The membrane-bound F1 sector of the H+–ATPase complex (F-type ATPase) in dark-adapted photosynthetic chromatophores is endowed with MgATP- and CaATP-dependent ATPase
activities, both sensitive to inhibitors such as oligomycin and venturicidin. Because of contatamination of free Mg2+ and Ca2+ ions in chromatophore preparations, kinetic characterization of the two hydrolitic reactions can be performed only in the
presence of both substrates, using a model for two alternative substrates. The two activities are characterized by similar
maximal rates and affinity constants [VMgATP and VCaATP: 13±1 and 10±1 nmol s–1 ATP hydrolyzed (μmol BChl)–1; KMgATP and KCaATP: 0.22±0.06 and 0.20±0.05 mm]. However, only the MgATP-dependent ATPase is coupled to Δ*H+ generation. In this process CaATP acts as an alternative substrate and a competitive inhibitor of the proton pump, with a
KI coincident with KCaATP for the hydrolytic activity. This finding highlights the central role that the coordination chemistry of the ion-nucleotide
complex plays in determining the proton gating mechanism at the catalytic site(s) of the enzyme complex. These results are
discussed on the basis of the coordination properties of the ions and of the available information on the protein structure.
Received: 5 December 1995 / Accepted: 7 March 1996 相似文献
In 1976, Crump, Hoel, Langley, and Peto described how almost any dose‐response relationship for carcinogens becomes linear at low doses when background cancers are taken into account. This has been used, by the U.S. Environmental Protection Agency, USEPA, as partial justification for a regulatory posture that assumes low‐dose linearity, as is illustrated by a discussion of regulation of benzene as a carcinogen. The argument depends critically on the assumption that the pollutant and the background proceed by the same biological mechanism. In this paper we show that the same argument applies to noncancer end points also. We discuss the application to a number of situations: reduction in lung function and consequent increase in death rate due to (particulate) air pollution; reduction in IQ and hence (in extreme cases) mental deficiency due to radiation in utero; reduction of sperm count and hence increase in male infertility due to DBCP exposure. We conclude that, although the biological basis for the health effect response is different, in each case low‐dose linearity might arise from the same mathematical effect discussed by Crump et al. (1976). We then examine other situations and toxic end points where low‐dose linearity might apply by the same argument. We urge that biologists and chemists should concentrate efforts on comparing the biological and pharmacokinetic processes that apply to the pollutant and the background. Finally, we discuss some public policy implications of the possibility that low dose linearity may be the rule rather than the exception for environmental exposures. 相似文献
Based on imperfect data and theory, agencies such as the United States Environmental Protection Agency (USEPA) currently derive “reference doses” (RfDs) to guide risk managers charged with ensuring that human exposures to chemicals are below population thresholds. The RfD for a chemical is typically reported as a single number, even though it is widely acknowledged that there are significant uncertainties inherent in the derivation of this number.
In this article, the authors propose a probabilistic alternative to the EPA's method that expresses the human population threshold as a probability distribution of values (rather than a single RfD value), taking into account the major sources of scientific uncertainty in such estimates. The approach is illustrated using much of the same data that USEPA uses to justify their current RfD procedure.
Like the EPA's approach, our approach recognizes the four key extrapolations that are necessary to define the human population threshold based on animal data: animal to human, human heterogeneity, LOAEL to NOAEL, and subchronic to chronic. Rather than using available data to define point estimates of “uncertainty factors” for these extrapolations, the proposed approach uses available data to define a probability distribution of adjustment factors. These initial characterizations of uncertainty can then be refined when more robust or specific data become available for a particular chemical or class of chemicals.
