). Less significant inhibition at the same concentrations was seen with the other glycosaminoglycans tested: hyaluronic acid and chondroitin sulphates A-C. 2. Heparin and heparan sulphate also strongly inhibited human leucocyte elastase activity towards insoluble human lung elastin, as determined by an e.l.i.s.a. for soluble elastin-derived peptides released by elastolytic activity on the elastin. This inhibition was shown not to be due to a direct interference of the glycosaminoglycans in the e.l.i.s.a. nor to the inhibition causing a change in the size of the elastin-derived peptides. However, unlike the chromogenic assay with n-succinyl-(L-alanine)3-p-nitroanilide as substrate, where heparin was the more effective inhibitor, in this assay system heparan sulphate was the more effective inhibitor (50% inhibition of 100 microliters of 50 ng of human leucocyte elastase/ml was obtained with 100 microliters of 4.5 micrograms of heparin/ml and 0.8 microgram of heparan sulphate/ml). These results suggest that heparin and heparan sulphate, as components of cellular and basement membranes, are likely to have a role in protecting structural proteins, such as elastin, from the proteolytic activity of human leucocyte elastase.(ABSTRACT TRUNCATED AT 250 WORDS)