共查询到20条相似文献,搜索用时 15 毫秒
1.
The effects of six years treatment with elevated [CO2] (350, 525, and 700 μl l-1) and nitrogen (N) (0, 10, and 20 g N m-2 yr-1) on soils, soil solution, and CO2 efflux in an open-top chamber study with ponderosa pine (Pinus ponderosa Laws.) are described. The clearest [CO2] effect was in year 6, when a pattern of lower soil N concentration and higher C/N ratio with elevated [CO2] emerged. Statistically significant effects of elevated [CO2] on soil total C, extractable P, exchangeable Mg2+, exchangeable Ca2+, base saturation, and soil solution HCO3
- and NO3
- were also found in various treatment combinations and at various times; however, these effects were inconsistent among treatments
and years, and in many cases (P, Mg2+, Ca2+, base saturation) reflected pre-treatment differences. The use of homogenized buried soil bags did not improve the power
to detect changes in soil C and N or help resolve the inconsistencies in soil C patterns. Nitrogen fertilization had the expected
negative effects on exchangeable Ca2+, K+, and Mg2+ in year 6, presumably because of increased NO3
- leaching, but had no consistent effect on soil C, N, or extractable P.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
2.
Mild proteolysis by trypsin activates the purified (Ca2+ + Mg2+) - ATPase protein from human red cells in a way which is similar to the effect obtained by addition of calmodulin. The trypsin concentration required to reach half maximal effect in 3 minutes at 37°C is 2.5 – 3.5 μg/ml. SDS-poly-acrylamide gel electrophoresis reveals a degradation of the main protein (150'000 Dalton) into a large fragment (95'000 – 100'000 Dalton) and a small fragment (35'000 – 40'000 Dalton). Increasing ATPase activity correlates with the degree of proteolysis.The
Ca of the digested (Ca2+ + Mg2+)-ATPase is 0.85 ± 0.1 μM Ca2+ as compared to 8.0 ± 0.75 μM Ca2+ before digestion and is statistically significantly different from
Ca = 1.66 ± 0.22 μM Ca2+ observed in activation by a saturating calmodulin concentration. Addition of calmodulin to the trypsinized enzyme has neither an effect on the Ca2+-affinity nor achieves any large increase of the maximal rate.High Ca2+ concentrations (above 0.05 – 0.1 mM) after trypsin treatment still inhibit the (Ca2+ + Mg2+)-ATPase activity. Mg2+ activates in the same concentration range (
Mg = 25 μM) as in the undigested preparation (
Mg = 27 μM) and retains its competitive behaviour towards Ca2+ after trypsin treatment.It is concluded that (1) trypsin treatment unmasks high affinity sites for Ca2+ (
Ca 1 μM) and that, therefore, such sites are not added to the system by calmodulin, and (2) that inhibition by high Ca2+-concentrations is not due to Ca - Mg competition at sites located on the calmodulin molecule. 相似文献
3.
The 31P nuclear magnetic resonance (nmr) spectra of product (phosphoenolpyruvate) and substrate (2-phosphoglycerate) binding to 1:1 molar ratios ot yeast enolase were obtained as functions of the level of various metal ions. Levels sufficient to produce substrate and product binding but not catalysis ( 1 equivalent/subunit), produced shifts (with respect to 86% H3PO4) to lower shielding of ca. 30 ppm in the case of Co2+, 5–8 ppm in the case of Mg2+, and 2–3 ppm in the case ofCa2+, but virtual obliteration in the case of Mn2+. The effects of Mn2+ and Co2+ are consistent with a close approach of the metal ions to the phosphate groups. The effects of the physiological cofactor and optimum activator Mg2+ and the nonactivator Ca2+ are interpreted as indicating different degrees of distortion of the R-O-P bond angle in the two metal-enzyme-substrate complexes. Levels of Mg2+ sufficient for optimal or near optimal catalysis (2 equivalents/subunit) produce shifts to higher shielding in the 31P resonances of both substrate and product. These shifts are intermediate between those in the presence of 1 equivalent/subunit and those of the free ligands. Addition of a second equivalent of Ca2+ produces a slight shift to lower shielding of the phosphoenolpyruvate resonance and a small shift to higher shielding in the resonance for 2-phosphoglycerate. Similar levels of Co2+ eliminate the resonances for both substrate and product. These effects are interpreted as arising from direct coordination between substrate-dependent metal ion binding and the phosphate esters. Higher levels of Ca2+, Mg2+, or Co2+ or addition of KF, all of which inhibit enzyme activity, have only minor effects on the spectra. The spectrum of inorganic phosphate, a competitive inhibitor, was also examined. KF strongly enhances binding, as does excess Mg2+, and the binding is accompanied by a chemical shift to lower shielding of ca 2 ppm. This is not due to formation of a magnesium-fluorophosphate complex, consistent with the findings of other workers. 相似文献
4.
