MgATP-independent hydrogen evolution catalysed by nitrogenase: an explanation for the missing electron(s) in the MgADP-AlF4 transition-state complex.
(1/204)
When the MoFe (Kp1) and Fe (Kp2) component proteins of Klebsiella pneumoniae nitrogenase are incubated with MgADP and AlF4(-) in the presence of dithionite as a reducing agent, a stable putative transition-state complex is produced [Yousafzai and Eady (1997) Biochem. J. 326, 637-640]. Surprisingly, the EPR signal associated with reduced Kp2 is not detectable, but Kp1 retains the S=3/2 EPR signal arising from the dithionite reduced state of the MoFe cofactor centre of the protein. This is consistent with the [Fe4S4] centre of the Fe protein in the complex being oxidized, and similar observations have been made with the complex of Azotobacter vinelandii [Spee, Arendsen, Wassink, Marritt, Hagen and Haaker (1998) FEBS Lett. 432, 55-58]. No satisfactory explanation for the fate of the electrons lost by Kp2 has been forthcoming. However, we report here that during the preparation of the MgADP-AlF4 K. pneumoniae complex under argon, H2 was evolved in amounts corresponding to one half of the FeMoco content of the Kp1 (FeMoco is the likely catalytic site of nitrogenase with a composition Mo:Fe7:S9:homocitrate). This is surprising, since activity is observed during incubation in the absence of MgATP, normally regarded as being essential for nitrogenase function, and in the presence of MgADP, a strong competitive inhibitor of nitrogenase. The formation of H2 by nitrogenase in the absence of AlF4(-) was also observed in reaction mixtures containing MgADP but not MgATP. The reaction showed saturation kinetics when Kp1 was titrated with increasing amounts of Kp2 and, at saturation, the amount of H2 formed was stoichiometric with the FeMoco content of Kp1. The dependence of the rate of formation of H2 on [MgADP] was inconsistent with the activity arising from MgATP contamination. We conclude that MgATP is not obligatory for H+ reduction by nitrogenase since MgADP supports a very low rate of hydrogen evolution. (+info)
Bleeding from a tear in the gastric mucosa caused by transoesophageal echocardiography during cardiac surgery: effective haemostasis by endoscopic argon plasma coagulation.
(2/204)
We present an unusual complication of transoesophageal echocardiography (TOE) during cardiac surgery. Although the patient had no oesophageal or gastrointestinal disease, a gastric mucosal tear of approximately 2 cm long occurred in the mucosa just distal to the gastrooesophageal junction caused by the TOE probe. Bleeding from the tear was stopped using endoscopic argon plasma coagulation. (+info)
Influence of oxygen concentration on argon plasma coagulation-induced tissue damage in isolated pig tracheas.
(3/204)
BACKGROUND: Argon plasma coagulation (APC) is used during bronchoscopy to restore passage of central airways. Concern has been raised about the possibility that elevated oxygen concentration may increase the tissue damage by APC. OBJECTIVE: to examine the extent of tissue damage at different concentrations of oxygen in isolated pig tracheas. METHODS: The APC-induced tissue damage was investigated macroscopically and microscopically in isolated and artificially ventilated pig tracheas at oxygen concentrations of 21, 50, 75 and 100%. RESULTS: There was deep coagulation of all tissue layers reaching the adventitia independent of the oxygen concentration up to 75% O(2). With 100% O(2), only tissue damage increased and combustion occurred. CONCLUSIONS: APC-induced tissue damage in ex vivo pig tracheas was independent of the oxygen concentration relevant for clinical practice. (+info)
Solar wind record on the moon: deciphering presolar from planetary nitrogen.
(4/204)
Ion microprobe analyses show that solar wind nitrogen associated with solar wind hydrogen implanted in the first tens of nanometers of lunar regolith grains is depleted in 15N by at least 24% relative to terrestrial atmosphere, whereas a nonsolar component associated with deuterium-rich hydrogen, detected in silicon-bearing coatings at the surface of some ilmenite grains, is enriched in 15N. Systematic enrichment of 15N in terrestrial planets and bulk meteorites relative to the protosolar gas cannot be explained by isotopic fractionation in nebular or planetary environments but requires the contribution of 15N-rich compounds to the total nitrogen in planetary materials. Most of these compounds are possibly of an interstellar origin and never equilibrated with the 15N-depleted protosolar nebula. (+info)
Stereoisomeric specificity and soil gas disequilibria: implications for martian life detection.
(5/204)
Variations in the atmospheric composition of soil samples were monitored by mass spectrometry after the addition of mixtures of D- and L-carbohydrates and/or amino acids. The changes in concentrations of CO2 in these experiments were found to be related to the steroisomeric configurations of the compounds with which the soil samples were enriched. The potential of this relationship provides a comparatively simple approach for detecting life in extraterrestrial soils. (+info)
[Is argon plasma coagulation an efficient treatment for digestive system vascular malformation and radiation proctitis?].
