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1.
Y A Shin 《Biopolymers》1973,12(11):2459-2475
Changes in the conformation of poly(G), poly(C), poly(U), and poly(I) in the presence of divalent metal ions Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Cd2+, and Zn2+ have been measured by means of ORD and u.v. spectra. Mg2+ and Ca2+ ions stabilize helical structures of all the polynucleotides very effectively at concentrations several orders of magnitude lower than the effective concentration of Na+ion. Cu2+ and Cd2+ destabilize the helical structure of polynucleotides to form random coils. Zn2+, Ni2+, Co2+, and Mn2+ions do not behave in such a clear-cut manner: they selectively stabilize some ordered structures, while destabilizing others, depending on the ligand strength of the nucleotide base as well as the preferred conformation of that polynucleotide. 相似文献
2.
Vladimir N. Potaman Valeiy N. Soyfer 《Journal of biomolecular structure & dynamics》2013,31(5):1035-1040
Abstract The PyPuPu triplexes consisting of CG*G triads are stabilized by alkaline earth cations (Ca2+, Mg2+) and transition metal cations (Mn2+, Co2+, Ni2+, Zn2+, Cd2+), while similar triplexes including TA*A triads are stabilized only by transition metal cations. We hypothesize that such a differential triplex stabilization by divalent metal cations can be the consequence of their coordination to the N7 of the third strand purines with concomitant polarization effects on the bases resulting in unequal Hoogsteen-type hydrogen bond enhancement. 相似文献
3.
Protoplasts prepared from yeast-like cells, hyphae and chlamydospores of Aureobasidium pullulans can take up heavy metals such as Zn2+, Co2+, Cd2+ and Cu2+. In relation to intact cells, the sensitivity of protoplasts to Cu2+ and Cd2+ was increased although chlamydospore protoplasts were more tolerant than yeast-like cell protoplasts. Surface binding of metals was reduced in protoplasts as compared with intact cells and this reduction was particularly evident for chlamydospore protoplasts. At the highest concentrations used, uptake of Zn2+, Co2+ and Cd2+ by yeast-like cell protoplasts was greater than that observed in intact cells which may have been due to toxicity, especially for Cd2+, resulting in increased membrane permeability, though for Zn2+ and Co2+ some barrier effect of the cell wall could not be completely discounted. Chlamydospore protoplasts were capable of intracellular metal uptake, unlike intact chlamydospores, and for Zn2+, uptake appeared to be via a different system less specific than that of the other cell types. For chlamydospores, the use of protoplasts confirmed the importance of the cell wall in preventing entry of metal ions into the cell. 相似文献
4.
5.
Summary The toxicity of chromium and tin on growth, photosynthetic carbon-fixation, oxygen evolution, heterocyst differentiation and nitrogenase activity ofAnabaena doliolum and its interaction with bivalent cations has been studied. Some interacting cations, viz. Ca2+, Mg2+ and Mn2+, substantially antagonised the toxic effects of chromium and tin with reference to growth, heterocyst differentiation and nitrogenase activity in the following hierarchal sequence: Ca2+ > Mg2+ > Mn2+. However, the sequence of hierarchy was Mg2+ > Ca2+ > Mn2+ for carbon fixation and Mn2+ > Mg2+ > Ca2+ for photosynthetic oxygen evolution. Synergistically inhibitory patterns were noticed for all the parameters, viz. growth,14CO2 uptake, oxygen evolution, heterocyst differentiation and nitrogenase activity ofA. doliolum when Ni2+, Co2+ and Zn2+ were combined with the test metals in the growth medium. These cations followed the following sequence of synergistic inhibition: Ni2+ > Co2+ > Zn2+. Among all the interacting cations, Ca2+, Mg2+ and Mn2+ exhibited antagonistic effects which relieved the test cyanobacterium from metal toxicity. In contrast to this, Ni2+, CO2+ and Zn2+ showed synergistic inhibition which potentiating the toxicity of test metals in the N2-fixing cyanobacteriumA. doliolum. It is evident from the present study that bivalent cations, viz. Ca2+, Mg2+, Mn2+, Ni2+, Co2+ and Zn2+, may appreciably regulate the toxicity of heavy metals in N2-fixing cyanobacteria if present in aquatic media. 相似文献
6.
