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1.
Among the various Ni‐based layered oxide systems in the form of LiNi1‐y‐zCoyAlzO2 (NCA), the compostions between y = 0.1–0.15, z = 0.05 are the most successful and commercialized cathodes used in electric vehicles (EVs) and hybrid electric vehicles (HEVs). However, tremendous research effort has been dedicted to searching for better composition in NCA systems to overcome the limitations of these cathodes, particularly those that arise when they are used use at high discharge/charge rates (>5C) and in high temperature (60 °C) environments. In addition, improving the thermal stability at 4.5 V is also very important in terms of the total amount of heat generated and the onset temperature. Here, a new NCA composition in the form of LiNi0.81Co0.1Al0.09O2 (y = 0.1, z = 0.09) is reported for the first time. Compared to the LiNi0.85Co0.1Al0.05O2 cathode, LiNi0.81Co0.1Al0.09O2 exhibits an excellent rate capability of 155 mAh g?1 at 10 C with a cut‐off voltage range between 3 and 4.5 V, corresponding to 562 Wh kg?1 at 24 °C. It additionally provides significantly improved thermal stability and electrochemical performance at the high temperature of 60 °C, with a discharge capacity of 122 mAh g?1 after 200 cycles with capacity retention of 59%. 相似文献
2.
Stomatal responses to leaf temperature (Tl) and to the mole fractions of water vapour in the ambient air (wa) and the leaf intercellular air spaces (wi) were determined in darkness to remove the potential effects of changes in photosynthesis and intercellular CO2 concentration. Both the steady‐state and kinetic responses of stomatal conductance (gs) to wa in darkness were found to be indistinguishable from those in the light. gs showed a steep response to the difference (Δw) between wa and wi when wa was varied. The response was much less steep when wi was varied. Although stomatal apertures responded steeply to Tl when Δw was held constant at 17 mmol mol?1, the response was much less steep when Δw was held constant at about zero. Similar results were obtained in the light for Δw = 15 mmol mol?1 and Δw ≈ 0 mmol mol?1. These results are discussed in the context of mechanisms for the stomatal response to humidity. 相似文献
3.
Consider the model yijk=u ± ai ± bi ± cij ± eijk i=1, 2,…, t; j=1, 2,…b; k=1, 2,…,nij where μ is a constant and ai, bi, cij are distributed independently and normally with zero means and variances Δ2 Δ2/bdij and δ2 respectively. It is assumed that di's, and dij's are known (positive) constants (for all i and j). In this paper procedures for estimating the variance components (Δ2, Δ2b and Δ2a) and for testing the hypothesis Hoc:Δ2c/Δ2 = y3 and Hoa:Δ2b/Δ2 = y4 (where y2, y3, and y4, are specified constants) are presented. A generalization for the mixed model case is discussed in the last section. 相似文献
4.