Vera Pingoud Wolfgang Wende Monika Reuter Albert Jeltsch Monika Fuxreiter 《Journal of molecular biology》2009,393(1):140-94
Restriction endonucleases of the PD…D/EXK family need Mg2+ for DNA cleavage. Whereas Mg2+ (or Mn2+) promotes catalysis, Ca2+ (without Mg2+) only supports DNA binding. The role of Mg2+ in DNA cleavage by restriction endonucleases has elicited many hypotheses, differing mainly in the number of Mg2+ involved in catalysis. To address this problem, we measured the Mg2+ and Mn2+ concentration dependence of DNA cleavage by BamHI, BglII, Cfr10I, EcoRI, EcoRII (catalytic domain), MboI, NgoMIV, PspGI, and SsoII, which were reported in co-crystal structure analyses to bind one (BglII and EcoRI) or two (BamHI and NgoMIV) Me2+ per active site. DNA cleavage experiments were carried out at various Mg2+ and Mn2+ concentrations at constant ionic strength. All enzymes show a qualitatively similar Mg2+ and Mn2+ concentration dependence. In general, the Mg2+ concentration optimum (between ∼ 1 and 10 mM) is higher than the Mn2+ concentration optimum (between ∼ 0.1 and 1 mM). At still higher Mg2+ or Mn2+ concentrations, the activities of all enzymes tested are reduced but can be reactivated by Ca2+. Based on these results, we propose that one Mg2+ or Mn2+ is critical for restriction enzyme activation, and binding of a second Me2+ plays a role in modulating the activity. Steady-state kinetics carried out with EcoRI and BamHI suggest that binding of a second Mg2+ or Mn2+ mainly leads to an increase in Km, such that the inhibitory effect of excess Mg2+ or Mn2+ can be overcome by increasing the substrate concentration. Our conclusions are supported by molecular dynamics simulations and are consistent with the structural observations of both one and two Me2+ binding to these enzymes. 相似文献
5.
Magnus Monné Lucia Daddabbo Lorena Carla Giannossa Maria Cristina Nicolardi Luigi Palmieri Daniela Valeria Miniero Annarosa Mangone Ferdinando Palmieri 《Journal of bioenergetics and biomembranes》2017,49(5):369-380
The ATP-Mg/phosphate carriers (APCs) modulate the intramitochondrial adenine nucleotide pool size. In this study the concentration-dependent effects of Mg2+ and other divalent cations (Me2+) on the transport of [3H]ATP in liposomes reconstituted with purified human and Arabidopsis APCs (hAPCs and AtAPCs, respectively, including some lacking their N-terminal domains) have been investigated. The transport of Me2+ mediated by these proteins was also measured. In the presence of a low external concentration of [3H]ATP (12 μM) and increasing concentrations of Me2+, Mg2+ stimulated the activity (measured as initial transport rate of [3H]ATP) of hAPCs and decreased that of AtAPCs; Fe2+ and Zn2+ stimulated markedly hAPCs and moderately AtAPCs; Ca2+ and Mn2+ markedly AtAPCs and moderately hAPCs; and Cu2+ decreased the activity of both hAPCs and AtAPCs. All the Me2+-dependent effects correlated well with the amount of ATP-Me complex present. The transport of [14C]AMP, which has a much lower ability of complexation than ATP, was not affected by the presence of the Me2+ tested, except Cu2+. Furthermore, the transport of [3H]ATP catalyzed by the ATP/ADP carrier, which is known to transport only free ATP and ADP, was inhibited by all the Me2+ tested in an inverse relationship with the formation of the ATP-Me complex. Finally, direct measurements of Mg2+, Mn2+, Fe2+, Zn2+ and Cu2+ showed that they are cotransported with ATP by both hAPCs and AtAPCs. It is likely that in vivo APCs transport free ATP and ATP-Mg complex to different degrees, and probably trace amounts of other Me2+ in complex with ATP. 相似文献
6.