In this study amphotericin B released the divalent trace metals Zn2+, Co2+, Cu2+, Ni2+, Mn2+, Fe2+, Cd2+ and Pb2+ from multilamellar liposomes containing cholesterol. This observation is consistent with amphotericin B channels being permeable to these metals, and it is proposed, therefore, that the antibiotic may be useful in investigating the metabolism of these elements. 相似文献
7.
Characterization of the metal ion binding site in the anti-terminator protein, HutP, of Bacillus subtilis 总被引:1,自引:0,他引:1
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HutP is an RNA-binding protein that regulates the expression of the histidine utilization (hut) operon in Bacillus subtilis, by binding to cis-acting regulatory sequences on hut mRNA. It requires L-histidine and an Mg2+ ion for binding to the specific sequence within the hut mRNA. In the present study, we show that several divalent cations can mediate the HutP–RNA interactions. The best divalent cations were Mn2+, Zn2+ and Cd2+, followed by Mg2+, Co2+ and Ni2+, while Cu2+, Yb2+ and Hg2+ were ineffective. In the HutP–RNA interactions, divalent cations cannot be replaced by monovalent cations, suggesting that a divalent metal ion is required for mediating the protein–RNA interactions. To clarify their importance, we have crystallized HutP in the presence of three different metal ions (Mg2+, Mn2+ and Ba2+), which revealed the importance of the metal ion binding site. Furthermore, these analyses clearly demonstrated how the metal ions cause the structural rearrangements that are required for the hut mRNA recognition. 相似文献
8.
《Bioscience, biotechnology, and biochemistry》2013,77(10):1688-1692
Thermostable dipeptidase from Bacillus stearothermophilus, a typical metalloenzyme containing 1.0g atom of Zn per mole of subunit of the dimeric enzyme was markedly activated by exogenous divalent metal ions such as Mn2+, Co2+, and Cd2+ . In contrast, several others including Ba2+, Hg2+, and Cu2+ considerably inhibited the enzyme, even the inherent metal, Zn2+, being slightly inhibitory. To study the metal-binding properties of this dipeptidase, the enzyme was completely resolved to the inactive, Zn-free apoenzyme by treatment with EDTA in the presence of guanidine hydrochloride in a weakly acidic buffer. The apoenzyme was readily reconstituted by incubation with either Zn2+, Mn2+, or Co2+, restoring the catalytic activity. The Mn-reconstituted enzyme had nearly twice the activity of the original Zn-enzyme. Combined with kinetic analyses of reconstitution of the apoenzyme with metal ions, these results show that the enzyme has two non-identical metal-binding sites, each with a different property. Furthermore, substitution of Mn2+ or Co2+ for Zn2+ considerably lowered the thermostability of the enzyme without affecting the overall conformation of the enzyme protein, suggesting that the prosthetic Zn is playing dual roles in conformational stability and catalysis of the thermostable dipeptidase. 相似文献
9.
The divalent metal ions Cu2+, Co2+, Mn2+, and Zn2+ form complexes with the fluorescent etheno analogs of the adenine nucleotides. The fluorescence intensity is thereby diminished. The binding strength of the metals to etheno-adenosine triphosphate is higher than to etheno-adenosine di- and monophosphate. The quenching effect of the divalent metal ions can be exploited as a simple routine activity measurement for various kinases and phosphohydrolases. 相似文献
10.
The divalent cation requirements of NOS activity in bovine retina homogenate supernatant were investigated. Supernatants were assayed under standard conditions (in mM: EDTA 0.45, Ca2+ 0.25, Mg2+ 4.0). In order to investigate the enzyme's dependence on divalent cations, the tissue homogenate was depleted of di- and trivalent cations by passing it over a cation-exchange column (Chelex 100). Surprisingly, NOS activity was 50-100% higher in this preparation. However, addition of either EDTA (33 M) or EGTA (1 mM) almost fully inhibited NOS activity, suggesting a requirement for residual divalent metal cation(s). Phenanthroline or iminodiacetic acid at low concentrations had little effect on activity, suggesting no requirement for Fe2+, Zn2+ or Cu2+. Ca2+ had a moderate stimulatory effect, with an optimum activity around 0.01 mM. Mg2+ or Mn2+ had little effect at concentrations < 0.25 mM. However, in the presence of EDTA, Mn2+ or Ca2+ markedly stimulated NOS activity with the optimum at 0.1 mM. At high concentrations (> 0.1-0.2 mM), all divalent cations tested (Ba2+, Zn2+, Co2+, Mn2+, Mg2+, Ca2+), as well as La3+, dose-dependently inhibited NOS activity. We propose that retinal NOS requires low concentrations of naturally occurring divalent metal ions, most probably Ca2+, for optimal activity and is inhibited by high di- and trivalent metal concentrations, probably by competition with Ca2+. 相似文献
11.