Terrestrial ecosystems and the carbon cycle 总被引:43,自引:0,他引:43
DAVID S. SCHIMEL 《Global Change Biology》1995,1(1):77-91
The terrestrial biosphere plays an important role in the global carbon cycle. In the 1994 Intergovernmental Panel Assessment on Climate Change (IPCC), an effort was made to improve the quantification of terrestrial exchanges and potential feedbacks from climate, changing CO2, and other factors; this paper presents the key results from that assessment, together with expanded discussion. The carbon cycle is the fluxes of carbon among four main reservoirs: fossil carbon, the atmosphere, the oceans, and the terrestrial biosphere. Emissions of fossil carbon during the 1980s averaged 5.5 Gt y?1. During the same period, the atmosphere gained 3.2 Gt C y?1 and the oceans are believed to have absorbed 2.0 Gt C y?1. The regrowing forests of the Northern Hemisphere may have absorbed 0.5 Gt C y?1 during this period. Meanwhile, tropical deforestation is thought to have released an average 1.6 Gt C y?1 over the 1980s. While the fluxes among the four pools should balance, the average 198Ds values lead to a ‘missing sink’ of 1.4 Gt C y?1 Several processes, including forest regrowth, CO2 fertilization of plant growth (c. 1.0 Gt C y?1), N deposition (c. 0.6 Gt C y?1), and their interactions, may account for the budget imbalance. However, it remains difficult to quantify the influences of these separate but interactive processes. Uncertainties in the individual numbers are large, and are themselves poorly quantified. This paper presents detail beyond the IPCC assessment on procedures used to approximate the flux uncertainties. Lack of knowledge about positive and negative feedbacks from the biosphere is a major limiting factor to credible simulations of future atmospheric CO2 concentrations. Analyses of the atmospheric gradients of CO2 and 13 CO2 concentrations provide increasingly strong evidence for terrestrial sinks, potentially distributed between Northern Hemisphere and tropical regions, but conclusive detection in direct biomass and soil measurements remains elusive. Current regional-to-global terrestrial ecosystem models with coupled carbon and nitrogen cycles represent the effects of CO2 fertilization differently, but all suggest longterm responses to CO2 that are substantially smaller than potential leaf- or laboratory whole plant-level responses. Analyses of emissions and biogeochemical fluxes consistent with eventual stabilization of atmospheric CO2 concentrations are sensitive to the way in which biospheric feedbacks are modeled by c. 15%. Decisions about land use can have effects of 100s of Gt C over the next few centuries, with similarly significant effects on the atmosphere. Critical areas for future research are continued measurements and analyses of atmospheric data (CO2 and 13CO2) to serve as large-scale constraints, process studies of the scaling from the photosynthetic response to CO2 to whole-ecosystem carbon storage, and rigorous quantification of the effects of changing land use on carbon storage. 相似文献
5.
Monte Carlo simulations are employed to investigate the thermodynamics of the first transition in writhe of a circular model filament corresponding to a 468 base-pair DNA. Parameters employed in these simulations are the torsional rigidity, C = 2.0 × 10−19 dyne cm2, and persistence length, P = 500 Å. Intersubunit interactions are modeled by a screened Coulomb potential. For a straight line of subunits this accurately approximates the nonlinear Poisson-Boltzmann potential of a cylinder with the linear charge density of DNA. Curves of relative free energy vs writhe at fixed linking difference (Δ1) exhibit two minima, one corresponding to slightly writhed circles and one to slightly underwrithed figure-8's, whenever Δ1 lies in the transition region. The free energies of the two minima are equal when Δ1c = 1.35, which defines the midpoint of the transition. At this midpoint, the free energy barrier between the two minima is found to be ΔGbar = (0.20) kBT at 298 K. Curves of mean potential energy vs writhe at fixed linking difference similarly exhibit two minima for Δ1 values in the transition region, and the two minimum mean potential energies are equal when Δ1 = 1.50. At the midpoint writhe, Δ1c = 1.35, the difference in mean potential energy between the minimum free energy figure-8 and circle states is (1.3) kBT, and the difference in their entropies is 1.3 kB. Thus, the entropy of the minimum free energy figure-8 state significantly exceeds that of the circle at the midpoint of the transition. The first transition in writhe is found to occur over a rather broad range of Δ1 values from 0.85 to 1.85. The twist energy parameter (ET), which governs the overall free energy of supercoiling, undergoes a sigmoidal decrease, while the translational diffusion coefficient undergoes a sigmoidal increase, over this same range. The static structure factor exhibits an increase, which reflects a decrease in radius of gyration associated with the circle to figure-8 transition. © 1996 John Wiley & Sons, Inc. 相似文献
6.
