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961.
962.
System N transporters are critical for glutamine release and modulate metabolic fluxes of glucose and acetate in cultured cortical astrocytes: changes induced by ammonia 下载免费PDF全文
963.
964.
Damian Miles Bailey Bruce Davies Lee Romer Lindy Castell Eric Newsholme George Gandy 《European journal of applied physiology and occupational physiology》1998,78(4):360-368
Elite distance runners participated in one of two studies designed to investigate the effects of moderate altitude training
(inspiratory partial pressure of oxygen ≈115–125 mmHg) on submaximal, maximal and supramaximal exercise performance following
return to sea-level. Study 1 (New Mexico, USA) involved 14 subjects who were assigned to a 4-week altitude training camp (1500–2000 m)
whilst 9 performance-matched subjects continued with an identical training programme at sea-level (CON). Ten EXP subjects
who trained at 1640 m and 19 CON subjects also participated in study 2 (Krugersdorp, South Africa). Selected metabolic and
cardiorespiratory parameters were determined with the subjects at rest and during exercise 21 days prior to (PRE) and 10 and
20 days following their return to sea-level (POST). Whole blood lactate decreased by 23% (P < 0.05 vs PRE) during submaximal exercise in the EXP group only after 20 days at sea-level (study 1). However, the lactate
threshold and other measures of running economy remained unchanged. Similarly, supramaximal performance during a standardised
track session did not change. Study 2 demonstrated that hypoxia per se did not alter performance. In contrast, in the EXP
group supramaximal running velocity decreased by 2% (P < 0.05) after 20 days at sea-level. Both studies were characterised by a 50% increase in the frequency of upper respiratory
and gastrointestinal tract infections during the altitude sojourns, and two male subjects were diagnosed with infectious mononucleosis
following their return to sea-level (study 1). Group mean plasma glutamine concentrations at rest decreased by 19% or 143
(74) μM (P < 0.001) after 3 weeks at altitude, which may have been implicated in the increased incidence of infectious illness.
Accepted: 19 March 1998 相似文献
965.
Artificial neural networks for computer-based molecular design 总被引:6,自引:0,他引:6
966.
Impact of gaseous nitrogen deposition on plant functioning 总被引:5,自引:0,他引:5
Dry deposition of NH3 and NOx (NO and NO2 ) can affect plant metabolism at the cellular and whole-plant level. Gaseous pollutants enter the plant mainly through the stomata, and once in the apoplast NH3 dissolves to form NH4 + , whereas NO2 dissolves to form NO3 − and NO2 − . The latter compound can also be formed after exposure to NO. There is evidence that NH3 -N and NOx -N can be reversibly stored in the apoplast. Temporary storage might affect processes such as absorption rate, assimilation and re-emission. Once formed, NO3 − and NO2 − can be reduced, and NH4 + can be assimilated via the normal enzymatic pathways, nitrate reductase (NR), nitrite reductase and the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. Fumigation with low concentrations of atmospheric NH3 increases in vitro glutamine synthetase activity, but whether this involves both or only one of the GS isoforms is still an open question. There seems to be no correlation between fumigation with low concentrations of NH3 and in vitro GDH activity. The contribution of atmospheric NH3 and NO2 deposition to the N budget of the whole plant has been calculated for various atmospheric pollutant concentrations and relative growth rates ( RGRs ). It is concluded that at current ambient atmospheric N concentrations the direct impact of gaseous N uptake by foliage on plant growth is generally small. 相似文献
967.
968.
Mutants of Anabaena 7120 defective in glutamine synthetase (GS) activity were isolated following transposon mutagenesis. Mutants M11, M55 and M73 showed about 60% less GS activity in N2-grown aerobic cultures than the wild-type strain and were resistant to the glutamate analogue l-methionine-dl-sulphoximine (MSX). These mutants had the capacity to excrete N2-fixed ammonia continuously into the culture medium and showed an enhanced level of aerobic nitrogenase activity. The intracellular ammonium pool generated in N2-grown cells of mutants was found to be less than that of the wild-type strain. Similarly, ammonium uptake by these mutants was 50% less in mutants compared to the wild-type, suggesting a possible role of GS in controlling this function. 相似文献
969.
Jose Neptuno Rodriguez-Lopez Andrew T. Smith R. N. F. Thorneley 《Journal of biological inorganic chemistry》1996,1(2):136-142
Horseradish peroxidase isoenzyme C (HRPC) mutants were constructed in order to understand the role of two key distal haem
cavity residues, histidine 42 and arginine 38, in the formation of compound I and in substrate binding. The role of these
residues as general acid-base catalysts, originally proposed for cytochrome c peroxidase by Poulos and Kraut in 1980 was assessed for HRPC. Replacement of histidine 42 by leucine [(H42L)HRPC*] decreased
the apparent bimolecular rate constant for the reaction with hydrogen peroxide by five orders of magnitude (k
1 = 1.4×102 M–1s–1) compared with both native-glycosylated and recombinant forms of HRPC (k
1 = 1.7×107 M–1s–1). The first-order rate constant for the heterolytic cleavage of the oxygen-oxygen bond to form compound I was estimated to
be four orders of magnitude slower for this variant. Replacement of arginine 38 by leucine [(R38L)HRPC*] decreased the observed
pseudo-first-order rate constant for the reaction with hydrogen peroxide by three orders of magnitude (k
1 = 1.1×104 M–1s–1), while the observed rate constant of oxygen bond scission was decreased sixfold (k
2 = 142 s–1). These rate constants are consistent with arginine 38 having two roles in catalysing compound I formation: firstly, promotion
of proton transfer to the imidazole group of histidine 42 to facilitate peroxide anion binding to the haem, and secondly,
stabilisation of the transition state for the heterolytic cleavage of the oxygen-oxygen bond. These roles for arginine 38
explain, in part, why dioxygen-binding globins, which do not have an arginine in the distal cavity, are poor peroxidases.
Binding studies of benzhydroxamic acid to (H42L)HRPC* and (R38L)HRPC* indicate that both histidine 42 and arginine 38 are
involved in the modulation of substrate affinity.
Received: 21 July 1995 / Accepted: 27 November 1995 相似文献
970.
Marie Louise Champigny 《Photosynthesis research》1995,46(1-2):117-127