Quantitative characterization of uncertainty in noncancer risk assessment will be useful to risk managers who face complex trade-offs between control costs and protection of public health. The new approach can help decision-makers understand how much extra control cost must be expended to achieve a specified increase in confidence that the human population threshold is not being exceeded. 相似文献
Summary Plasma membranes of maize (Zea mays L., cv. Sil Anjou 18) roots were isolated by aqueous two-phase partitioning. Multi elemental analysis by proton induced X-ray emission (PIXE) was used for the investigation of elemental composition of plasma membranes. Fe, Cu, and Zn as well as P, S, and Ca were identified. We did not find significant amounts of V, Mn, Se, Mo, or W.Abbreviations EDTA
ethylenediaminetetraacetic acid
- HCF III
hexacyanoferrate III (ferricyanide, K3[FeCN6])
- Hepes
2-[4-(2-hydroxyethyl)-1-piperazine]-ethanesulfonic acid
- PIXE
proton induced X-ray emission (proton microprobe)
- STA
siliciotungstic acid
- Tris
tris (hydroxymethyl)aminomethane 相似文献
This report describes the effect of different dose levels of infection upon worm burdens and development and fecundity of the parasites. Three groups each of 40, 9-week-old, helminth naïve pigs were inoculated once with either 2000 (group A), 20,000 (group B), or 200,000 (group C) infective third stage larvae of Oesophagostomum dentatum. Subgroups of 5 pigs from each major group were killed 3, 6, 11, 14, 18, 25, 34 and 47 days post inoculation (p.i.) and the large intestinal worm burdens were determined. Faecal egg counts were determined at frequent intervals after day 13 p.i. There were no overt clinical signs of gastrointestinal helminthosis during the experiment. Faecal egg counts became positive in groups A and B at around day 19 p.i., whereas most pigs in the high dose group C did not have positive egg counts until day 27–33 p.i. and some pigs remained with zero egg counts until the end of the study. Throughout the experiment the worm populations in group C consisted mainly of immature larval stages, while those in groups A and B were predominantly adult stages after days 14–18. Adult worms from the low dose group A were significantly longer than those from group C. At high population densities, stunted development of worms and reduced fecundity among female worms were found. Furthermore, there was a tendency for the distribution of the worms within the intestine to be altered with increasing population size. 相似文献
The influence of solar activity (SA) and geomagnetic activity (GMA) on human homeostasis has long been investigated. The
aim of the present study was to analyse the relationship between monthly proton flux (>90 MeV) and other SA and GMA parameters
and between proton flux and temporal (monthly) distribution of total and cardiovascular-related deaths. The data from 180
months (1974–1989) of distribution in the Beilinson Campus of the Rabin Medical Centre, Israel, and of 108 months (1983–1991)
from the Kaunas Medical Academy, were analysed and compared with SA, GMA and space proton flux (>90 MeV). It was concluded:
(1) monthly levels of SA, GMA and radiowave propagation (Fof2) are significantly and adversely correlated with monthly space
proton flux (>90 MeV); (2) medical-biological phenomena that increase during periods of low solar and/or geomagnetic activity
may be stimulated by physical processes provoked by the concomitant increase in proton flux; (3) the monthly number of deaths
related (positively or negatively) to SA are significantly and adversely related to the space proton flux (>90 MeV).
Received: 14 January 1996 / Accepted: 14 October 1996 相似文献
Secondary metabolic-energy-generating systems generate a proton motive force (pmf) or a sodium ion motive force (smf) by a
process that involves the action of secondary transporters. The (electro)chemical gradient of the solute(s) is converted into
the electrochemical gradient of protons or sodium ions. The most straightforward systems are the excretion systems by which
a metabolic end product is excreted out of the cell in symport with protons or sodium ions (energy recycling). Similarly,
solutes that were accumulated and stored in the cell under conditions of abundant energy supply may be excreted again in symport
with protons when conditions become worse (energy storage). In fermentative bacteria, a proton motive force is generated by
fermentation of weak acids, such as malate and citrate. The two components of the pmf, the membrane potential and the pH gradient,
are generated in separate steps. The weak acid is taken up by a secondary transporter either in exchange with a fermentation
product (precursor/product exchange) or by a uniporter mechanism. In both cases, net negative charge is translocated into
the cell, thereby generating a membrane potential. Decarboxylation reactions in the metabolic breakdown of the weak acid consume
cytoplasmic protons, thereby generating a pH gradient across the membrane. In this review, several examples of these different
types of secondary metabolic energy generation will be discussed. 相似文献
Electron transport, phosphorylation and internal proton concentration were measured in illuminated spinach chloroplast thylakoid membranes under a number of conditions. Regardless of the procedure used to vary these parameters, the data fit a simple chemiosmotic model. Protons from Photosystem II did not appear to be utilized differently from those derived from Photosystem I. The maximal phosphorylation efficiency () for photophosphorylation in washed thylakoids under oxidizing conditions is likely to be . This value is consistent with a proton-to-electron-pair ratio of 4 for electron flow through both photosystems and a proton-to-ATP ratio of 3 for the chloroplast proton-ATPase. 相似文献
Sulfate uptake into duckweed (Lemna gibba G1) was studied by means of [35S]sulfate influx and measurements of electrical membrane potential. Uptake was strongly regulated by the intracellular content of soluble sulfate. At the onset of sulfate uptake the membrane potential was transiently depolarized. Fusicoccin stimulated uptake up to 165% of the control even at pH 8. It is suggested that sulfate uptake is energized in the whole pH range by a 3H+/sulfate cotransport mechanism. Kinetics of sulfate uptake and sulfate-induced membrane depolarization in the concentration range of 5 M to 1 mM sulfate at pH 5.7 was best described by two Michaelis-Menten terms without any linear component. The second system had a lower affinity for sulfate and was fully active only at sufficiently high proton concentrations.Abbreviations
co
extracellular sulfate concentration
-
ci
intracellular sulfate concentration
-
Em
electrical membrane potential difference
- Em
sulfate-induced, maximal membrane depolarization
-
electrochemical proton gradient
- FW
fresh weight 相似文献