Xing Xu Hua Huo Jiyuan Jian Liguang Wang He Zhu Sheng Xu Xiaoshu He Geping Yin Chunyu Du Xueliang Sun 《Liver Transplantation》2019,9(15)
Ni‐rich Li[NixCoyMn1?x?y]O2 (x ≥ 0.8) layered oxides are the most promising cathode materials for lithium‐ion batteries due to their high reversible capacity of over 200 mAh g?1. Unfortunately, the anisotropic properties associated with the α‐NaFeO2 structured crystal grains result in poor rate capability and insufficient cycle life. To address these issues, a micrometer‐sized Ni‐rich LiNi0.8Co0.1Mn0.1O2 secondary cathode material consisting of radially aligned single‐crystal primary particles is proposed and synthesized. Concomitant with this unique crystallographic texture, all the exposed surfaces are active {010} facets, and 3D Li+ ion diffusion channels penetrate straightforwardly from surface to center, remarkably improving the Li+ diffusion coefficient. Moreover, coordinated charge–discharge volume change upon cycling is achieved by the consistent crystal orientation, significantly alleviating the volume‐change‐induced intergrain stress. Accordingly, this material delivers superior reversible capacity (203.4 mAh g?1 at 3.0–4.3 V) and rate capability (152.7 mAh g?1 at a current density of 1000 mA g?1). Further, this structure demonstrates excellent cycling stability without any degradation after 300 cycles. The anisotropic morphology modulation provides a simple, efficient, and scalable way to boost the performance and applicability of Ni‐rich layered oxide cathode materials. 相似文献
7.
Leaf‐level measurements have shown that mesophyll conductance (gm) can vary rapidly in response to CO2 and other environmental factors, but similar studies at the canopy‐scale are missing. Here, we report the effect of short‐term variation of CO2 concentration on canopy‐scale gm and other CO2 exchange parameters of sunflower (Helianthus annuus L.) stands in the presence and absence of abscisic acid (ABA) in their nutrient solution. gm was estimated from gas exchange and on‐line carbon isotope discrimination (Δobs) in a 13CO2/12CO2 gas exchange mesocosm. The isotopic contribution of (photo)respiration to stand‐scale Δobs was determined with the experimental approach of Tcherkez et al. Without ABA, short‐term exposures to different CO2 concentrations (Ca 100 to 900 µmol mol?1) had little effect on canopy‐scale gm. But, addition of ABA strongly altered the CO2‐response: gm was high (approx. 0.5 mol CO2 m?2 s?1) at Ca < 200 µmol mol?1 and decreased to <0.1 mol CO2 m?2 s?1 at Ca >400 µmol mol?1. In the absence of ABA, the contribution of (photo)respiration to stand‐scale Δobs was high at low Ca (7.2‰) and decreased to <2‰ at Ca > 400 µmol mol?1. Treatment with ABA halved this effect at all Ca. 相似文献
8.
9.
《Free radical research》2013,47(1-5):173-184
The sources and steady-state concentration of singlet oxygen in the atmosphere are assessed in view of potential effects on the biosphere. Collision-induced absorption of sunlight by molecular oxygen in 1 atm of air produces O2(a'δg) at a rate P = 1.6 × 10'cm's?1 in bright sunlight. Less than 10% are added to this purely natural source by the photolysis of ozone, and by anthropogenic sensitizers (SO2, NO2, volatile aromatics). Collisional quenching of O2(a'δg) by ground state oxygen establishes a steady-state concentration of ca. 1.7 × 108cm?1'. Reactions of singlet oxygen with other atmospheric pollutants are entirely negligible when compared with the concurrent reactions of ambient OH and 03. Potential effects of atmospheric singlet oxygen on the biosphere are limited by the deposition rate F< 0.051 P which depends on the production rate P of O2(a' δg) in the air layer immediately above the flat surface. 相似文献
10.