Nitrate absorption and assimilation in ryegrass as influenced by calcium and magnesium 总被引:3,自引:2,他引:1
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The absorption and assimilation patterns of 15NO3− supplied as the Ca2+ and Mg2+ salts to intact ryegrass (Lolium perenne) seedlings were compared. No statistically significant effect of ambient cation on the amounts of 15NO3− absorbed was observed in the initial six hours, but during the subsequent six hours, absorption from Ca(15NO3)2 exceeded that from Mg (15NO3)2. 相似文献
7.
The interaction of two DNA octamers, d(m5CG)4 and d(GGAATTCC), with the polyamines spermine4+ and spermidine3+, has been studied by means of 1H‐nmr nuclear Overhauser effect (NOE) difference measurements. The experiments were performed at 10°C and for a polyamine charge to DNA charge (i.e., phosphate) ratio of 0.4, where the solution of d(m5CG)4 contains about 50% Z‐form of the DNA. The results show that the polyamine intramolecular NOEs for the protons on the propyl chains are similarly negative with the two oligonucleotides, while those on the butyl chain show slightly more negative NOE with d(m5CG)4 than with d(GGAATTCC). The fully N‐methylated analogues of spermine (Me10Spn4+) and spermidine (Me8Spd3+) as well as the diamines 1,3‐diaminopropane (DAP2+) and 1,4‐diaminobutane (putrescine2+) have been studied for the ability to transform d(m5CG)4 from the B‐ to the Z‐form. 1H‐nmr spectra showed the order spermine4+ > spermidine3+ > Me10Spn4+ > Me8Spd3+ > 1,3‐diaminopropane2+ > putrescine2+, with spermine showing the largest relative amount of Z‐DNA. 1H‐nmr pulsed‐gradient self‐diffusion measurements of the triamines showed a large difference in the interaction of Spd and Me8Spd with the two different duplexes. With the same duplex (either of the two), however, no difference between Spd and Me8Spd can be seen. Within a two‐state model this is interpreted as a larger fraction of bound polyamines with d(m5CG)4 than with d(GGAATTCC). © 1999 John Wiley & Sons, Inc. Biopoly 49: 41–53, 1999 相似文献
8.
9.
Long-term and seasonal changes in the chemical composition of precipitation and surface waters of lakes and tarns in the English Lake District 总被引:4,自引:0,他引:4
D. W. SUTCLIFFE T. R. CARRICK J. HERON E. RIGG J. F. TALLING C. WOOF J. W. G. LUND 《Freshwater Biology》1982,12(5):451-506
SUMMARY. The composition of bulk (wet and dry) precipitation in 1975 and 1976 was similar to that found 22 years previously. In 1975, mean values for nine precipitation samplers at one site (Wraymires) ranged from pH 4.3 to 4.5; similar values were obtained in 1976. Samplers covered with fine-mesh plastic gauze caught substantially more Ca2+ and K+ than open samplers, but pH and SO2-4+ NO3- concentrations were similar in open and covered samplers. In precipitation, c. 50% of H+ was balanced by NO3- and c. 50% by SOi; 80% of the SO2-4 was balanced by Ca2+ and Mg2+. Conccntrations of major cations (H+, Na+, K+,Ca2+, Mg2+) and anions (CI-, NO3- SO2-4 and alkalinity [Alk—largely HCO3-]) in upland water- bodies were similar to those found in precipitation, but pH levels were generally higher and above 6.0 m some tarns. At lower altitudes, on base-rich roeks and soils, Ca2+ and Alk become dominant. Results of a survey of lakes and tarns in 1974–78 are compared with a survey in 1953–56 and published data (chiefly for pH and Alk) for 1947–50, 1932 and 1928. Comparisons are also made with other measurements of Alk in three productive lakes (Blelham Tarn, Esthwaite Water and Windermere) for 1936–39 and 1945–80. Winter levels of NO3-N, PO4-P and Si are given for these lakes; although the first two have increased during the late 1960s and the 1970s there has been no significant change in the last. NO3- and probably some SO2-4. In productive lakes a substantial (c. 50%) rise in mean Alk occurred during the late 1960s and the 1970s, possibly related to increased winter levels of NO3-N and PO4-P derived from sewage and fertilizers. In this period the maximum pH levels reached in summer were notably high, sometimes exceeding pH 10. The rise in Alk, conductivity and pH of surface waters is influenced by climatic factors (a decade of drier years), sewage input and biological productivity within the lakes. Considerable seasonal fluctuations in the concentrations of major ions, a characteristic feature of surface waters in the English Lake District, are illustrated and some implications for cation-anion balance briefly discussed. Mid-winter concentrations are usually high forNa+, K+, Cl-. NO3-and low for Ca2+, Mg2+, Alk. SO2-4. Alkalinity. pH and conductivity of Lake District tarns and lakes show no signs of acidification during the period 1928–80. On the contrary, productive lakes have become more alkaline and some unproductive low-alkalinity (< 100 μ-equiv. 1-1) lakes also show signs of alkalization, with increased mean concentrations of Na+. Ca2+ and Mg2+, balanced by Alk. 相似文献
10.