The relation that exist between the Pi-PPi exchange reaction and pyrophosphate hydrolysis by the membrane-bound pyrophosphatase of chromatophores ofRhodospirillum rubrum was studied. The two reactions have a markedly different requirement for pH. The optimal pH for hydrolysis was 6.5 while the Pi-PPi exchange reaction was at 7.5; the pH affects mainly theK
m
of Mg2+ or Pi for the enzyme; Mn2+ and Co2+ support the Pi-PPi exchange reaction partially (50%), but the reaction is slower than with Mg2+; other divalent cations like Zn2+ or Ca2+ do not support the exchange reaction. In the hydrolytic reaction, Zn2+, at low concentration, substitutes for Mg2+ as substrate, and Co2+ also substitutes in limited amount (50%). Other cations (Ca2+, Cu2+, Fe2+, etc.) do not act as substrates in complex with PPi. The Zn2+ at high concentrations inhibited the hydrolytic reaction, probably due to uncomplexed free Zn2+. In the presence of high concentration of substrate for the hydrolysis (Mg-PPi) the divalent cations are inhibitory in the following order: Zn2+>Mn2+>Ca2+Co2+>Fe2+>Cu2+>Mg2+. The data in this work suggest that H+ and divalent cations in their free form induced changes in the kinetic properties of the enzyme. 相似文献
12.
Ramesh C. Gupta Ramji L. Khandelwal Prakash V. Sulakhe 《Molecular and cellular biochemistry》1990,97(1):53-66
Summary The effects of divalent metals, metal chelators (EDTA, EGTA) and sodium dodecyl sulfate were investigated on the phosphatase activity of isolated bovine brain calcineurin assayed in the absence (called intrinsic) and presence of calmodulin. Intrinsic phosphatase was increased by Mn2+, was unaffected by Mg2+, Ca2–, and Ba+, and was markedly inhibited by Ni2–, Fe2+, Zn2+ and Cu2–. When assayed in the presence of calmodulin, many divalent metals (Ni2–, Zn2+, Pb2+, Cd2+), besides Mn2+, increased modestly the phosphatase activity at low concentrations (10–100 M) and inhibited it markedly at high concentrations. Ca2–-calmodulin stimulated phosphatase activity was antagonized by Ni2+, Zn2+, Fe2+, Cu2+, Pb2+, at low concentrations (50 M), and by Ba2+, Cd2+ at slightly higher concentrations (> 100 M); Mn2+ and Co2– (50 M to 1 mM) in fact augmented it. EDTA and EGTA in a concentration and time dependent fashion inhibited the intrinsic phosphatase activity, particularly that of trypsinized calcineurin. SDS in low concentrations (0.005%) augmented the phosphatase activity and inhibited it at high concentrations. Mn2+ (± calmodulin) and Ca2+ only with calmodulin present increased the phosphatase activity assayed with low concentrations of SDS. The EDTA dependent inhibition of intrinsic phosphatase was almost abolished in assays containing SDS. Prior exposure of calcineurin to Mn2+ led to a high activity conformation state of calcineurin that was long-lived or pseudo-irreversible. Such Mn2+-activated state of calcineurin exhibited no discerbible change in the affinity towards myelin basic protein or its inhibition by trifluoperazine. At alkaline pH, Mg2+ supported the intrinsic phosphatase activity, although to a lesser degree than Mn2+. The latter cation, compared to Mg2+ and Ni2+, was also a more powerful stimulator of the calcineurin phosphatase assayed with histone (III-S) and myosin light chain as substrates. 相似文献
13.