Thermodynamics of the B to Z transition in poly(dGdC) 总被引:1,自引:0,他引:1
The thermodynamics of the B to Z transition in poly(dGdC) was examined by differential scanning calorimetry, temperature-dependent absorbance spectroscopy, and CD spectroscopy. In a buffer containing 1 mM Na cacodylate, 1 mM MgCl2, pH 6.3, the B to Z transition is centered at 76.4°C, and is characterized by ΔHcal = 2.02 kcal (mol base pair)?1 and a cooperative unit of 150 base pairs (bp). The tm of this transition is independent of both polynucleotide and Mg2+ concentrations. A second transition, with ΔHcal = 2.90 cal (mol bp)?1, follows the B to Z conversion, the tm of which is dependent upon both the polynucleotide and the Mg2+ concentrations. Turbidity changes are concomitant with the second transition, indicative of DNA aggregation. CD spectra recorded at a temperature above the second transition are similar to those reported for ψ(–)-DNA. Both the B to Z transition and the aggregation reaction are fully and rapidly reversible in calorimetric experiments. The helix to coil transition under these solution conditions is centered at 126°C, and is characterized by ΔHcal = 12.4 kcal (mol bp)?1 and a cooperative unit of 290 bp. In 5 mM MgCl2, a single transition is seen centered at 75.5°C, characterized by ΔHcal = 2.82 kcal (mol bp)?1 and a cooperative unit of 430 bp. This transition is not readily reversible in calorimetric experiments. Changes in turbidity are coincident with the transition, and CD spectra at a temperature just above the transition are characteristic of ψ(–)-DNA. A transition at 124.9°C is seen under these solution conditions, with ΔHcal = 10.0 kcal (mol bp)?1 and which requires a complex three-step reaction mechanism to approximate the experimental excess heat capacity curve. Our results provide a direct measure of the thermodynamics of the B to Z transition, and indicate that Z-DNA is an intermediate in the formation of the ψ-(–) aggregate under these solution conditions. 相似文献
11.
ROBERT E. STAUFFER 《Freshwater Biology》1992,27(3):307-326
1. Factorial designs were used to investigate the spatial and temporal components of temperature sampling error in two medium-sized (maximum fetch = 9 and 11 km; maximum depth = 24 and 70m) Wisconsin lakes (Mendota and Green). Sampling designs were then optimized for estimating temperature distributions, thermocline migration, lake heat content, and vertical eddy conductivities in these and similar stratified lakes. 2. Errors were at a minimum for: (i) isotherms positioned 1/3 - 1/2 of the way down through the seasonal thermocline; (ii) at lake stations located near the node of the principal internal seiche; (iii) after an extended interval of low windpower. If seven spatially distributed lake stations were sampled n1, n2 times during the two low-power intervals bracketing a weather front, and the rime delay between revisits was randomized with respect to the period of the uninodal temperature seiches, the resulting standard error (cm) in ΔZ?17 (depth of 17° isotherm; an unbiased, minimum-variance estimator for main thermocline migration) was (144/n1+ 144/n2)1/2. If n1=n2= 2 ‘revisits’, the resulting CV (coefficient of variation) was 5–10% for ΔZ?17 accompanying major individual cold fronts in early summer. 3. When estimating Kz, the vertical eddy conductivity, the most important component of error relates to ΔHz, the change in heat content below depth z. The error in ΔHz is minimized in the same manner as for lake-mean isotherm depth. Using the Mendota sampling design described above, the RMS error in ΔHz decreases from ~90 cal cm?2 in the upper metalimnion to ~35calcm?2 near the base of the metalimnion. For seven take stations, and n1= n2= 2, Kz can be estimated with a CV ~10% bracketing a single major cold front. The CV decreases approximately as ΔHz?1, hence is roughly proportional to Δt?1 or to (cumuiative windpower)?1. 相似文献
12.
Stomatal conductance and transpiration were measured concurrently in an irrigated Eucalyptus globulus Labill. plantation. Canopy stomatal conductance, canopy boundary layer conductance and the dimensionless decoupling coefficient (Ω) were calculated (a) summing the conductance of three canopy layers (gc) and (b) weighting the contribution of foliage according to the amount of radiation received (gc′). Canopy transpiration was then calculated from gc and gc′ for Ω = 1 (Eeq), Ω = 0 (Eimp) and by weighting Eeq and Eimp using Ω (EΩ). Eeq, Eimp and EΩ were compared to transpiration estimated from measurements of heat pulse velocity. The mean value of Ω was 0·63. Transpiration calculated using gc and assuming perfect coupling (12·5 ± 0·9 mmol m?2 s?1) significantly overestimated measured values (8·7 ± 0·8 mmol m?2 s?1). Good estimates of canopy transpiration were obtained either (a) calculating EΩ separately for the individual canopy layers or (b) treating the canopy as a single layer and using gc′ in a calculation of Eimp (Ω = 0). The latter approach only required measurement of stomatal conductance at a single canopy position but would be unsuitable for use in combined models of canopy transpiration and assimilation. It should however, be suitable for estimating transpiration in forests regardless of the degree of coupling. 相似文献
13.