模拟酸沉降对鼎湖山季风常绿阔叶林地表径流水化学特征的影响 总被引:2,自引:0,他引:2
通过模拟酸沉降实验,研究了旱季期间(10-3月份)鼎湖山季风常绿阔叶林在4种不同pH模拟酸雨处理(对照、pH 4.0、pH 3.5、pH 3.0)下地表径流水化学输出特征.结果显示:(1)地表径流pH随酸处理强度增强呈“U”型变化模式,酸沉降对地表径流pH的影响不显著(P>0.05),表明模拟酸沉降尚未引起地表水的酸化.(2)地表径流中NO3-、SO24-浓度随酸处理强度增强略有增加;HCO3-浓度的变化模式与地表径流pH类似.酸根离子浓度与地表径流pH相关性分析表明,SO24-、HCO3-有助于提高地表水抗酸化能力而NO3-则有助于促进地表水酸化.(3)地表径流中盐基离子对酸沉降的响应不尽相同.pH 3.0处理显著提高地表径流中Ca2+、Na+浓度;Mg2+浓度具有随酸处理梯度增强而增加的趋势;K+受模拟酸度的影响小.表明强酸(pH3.0)处理将导致土壤Na+、Ca2+、Mg2+盐基离子流失.(4)酸沉降具有诱发土壤可溶性有机碳(DOC)流失的倾向,增加地表水受有机污染的风险. 相似文献
11.
The effects of omission of Ca2+ and Mg2+ from the incubation medium on three aspects of insulin action in isolated fat cells have been investigated. In the (Ca2+ + Mg2+)-free incubation medium incorporation of L-[14C]leucine into fat cell protein was reduced in the absence of insulin. Insulin stimulated L-[14C]-leucine incorporation only in the presence of added CaCl2 or MgCl2. Incubation of the cells in the (Ca2+ + Mg2+)-free medium reduced but did not abolish the ability of adrenaline to stimulate lipolysis or the ability of insulin to inhibit the adrenaline-stimulated lipolysis. Specific binding of 125I-labelled insulin to the fat cells was reduced in the absence of Ca2+ and Mg2+ but was not abolished, even in the presence of EDTA. Ca2+ was routinely the most effective divalent cation in supporting these aspects of insulin action, but similar responses were obtained with Mg2+, Sr2+ and Ba2+.Since insulin still binds to the cells under conditions in which some of the cellular effects of the hormone are abolished, it is suggested that divalent cations may have a role, either direct or indirect, in the processes linking the insulin-insulin receptor complex to certain effector systems in the cells. It is tentatively suggested that this action occurs at the level of the fat cell plasma membrane. 相似文献
12.