(6/204)
BACKGROUND AND AIMS: Argon beam coagulation is an innovative no-touch electrocoagulation technique in which high-frequency monopolar alternating current is delivered to the tissue through ionized argon gas. The aim of this prospective study was to evaluate the efficacy and safety of argon plasma coagulation (APC) for the treatment of hemorrhagic digestive vascular malformations and hemorrhagic radiation proctosigmoiditis. METHODS AND PATIENTS: From March 1998 through April 1999, we used endoscopic APC (ERBE, Lyon, France, argon gas source ICC 300, high-frequency electrosurgical generator ICC 200, gas flow 1 L/min, power setting 50 W) to treat 39 consecutive patients (mean age 70.3 +/- 10 years). The indications for treatment were anemia (n =10), active or oozing haemorrhage (n =15) from digestive angiodysplastic lesions (n =25), hemorrhagic antral telangiectatic vascular lesions (n =2), and hemorrhagic radiation proctosigmoiditis (n =12) after failure of medical treatments (5-aminosalicylic acid, corticosteroids, or sucralfate enemas). The efficacy of APC treatment was evaluated on symptoms, transfusion requirement, bleeding recurrence, hemoglobin value before and 6 months after APC therapy. RESULTS: On the average, 1 +/- 0.5 sessions per patient was required to treat digestive vascular malformations. Definitive haemostasis of digestive angiodysplastic lesions with active or oozing haemorrhage was achieved in one session in all patients. No bleeding recurrence was observed during the follow-up period of 6 months. Anemia recurrence was observed in 2 patients (7%). Average hemoglobin levels recorded before and 6 months after APC therapy were 78.8 +/- 21.2 g/L and 108 +/- 13.7 g/L, respectively (P<0.05). On the average, 2.8 +/- 0.8 sessions per patient were required to treat hemorrhagic radiation proctosigmoiditis. Ten patients (83%) reported improvement or cessation of rectal bleeding, most of them immediately after APC therapy. Endoscopic control was performed one month after APC therapy and showed complete disappearance of lesions in 8 patients (66%). Average hemoglobin levels recorded before and 6 months after APC therapy were of 102.7 +/- 21 g/L and 120 +/- 19.5 g/L, respectively (P <0.05). Complications were observed in 5 cases (13%): pneumoperitoneum in 2 cases, chronic rectal ulcerations in 2 cases, and nonsymptomatic rectal stenosis in 1 case. CONCLUSION: APC appears to be a simple, safe, and effective technique in the management of hemorrhagic radiation-induced proctosigmoiditis and hemorrhagic lesions. (+info)
Impact event at the Permian-Triassic boundary: evidence from extraterrestrial noble gases in fullerenes.
(7/204)
The Permian-Triassic boundary (PTB) event, which occurred about 251.4 million years ago, is marked by the most severe mass extinction in the geologic record. Recent studies of some PTB sites indicate that the extinctions occurred very abruptly, consistent with a catastrophic, possibly extraterrestrial, cause. Fullerenes (C60 to C200) from sediments at the PTB contain trapped helium and argon with isotope ratios similar to the planetary component of carbonaceous chondrites. These data imply that an impact event (asteroidal or cometary) accompanied the extinction, as was the case for the Cretaceous-Tertiary extinction event about 65 million years ago. (+info)
The microscopic protein structure of the lens with a theory for cataract formation as determined by Raman spectroscopy of intact bovine lenses.
(8/204)
Intact bovine lenses have been studied using the polarized Raman spectroscopic technique. A brief theoretical and experimental review of Raman spectroscopy is presented. From the dependence of the Raman depolarization ratio on the propagation direction of the incident radiation we have determined that the uniaxial qualities of the lens result from microscopic anisotropy and have established the quantitative positional correlation of specific chemical bonds with respect to the lens optic axis. In particular, the hydrogen bonded linear CONH groups of the antiparallel beta-pleated sheet are preferentially oriented in directions orthogonal to the lens optic axis. The Raman spectra of intact lenses do not exhibit bands at positions characteristic of either the alpha-helix or the random coil protein structure. The antiparallel beta-pleated sheet protein microstructure and the lens fiber cross-sectional macrostructure exhibit a remarkable similarity. This similarity may be causal and is consistent with the protein concentration of the lens, the birefringent properties observed by both Lenhard and Brewster, the CONH bond angle distribution with respect to the optic axis, and the lens anatomy. It is suggested that cortical cataracts are caused by fluctuations in protein orientational order. (+info)