A thorough spectral investigation of the copper(II) complex of the antitumor compound, bleomycin, has been carried out in solution employing optical, difference optical, electron spin resonance, and circular dichroism techniques. The optical spectrum of a pH = 7 solution of the 1:1 complex between copper(II) and bleomycin is characterized by a broad weak band in the visible region (λmax = 610 nm) that cannot be resolved and intense ultraviolet bands at 317 (? = 2800), 327 (shoulder), 250 (? = 4700), and 257 nm (shoulder). The circular dichroism spectrum in the visible region shows the broad and weak visible absorption band contains at least three components (558, 675, and 880 nm) that are likely to be “d-d” in origin. The electron spin resonance spectrum is characteristic of a tetragonal d9 copper(II) system showing no rhombic distoritions at X-band frequencies (gx = gy ± 0.002). The spin Hamiltonian parameters for the pH = 7.0 solution corrected for second order effects are A∥ = 177 × 10?4 cm?1, A⊥ ? 15 × 10?4 cm?1, g∥ = 2.214, g⊥ = 2.039. Most interesting was the observation of extra hyperfine splitting due to endogenous nitrogen coordination in a 30% glycerol glass (AN = 12.0 × 10?4 cm?1). That pattern is best interpreted as a seven-line sequence associated with three liganded nitrogens. A dramatic change in all spectral properties occurs when the pH of the copper(II)-bleomycin complex is lowered to 2.5. All these data taken together suggest a CuN3O coordination complex in solution. Details and justifications as well as a discussion of the limitations of the interpretations are presented. 相似文献
14.
Sol Yun Seong‐Min Bak Sanghyeon Kim Jeong Seok Yeon Min Gyu Kim Xiao‐Qing Yang Paul V. Braun Ho Seok Park 《Liver Transplantation》2019,9(6)
Controlling the internal microstructure and overall morphology of building blocks used to form hybrid materials is crucial for the realization of deterministically designed architectures with desirable properties. Here, integrative spray‐frozen (SF) assembly is demonstrated for forming hierarchically structured open‐porous microspheres (hpMSs) composed of Fe3O4 and reduced graphene oxide (rGO). The SF process drives the formation of a radially aligned microstructure within the sprayed colloidal droplets and also controls the overall microsphere morphology. The spherical Fe3O4/rGO hpMSs contain interconnected open pores, which, when used as a lithium‐ion battery anode, enables them to provide gravimetric and volumetric capacities of 1069.7 mAh g?1 and 686.7 mAh cm?3, much greater than those of samples with similar composition and different morphologies. The hpMSs have good rate and cycling performance, retaining 78.5% capacity from 100 to 1000 mA g?1 and 74.6% capacity over 300 cycles. Using in situ synchrotron X‐ray absorption spectroscopy, the reaction pathway and phase evolution of the hpMSs are monitored enabling observation of the very small domain size and highly disordered nature of FexOy. The reduced capacity fade relative to other conversion systems is due to the good electrical contact between the pulverized FexOy particles and rGO, the overall structural integrity of the hpMSs, and the interconnected open porosity. 相似文献
15.