The activation of muscle pyruvate kinase by divalent cations was studied by steady-state kinetics. Under experimental conditions the enzyme exhibits activation by Mg2+, Co2+, Mn2+, Ni2+, and Zn2+ in descending order of maximal velocity. Combinations of cations were also studied. A synergistic activation was observed with a fixed concentration of Mg2+ and varying concentrations of Mn2+ or of Co2+. This synergism indicates at least two roles for the cations for enzymatic activation and a differential specificity among the cations for the separate functions. Synergistic activation was also observed with fixed Co2+ and varying Mn2+. These results are consistent with a cation specifically required to activate the enzyme and a cation which serves as a cation-nucleotide complex which is a substrate for the reaction. The response observed suggests that Mn2+ is a better activator of the enzyme than is Mg2+, however, MgADP is a better substrate than is MnADP. The lack of a synergistic effect by Ni2+ or Zn2+ with Mg2+ suggests that Ni2+ and Zn2+ are poor activators either because they serve one catalytic function poorly but bind to that site tightly or they serve both catalytic functions poorly in contrast to Mg2+. These studies yield the first simple kinetic evidence that muscle pyruvate kinase, under catalytic conditions of the overall reaction, has a dual divalent cation requirement for activity. 相似文献
14.
The voltage-gated proton channel Hv1 functions as a dimer, in which the intracellular C-terminal domain of the protein is responsible for the dimeric architecture and regulates proton permeability. Although it is well known that divalent metal ions have effect on the proton channel activity, the interaction of divalent metal ions with the channel in detail is not well elucidated. Herein, we investigated the interaction of divalent metal ions with the C-terminal domain of human Hv1 by CD spectra and fluorescence spectroscopy. The divalent metal ions binding induced an obvious conformational change at pH 7 and a pH-sensitive reduction of thermostability in the C-terminal domain. The interactions were further estimated by fluorescence spectroscopy experiments. There are at least two binding sites for divalent metal ions binding to the C-terminal domain of Hv1, either of which is close to His244 or His266 residue. The binding of Zn2+ to the two sites both enhanced the fluorescence of the protein at pH 7, whereas the binding of other divalent metal ions to the two sites all resulted fluorescence quenching. The orders of the strength of divalent metal ions binding to the two sites from strong to weak are both Co2+, Ca2+, Ni2+, Mg2+, and Mn2+. The strength of Ca2+, Co2+, Mg2+, Mn2+ and Ni2+ binding to the site close to His244 is stronger than that of these divalent metal ions binding to the site close to His266. 相似文献
15.
Effects of Metal Ions on the Conformation and Activity of Acutolysin D from Agkistrodon Acutus Venom
Acutolysin D, isolated from the venom of Agkistrodon acutus, possesses marked haemorrhagic and proteolytic activities. The molecular weight and the absorption coefficients (A
1%
280) of acutolyisn D have been determined to be 47,850 ± 8 amu and 9.3 by mass spectrometer and UV spectrum, respectively. The effects of metal ions on the conformation and activity of acutolysin D have been studied by following fluorescence, circular dichroism and biological activity measurements. Acutolysin D contains two Ca2+-binding sites and two Zn2+-binding sites determined by atomic absorption spectrophotometer. Zn2+ is essential for the enzyme activities of acutolysin D, however, the presence of 1 mM Zn2+ significantly decreases its caseinolytic activity and intrinsic fluorescence intensity at pH 9.0 due to Zn(OH)2 precipitate formation. Ca2+ is important for the structural integrity of acutolysin D, and the presence of 1 mM Ca2+ markedly enhances its caseinolytic activity. Interestingly, the caseinolytic activity which is inhibited partly by Cu2+, Co2+, Mn2+ or Tb3+ and inhibited completely by Cd2+, is enhanced by Mg2+. The fluorescence intensity of the protein decreases in the presence of Cu2+, Co2+, Cd2+ or Mn2+, but neither for Ca2+, Mg2+ nor for Tb3+. Zn2+, Ca2+, Mg2+, Cu2+, Mn2+, Co2+ and Tb3+ have slight effects on its secondary structure contents. In addition, Cd2+ causes a marked increase of antiparallel β-sheet content from 45.5% to 60.2%. 相似文献
16.
