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51.
R.M. Krupka 《The Journal of membrane biology》1999,167(1):35-41
Control of the coupled reaction sequence in active transport depends on systematic changes in the properties of the carrier
protein as the reaction proceeds. These changes would have to be brought about by specific interactions with the substrate,
the binding forces being used to stabilize either (i) a carrier state with altered properties or (ii) the transition state
in a carrier transformation. In the first case the tightness of coupling (the ratio of the coupled rate to slippage) will
at first rise with the increment in binding energy in the altered state but will approach an upper limit when overly strong
binding forces retard substrate dissociation in a subsequent step in the coupled reaction sequence. Primary and secondary
active transport are subject to this limitation because the coupling mechanism necessarily involves intermediates in which
the substrate is strongly bound. Exchange-only transport is not necessarily subject to the same limitation because the mechanism
can involve only a substrate-catalyzed change in carrier state. The available data, although scant, agree with these conclusions.
Received: 3 June 1998/Revised: 22 September 1998 相似文献
52.
53.
Wen-Yeh Hsieh Tang-Ching Kuan Kun-Shan Cheng Yan-Chiou Liao Mu-Yuan Chen Pei-Heng Lin Yuan-Chang Hsu Chen-Yi Huang Wei-Hua Hsu Sheng-Yao Yu Chih-Sheng Lin 《International journal of biological sciences》2012,8(8):1197-1205
Objective: Pleural effusion is common problem, but the rapid and reliable diagnosis for specific pathogenic effusions are lacking. This study aimed to identify the diagnosis based on clinical variables to differentiate pleural tuberculous exudates from other pleural effusions. We also investigated the role of renin-angiotensin system (RAS) and matrix metalloproteinase (MMPs) in the pathogenesis of pleural exudates.Experimental design: The major components in RAS and extracellular matrix metabolism, including angiotensin converting enzyme (ACE), ACE2, MMP-2 and MMP-9 activities, were measured and compared in the patients with transudative (n = 45) and exudative (n = 80) effusions. The exudative effusions were come from the patients with tuberculosis (n = 20), pneumonia (n = 32), and adenocarcinoma (n = 28).Results: Increased ACE and equivalent ACE2 activities, resulting in a significantly increased ACE/ACE2 ratio in exudates, were detected compared to these values in transudates. MMP-9 activity in exudates was significantly higher than that in transudates. The significant correlation between ACE and ACE2 activity that was found in transudates was not found in exudates. Advanced analyses showed significantly increased ACE and MMP-9 activities, and decreased ACE2 activity in tuberculous pleural effusions compared with those in pneumonia and adenocarcinoma effusions. The results indicate that increased ACE and MMP-9 activities found in the exudates were mainly contributed from a higher level of both enzyme activities in the tuberculous pleural effusions.Conclusion: Interplay between ACE and ACE2, essential functions in the RAS, and abnormal regulation of MMP-9 probably play a pivotal role in the development of exudative effusions. Moreover, the ACE/ACE2 ratio combined with MMP-9 activity in pleural fluid may be potential biomarkers for diagnosing tuberculous pleurisy. 相似文献
54.
Oxygen uptake of growth hormone transgenic coho salmon during starvation and feeding 总被引:3,自引:0,他引:3
R. A. Leggatt R. H. Devlin† A. P. Farrell‡§ D. J. Randall¶ 《Journal of fish biology》2003,62(5):1053-1066
Oxygen uptake of growth hormone transgenic coho salmon Oncorhynchus kisutch was measured in individual fish with a closed-system respirometer and was compared with that of similar-sized non-transgenic control coho salmon during starvation and when fed a fixed ration or to satiation. Transgenic and control fish did not differ in their standard oxygen uptake after 4 days of starvation, although control fish had a higher routine oxygen uptake, scope for spontaneous activity and initial acclimation oxygen uptake. During feeding, transgenic fish ate significantly more than control fish, and had an overall oxygen uptake that was 1·7 times greater than control fish. When fish that had eaten the same per cent body mass were compared, transgenic fish had an oxygen uptake that was 1·4 times greater than control fish. Differences in oxygen uptake in growth hormone transgenic coho salmon and non-transgenic fish appear to be due to the effects of feeding, acclimation and activity level, and not to a difference in basal metabolism. 相似文献
55.
56.
F. H. Bronson 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2009,364(1534):3331-3340
Seasonal reproduction is common among mammals at all latitudes, even in the deep tropics. This paper (i) discusses the neuroendocrine pathways via which foraging conditions and predictive cues such as photoperiod enforce seasonality, (ii) considers the kinds of seasonal challenges mammals actually face in natural habitats, and (iii) uses the information thus generated to suggest how seasonal reproduction might be influenced by global climate change. Food availability and ambient temperature determine energy balance, and variation in energy balance is the ultimate cause of seasonal breeding in all mammals and the proximate cause in many. Photoperiodic cueing is common among long-lived mammals from the highest latitudes down to the mid-tropics. It is much less common in shorter lived mammals at all latitudes. An unknown predictive cue triggers reproduction in some desert and dry grassland species when it rains. The available information suggests that as our climate changes the small rodents of the world may adapt rather easily but the longer lived mammals whose reproduction is regulated by photoperiod may not do so well. A major gap in our knowledge concerns the tropics; that is where most species live and where we have the least understanding of how reproduction is regulated by environmental factors. 相似文献
57.
Summary In crassulacean acid metabolism (CAM) large amounts of malic acid are redistributed between vacuole and cytoplasm in the course
of night-to-day transitions. The corresponding changes of the cytoplasmic pH (pHcyt) were monitored in mesophyll protoplasts from the CAM plantKalanchoe daigremontiana Hamet et Perrier by ratiometric fluorimetry with the fluorescent dye 2′,7′-bis-(2-carboxyethyl)-5-(and-6-)carboxyfluorescein
as a pHcyt indicator. At the beginning of the light phase, pHcyt was slightly alkaline (about 7.5). It dropped during midday by about 0.3 pH units before recovering again in the late-day-to-early-dark
phase. In the physiological context the variation in pHcyt may be a component of CAM regulation. Due to its pH sensitivity, phosphoenolpyruvate carboxylase appears as a likely target
enzyme. From monitoring ΔpHcyt in response to loading the cytoplasm with the weak acid salt K-acetate a cytoplasmic H+-buffer capacity in the order of 65 mM H+ per pH unit was estimated at a pHcyt of about 7.5. With this value, an acid load of the cytoplasm by about 10 mM malic acid can be estimated as the cause of the
observed drop in pHcyt. A diurnal oscillation in pHcyt and a quantitatively similar cytoplasmic malic acid is predicted from an established mathematical model which allows simulation
of the CAM dynamics. The similarity of model predictions and experimental data supports the view put forward in this model
that a phase transition of the tonoplast is an essential functional element in CAM dynamics. 相似文献
58.
Chemical tools capable of detecting ferrous iron with oxidation-state specificity have only recently become available. Coincident with this development in chemical biology has been increased study and appreciation for the importance of ferrous iron during infection and more generally in host–pathogen interaction. Some of the recent findings are surprising and challenge long-standing assumptions about bacterial iron homeostasis and the innate immune response to infection. Here, we review these recent developments and their implications for antibacterial therapy. 相似文献
59.
60.
《Developmental cell》2021,56(16):2329-2347.e6