Hoon‐Hee Ryu Nam‐Yung Park Dae Ro Yoon Un‐Hyuck Kim Chong S. Yoon Yang‐Kook Sun 《Liver Transplantation》2020,10(25)
A new class of layered cathodes, Li[NixCoyB1?x?y]O2 (NCB), is synthesized. The proposed NCB cathodes have a unique microstructure in which elongated primary particles are tightly packed into spherical secondary particles. The cathodes also exhibit a strong crystallographic texture in which the a–b layer planes are aligned along the radial direction, facilitating Li migration. The microstructure, which effectively suppresses the formation of microcracks, improves the cycling stability of the NCB cathodes. The NCB cathode with 1.5 mol% B delivers a discharge capacity of 234 mAh g?1 at 0.1 C and retains 91.2% of its initial capacity after 100 cycles (compared to values of 229 mAh g?1 at 0.1 C and 78.8% for pristine Li[Ni0.9Co0.1]O2). This study shows the importance of controlling the microstructure to obtain the required cycling stability, especially for Ni‐rich layered cathodes, where the main cause of capacity fading is related to mechanical strain in their charged state. 相似文献
16.
The preparation of the planar yellow [Ni([8]aneN2)2](ClO4)2 is described. The complex dissociates in basic solution, with rate = kOH[NiL][OH?] (L = 1,5-diazacyclo-octane). At 25 °C, kOH = 4.5 x 10?2 M?1 s?1 and the corresponding activation parameters are ΔH≠ = 69.2 kJ mol?1 and ΔS298≠ = ?38.6 J K?1 mol?1. Acid catalysed dissociation in quite slow even in strongly acidic solutions. The kinetic data in this case can be fitted to the expression Kobs = ko + KH[H+], where ko relates to a solvolytic pathway and kH to the acid catalysed pathway. At 60 °C, Ko = 2 x 10?5 s?1 and kH is 2 x 10?5 M?1 s?1. Possible mechanisms for these reactions are considered.The Ni(II)/Ni(III) redox couple for NiLn+ is irreversible on Pt using MeCN as solvent. 相似文献
17.
In this paper, a statistical model for clinical trials is presented for the special situation that a varying and unstructered number of binary responses is obtained from each subject. The assumptions of the model are the following: 1.) For each subject there is a (constant) individual Bernoulli parameter determining the distribution of the binary responses of this subject. 2.) The Bernoulli parameters associated with the subjects are realizations of independent random variables with distributions Pg in treatment group g(g = 1, 2, …, G). 3.) Given the value of the Bernoulli parameter, the observations are stochastically independent within each subject. Under these assumptions, a test statistic is derived to test the hypothesis H0:E(P1) = E(P2) = … = E(PG). It is proven and demonstrated by simulations, that the test statistic asymptotically (i.e. for a large number of subjects) follows the X2-distribution. 相似文献
18.
Abstract According to computer energy balance simulations of horizontal thin leaves, the quantitative effects of stomatal distribution patterns (top vs. bottom surfaces) on transpiration (E) were maximal for sunlit leaves with high stomatal conductances (gs) and experiencing low windspeeds (free or mixed convection regimes). E of these leaves decreased at windspeeds > 50 cm s?1, despite increases in the leaf-to-air vapour density deficit. At 50 cm s?1 wind-speed, rapidly transpiring leaves had greater E when one-half of the stomata were on each leaf surface (amphistomaty; 10.16 mmol H2O m?2 s?1) than when all stomata were on either the top (hyperstomaty; 9.34 mmol m?2s?1) or bottom (hypostomaty; 7.02 mmol m?2s?1) surface because water loss occurred in parallel from both surfaces. Hyperstomatous leaves had larger E than hypostomatous leaves because free convection was greater on the top than on the bottom surface. Transpiration of leaves with large g, was greatest at windspeeds near zero when ~60–75% of the stomata were on the top surface, while at high windspeeds E was greatest with, 50% of the stomata on top. For leaves with low gs, stomatal distribution exerted little influence on simulated E values. Laboratory measurements of water loss from simulated hypo-, hyper-, and amphistomatous leaf models qualitatively supported these predictions. 相似文献
19.