Adil E. Shamoo 《The Journal of membrane biology》1978,43(2-3):227-242
Summary The purified 20,000-dalton fragment of sarcoplasmic reticulum (Ca2++Mg2+)-ATPase has been shown by us (A.E. Shamoo, T.E. Ryan, P.S. Stewart, D.H. MacLennan, 1976. J. Biol. Chem.251:4147) to have Ca2+-selective ionophoric activity. The Ca2+-ionophoric fragment has been purified by either SDS-column chromatography or SDS-preparative gel electrophoresis. The Ca2+-ionophoric fragment has been subjected to prolonged dialysis to insure the removal of bound SDS from the fragment. The selectivity sequence of this fragment in black lipid membranes (BLM) formed from either oxidized cholesterol or phosphatidylcholine/cholesterol is the same,P
Ba>P
Ca>P
Sr>P
Mg>P
Mn. This selectivity sequence is the same as that for the intact (Ca2+ +Mg2+)-ATPase. Treatment of the fragment with cholate to absolutely insure the removal of bound SDS resulted in the fragment having a selectivity sequence as above except thatP
Mn>P
Mg. This and other data indicate that the 20,000-dalton fragment is the site containing the Ca2+-ionophoric activity of the (Ca2++Mg2+)-ATPase. 相似文献
13.
Calmodulin stimulation of calcium uptake and (Ca2+-Mg2+)-ATPase activities in microsomes from canine tracheal smooth muscle 总被引:2,自引:0,他引:2
The role of calmodulin in the regulation of microsomal 45Ca2+ transport in canine tracheal smooth muscle was studied. Calmodulin stimulated ATP-dependent 45Ca2+ uptake and (Ca2+Mg2+)-ATPase activities in microsomes treated with 0.5 mM EDTA and 0.5 mM EGTA. Oxalate also stimulated ATP-dependent 45Ca2+ uptake and (Ca2+Mg2+)-ATPase activities and the stimulation was additive to the effects of calmodulin. The (Ca2+Mg2+)-ATPase and ATP-dependent 45Ca2+ uptake activities are probably related as they exhibited similar [Ca2+]free- and [calmodulin]-dependencies. These results indicate that calmodulin may play a role in the control of the cytosolic [Ca2+]free in canine tracheal smooth muscle. 相似文献
14.
Pérez-Vázquez V Ramírez J Aguilera-Aguirre L González-Hernández JC Clemente-Guerrero M Manzo-Avalos S Uribe S Saavedra-Molina A 《Amino acids》2002,22(4):405-416
Summary. The manganese superoxide dismutase (Mn-SOD) converts superoxide anions to hydrogen peroxide plus oxygen, providing the first
line of defense against oxidative stress in mitochondria. Heart mitochondria exhibited higher Mn-SOD activity than liver mitochondria.
In mitochondria from both tissues Mn-SOD activity decreased after incubation at low oxygen concentration (hypoxic mitochondria).
The effects of free Ca2+ ([Ca2+]f) and free Mg2+ ([Mg2+]f) on normoxic and hypoxic mitochondria from either organ were tested. In normoxic mitochondria from either tissue, both [Ca2+]f and [Mg2+]f activated the enzyme, although [Mg2+]f was less efficient as an activator and the effect was lower in heart than in liver mitochondria. When added simultaneously,
high [Ca2+]f and [Mg2+]f exhibited additive effects which were more pronounced in heart mitochondria and were observed regardless of whether mitochondria
had been incubated under normal or low oxygen. The data suggest that [Ca2+]f plays a role in regulating Mn-SOD in concert with the activation of aerobic metabolism.
Received April 2, 2001 Accepted August 16, 2001 相似文献
15.