Control of transpiration from the upper canopy of a tropical forest: the role of stomatal, boundary layer and hydraulic architecture components 总被引:6,自引:4,他引:6
F. C. MEINZER J. L. ANDRADE G. GOLDSTEIN N. M. HOLBROOK J. CAVELIER P. JACKSON 《Plant, cell & environment》1997,20(10):1242-1252
Concurrent, independent measurements of stomatal conductance (gs), transpiration (E) and microenvironmental variables were used to characterize control of crown transpiration in four tree species growing in a moist, lowland tropical forest. Access to the upper forest canopy was provided by a construction crane equipped with a gondola. Estimates of boundary layer conductance (gb) obtained with two independent methods permitted control of E to be partitioned quantitatively between gs and gb using a dimensionless decoupling coefficient (Ω) ranging from zero to 1. A combination of high gs (c. 300–600 mmol m?2 s?1) and low wind speed, and therefore relatively low gb (c. 100–800 mmol m?2 s?1), strongly decoupled E from control by stomata in all four species (Ω= 0.7–0.9). Photosynthetic water-use efficiency was predicted to increase rather than decrease with increasing gs because gb was relatively low and internal conductance to CO2 transfer was relatively high. Responses of gs to humidity were apparent only when the leaf surface, and not the bulk air, was used as the reference point for determination of external vapour pressure. However, independent measurements of crown conductance (gc), a total vapour phase conductance that included stomatal and boundary layer components, revealed a clear decline in gc with increasing leaf-to-bulk air vapour pressure difference (Va because the external reference points for determination of gc and Va were compatible. The relationships between gc and Vc and between gs and Vs appeared to be distinct for each species. However, when gs and gc were normalized by the branch-specific ratio of leaf area to sapwood area (LA/SA), a morphological index of potential transpirational demand relative to water transport capacity, a common relationship between conductance and evaporative demand for all four species emerged. Taken together, these results implied that, at a given combination of LA/SA and evaporative demand scaled to the appropriate reference point, the vapour phase conductance and therefore transpiration rates on a leaf area basis were identical in all four contrasting species studied. 相似文献
20.
Δ53β hydroxysteroid dehydrogenase activity transforms biologically inactive Δ53β hydroxy steroids into the active Δ43-keto products (e.g. pregnenolone to progesterone). Using a cytochemical procedure which allows for the continuous microdensitometric monitoring of an enzyme reaction as it proceeds and a well described cytochemical assay for Δ53β HSD we have analysed the initial velocity rates (Vo) for dehydroepiandrosterone (DHEA) binding to this enzyme in regressing (i.e. 20α hydroxy steroid dehydrogenase positive) corpus luteum (CL) cells in unfixed tissue sections (5 μm) of the dioestrous and proestrous rat ovary. The results are mean ± S.E.M. The relationship between DHEA concentration (0 to 50 μM) and Δ53β HSD activity in the dioestrous corpora lutea was sigmoidal and had an atypical 1/Vo versus 1/S plot, the x intercept being positive. Using a 1/Vo versus 1/S2 plot the Vmax was determined to be 1·0 ± 0·08 μmol min?1 mg?1 CL (n = 6). The Hill constant was 2·7 ± 0·02 (n = 6) suggesting a high degree of positive co-operativity for DHEA binding. The S concentration for half maximal activity was 17 ± 1 μmoles (n = 6). In the corpora lutea cells of the proestrous ovary, the Vmax for DHEA transformation was unchanged (0·95 ± 0·04 μmol min?1 mg?1, n = 3) whilst the S0·5 was significantly increased to 27 ± 0·1 (p < 0·01, n = 3). The Hill constant remained positive being 2·9 ± 0·2 (n = 3). NAD+ binding to 3β HSD in regressing corpora lutea of the proestrous ovary has been demonstrated previously to be hyperbolic and fit the classical Michaelis-Menten model.1 Extending the analysis of NAD+ binding to the regressing corpus luteum of the dioestrous rat ovary revealed similar kinetic characteristics to that seen with the proestrous enzyme, the apparent Vmax and Km being 0·84 ± 0·04 μmol min?1 mg?1 CL (n = 3) and 27 ± 7 μmol 1?1 (n = 3) respectively. The Hill constant was 1·1 ± 0·03 (n = 3), indicating no co-operativity of co-factor binding. 相似文献