Singh Jaipaul Hustler Brenda I. Waring John J. Howarth Frank C. 《Molecular and cellular biochemistry》1997,176(1-2):127-134
This study employs both dietary and physiological studies to investigate the relationship between calcium (Ca2+) and magnesium (Mg2+) signalling in the mammalian myocardium. Rats maintained on a low Mg2+ diet (LMD; 39 mg Kg-1 Mg2+ in food) consumed less food and grew more slowly than control rats fed on a control Mg2+ diet (CMD; 500 mg Kg-1 Mg2+ in food). The Mg2+ contents of the heart and plasma were 85 ± 3% and 34 ± 6.5%, respectively relative to the control group. In contrast, Ca2+ contents in the heart and plasma were 177 ± 5% and 95 ± 3%. The levels of potassium (K+) was raised in the plasma (129 ± 16%) and slightly decreased in the heart (88 ± 6%) compared to CMD. Similarly, sodium (Na+) contents were slightly higher in the heart and lowered in the plasma of low Mg2+ diet rats compared to control Mg2+ diet rat. Perfusion of the isolated Langendorff's rat heart with a physiological salt solution containing low concentrations (0-0.6 mM) of extracellular magnesium [Mg2+]0 resulted in a small transient increase in the amplitude of contraction compared to control [Mg2+]0 (1.2 mM). In contrast, elevated [Mg2+]0 (2-7.2 mM) caused a marked and progressive decrease in contractile force compared to control. In isolated ventricular myocytes the L-type Ca2+ current (ICa,L was significantly (p < 0.001) attenuated in cells dialysed with 7.1 mM Mg2+ compared to cells dialysed with 2.9 µM Mg2+. The results indicate that hypomagnesemia is associated with decrease levels of Mg2+ and elevated levels of Ca2+ in the heart and moreover, internal Mg2+ is able to modulate the Ca2+ current through the L-type Ca2+ channel which in turn may be involved with the regulation of contractile force in the heart. 相似文献
16.
Sylvain Brunet Todd Scheuer William A. Catterall 《The Journal of general physiology》2009,134(2):81-94
L-type Ca2+ currents conducted by Cav1.2 channels initiate excitation–contraction coupling in cardiac myocytes. Intracellular Mg2+ (Mgi) inhibits the ionic current of Cav1.2 channels. Because Mgi is altered in ischemia and heart failure, its regulation of Cav1.2 channels is important in understanding cardiac pathophysiology. Here, we studied the effects of Mgi on voltage-dependent inactivation (VDI) of Cav1.2 channels using Na+ as permeant ion to eliminate the effects of permeant divalent cations that engage the Ca2+-dependent inactivation process. We confirmed that increased Mgi reduces peak ionic currents and increases VDI of Cav1.2 channels in ventricular myocytes and in transfected cells when measured with Na+ as permeant ion. The increased rate and extent of VDI caused by increased Mgi were substantially reduced by mutations of a cation-binding residue in the proximal C-terminal EF-hand, consistent with the conclusion that both reduction of peak currents and enhancement of VDI result from the binding of Mgi to the EF-hand (KD ≈ 0.9 mM) near the resting level of Mgi in ventricular myocytes. VDI was more rapid for L-type Ca2+ currents in ventricular myocytes than for Cav1.2 channels in transfected cells. Coexpression of Cavβ2b subunits and formation of an autoinhibitory complex of truncated Cav1.2 channels with noncovalently bound distal C-terminal domain (DCT) both increased VDI in transfected cells, indicating that the subunit structure of the Cav1.2 channel greatly influences its VDI. The effects of noncovalently bound DCT on peak current amplitude and VDI required Mgi binding to the proximal C-terminal EF-hand and were prevented by mutations of a key divalent cation-binding amino acid residue. Our results demonstrate cooperative regulation of peak current amplitude and VDI of Cav1.2 channels by Mgi, the proximal C-terminal EF-hand, and the DCT, and suggest that conformational changes that regulate VDI are propagated from the DCT through the proximal C-terminal EF-hand to the channel-gating mechanism. 相似文献
17.
We have compared effects of dimethylsulfoxide (Me2SO) and two polyols on the Ca2+-ATPase purified from human erythrocytes. As studied under steady-state conditions over a broad solute concentration range and temperature, Me2SO, glycerol, and xylitol do not inhibit the Ca2+-ATPase activity; this is in contrast to numerous other organic solutes that we have investigated. Under specific experimental conditions, Me2SO (but not glycerol) substantially increases Ca2+-ATPase activity, suggesting a possible facilitation of enzyme oligomerization. The activation is more pronounced at low Ca2+ concentrations. In contrast to glycerol, Me2SO shows no protective effect on enzyme structure as assessed by determining residual Ca2+-ATPase activity after exposing the enzyme to thermal denaturation at 45°C. Under these conditions several other organic solutes strongly enhance the denaturating effect of temperature. Because of the temperature dependence of its effect on the Ca2+-ATPase activity we believe that Me2SO activates the Ca2+-ATPase by indirect water-mediated interactions. 相似文献
18.
31P NMR chemical shifts of salts of adenosine 5′-triphosphate and diphosphate: ATPH2?22(Me4N+) · H2O, ATPH2?22 Na+ · 3.5 H2O, ATPH2?2Mg2+ · 4 H2O, ATPH2?2Ca2+ · 2 H2O, ADPH2?2(Me4N+) · H2O and ADPH2?Mg2+ · 4 H2O have been measured in 0.02 M 2H2O solutions at 145.7 MHz (22° C) at constant p2H values (8.20 and 6.20). The results are compared with those obtained from salts of adenosine 5′-monophosphate and other simpler phosphomonoesters, e.g. AMP2?2(Me4N+), AMP2?Mg2+, AMPH?Me4N+ and (AMPH?)2Mg2+. It is concluded that the effects exerted by Mg2+ and Ca2+ on the 31P NMR shifts of dipoly- and tripolyphosphates relative to monovalent cations are due mainly to changes in conformation of the polyphosphate chain rather than to purely electronic factors associated with the binding of divalent cations to the phospho-oxyanions. The data are consistent with the existence of the following complexes at p2H 8.20: (MgPαPβ)ADP? and (MgPαPγ)ATP2?af (MgPαPβ)ATP2?af (MgPβPγ)ATP2? with the latter equilibrium relatively fast in the NMR time scale. Monoprotonation of the terminal phosphate appears to weaken the Mg2+-polyphosphate binding, particularly at Pβ of MgADPH and at Pβ and Pγ of MgATPH?. The Mg2+-polyphosphate binding weakens further at p2H 3.70, i.e. in MgATPH2. Possible implications of the results in the mechanism of actomyosin Mg2+-ATPase in muscle contraction are discussed. 相似文献
19.
Enhancement of Nitrate Uptake and Growth of Barley Seedlings by Calcium under Saline Conditions 总被引:2,自引:0,他引:2
The effect of Ca2+ on NO3− assimilation in young barley (Hordeum vulgare L. var CM 72) seedlings in the presence and absence of NaCl was studied. Calcium increased the activity of the NO3− transporter under saline conditions, but had little effect under nonsaline conditions. Calcium decreased the induction period for the NO3− transporter under both saline and nonsaline conditions but had little effect on its apparent Km for NO3− both in the presence and absence of NaCl. The enhancement of NO3− transport by Ca2+ under saline conditions was dependent on the presence of Ca2+ in the uptake solution along with the salt, since Ca2+ had no effect when supplied before or after salinity stress. Although Mn2+ and Mg2+ enhanced NO3− uptake under saline conditions, neither was as effective as Ca2+. In longer studies, increasing the Ca2+ concentration in saline nutrient solutions resulted in increases in NO3− assimilation and seedling growth. 相似文献
20.
Ján Slezák Ludmila Okruhlicová Narcisa Tribulová Wolfgang Schulze Naranjan S. Dhalla 《Molecular and cellular biochemistry》1995,147(1-2):169-172
ATPases of cardiac cells are known to be among the most important enzymes to maintain the fluxes of vital cations by hydrolysis of the terminal high-energy phosphate of ATP. Biochemically the activities of Ca2+-pump ATPase, Ca2+/Mg2+-ecto ATPase, Na+,K+-ATPase and Mg2+-ATPase are determined in homogenates and isolated membranes as well as in myofibrillar and mitochondrial fractions of various purities. Such techniques permit estimation of enzyme activitiesin vitro under optimal conditions without precise enzyme topography. On the other hand, cytochemical methods demonstrate enzyme activityin situ, but not under optimal conditions. Until recently several cytochemical methods have been employed for each enzyme in order to protect its specific activity and precise localization but the results are difficult to interpret. To obtain more consistent data from biochemical and cytochemical point of view, we modified cytochemical methods in which unified conditions for each ATPase were used. The fixative solution (1% paraformaldehyde –0.2% glutaraldehyde in 0.1 M Tris Base buffer, pH 7.4), the same cationic concentrations of basic components in the incubation medium (0.1 M Tris Base, 2mM Pb(NO2)3, 5 mM MgSO4, 5 mM ATP) and selective stimulators or inhibitors were employed. The results reveal improved localization of Ca2+-pump ATPase, Na+–K+ ATPase and Ca2+/Mg2+-ecto ATPase in the cardiac membrane. 相似文献