Rochelle D. Schwartz Joseph P. Wagner Xiao Yu David Martin 《Journal of neurochemistry》1994,62(3):916-922
Abstract: The effects of the divalent cations Ca2+, Sr2+, Ba2+, Mg2+, Mn2+, and Cd2+ were studied on γ-aminobutyric acidA (GABAA) responses in rat cerebral cortical synaptoneurosomes. The divalent cations produced bidirectional modulation of muscimol-induced 36Cl? uptake consistent with their ability to permeate and block Ca2+channels. The order of potency for inhibition of muscimol responses was Ca2+ > Sr2+ > Ba2+, similar to the order for permeation of Ca2+ channels in neurons. The order of potency for enhancement of muscimol responses was Cd2+> Mn2+ > Mg2+, similar to the order for blockade of Ca2+channels in neurons. Neither Ca2+ nor Mg2+ caused accumulation of GABA in the extravesicular space due to increased GABA release or decreased reuptake of GABA by the synaptoneurosomes. The inhibition of muscimol responses by Ca2+ was most likely via an intracellular site of action because additional inhibition could be obtained in the presence of the Ca2+ ionophore, A23187. This confirms electrophysiologic findings in cultured neurons from several species. In contrast, the effects of Cd2+, Mn2+, and Mg2+ may be mediated via blockade of Ca2+ channels or by intracellular sites, although the results of these studies do not distinguish between the two loci. The effects of Zn2+ were also studied, because this divalent cation is reported to have widely divergent effects on GABAA responses. In contrast to other studies, we demonstrate that Zn2+ inhibits GABAA responses in an adult neuronal preparation. Zn2+ produced a concentration-dependent inhibition (limited to 40%) of muscimol responses with an EC50 of 60 μM. The inhibition of muscimol-induced 38Cl? uptake by Zn2+ was noncompetitive. The effect of Zn2+was reduced in the presence of Mg2+ in a competitive or allosteric manner. The portion of GABAA receptors sensitive to Zn2+ may reflect a specific subunit composition in cerebral cortex as previously observed for recombinant GABAA receptors in several expression systems. The modulation of GABAA receptor function by Ca2+ and other divalent cations may play an important role in the development and/or attenuation of neuronal excitability associated with pathologic conditions such as seizure activity and cerebral ischemia. 相似文献
17.
The effects of metal ions of the first-row transition and of alkaline earth metals on the DNA helix conformation have been studied by uv difference spectra, circular dichroism, and sedimentation measurements. At low ionic strength (10?3 M NaClO4) DNA shows a maximum in the difference absorption spectra in the presence of Zn2+, Mn2+, Co2+, Cd2+, and Ni2+ but not with Mg2+ or Ca2+. The amplitude of this maximum is dependent on GC content as revealed by detailed studies of the DNA-Zn2+ complex of eight different DNA's. Pronounced changes also occur in the CD spectra of DNA transition metal complexes. A transition appears up to a total ratio of approximately 1 Zn2+ per DNA phosphate at 10?3 M NaClO4; then no further change was observed up to high concentrations. The characteristic CD changes are strongly dependent on the double-helical structure of DNA and on the GC content of DNA. Differences were also observed in hydrodynamic properties of DNA metal complexes as revealed by the greater increase of the sedimentation coefficient of native DNA in the presence of transition metal ions. Spectrophotometric acid titration experiments and CD measurements at acidic pH clearly indicate the suppression of protonation of GC base-pair regions on the addition of transition metal ions to DNA. Similar effects were not observed with DNA complexes with alkaline earth metal ions such as Mg2+ or Ca2+. The data are interpreted in terms of a preferential interaction of Zn2+ and of other transition metal ions with GC sites by chelation to the N-7 of guanine and to the phosphate residue. The binding of Zn2+ to DNA disappears between 0.5 M and 1 M NaClO4, but complex formation with DNA is observable again in the presence of highly concentrated solutions of NaClO4 (3?7.2 M NaClO4) or at 0.5 to 2 M Mn2+. At relatively high cation concentration Mg2+ is also effective in changing the DNA comformation. These structural alterations probably result from both the shielding of negatively charged phosphate groups and the breakdown of the water structure along the DNA helix. Differential effects in CD are also observed between Mn2+, Zn2+ on one hand and Mg2+ on the other hand under these conditions. The greater sensitivity of the double-helical conformation of DNA to the action of transition metal ions is due to the affinity of the latter to electron donating sites of the bases resulting from the d electronic configuration of the metal ions. An order of the relative phosphate binding ability to base-site binding ability in native DNA is obtained as follows: Mg2+, Ba2+, < Ca2+ < Fe2+, Ni2+, Co2+ < Mn2+, Zn2+ < Cd2+ < Cu2+. The metal-ion induced conformational changes of the DNA are explained by alternation of the winding angle between base pairs as occurs in the transition from B to C conformation. These findings are used for a tentative molecular interpretation of some effects of Zn2+ and Mn2+ in DNA synthesis reported in the literature. 相似文献
18.
Maize seeds and five-day-old maize seedlings were incubated in media containing Pb2+ at concentrations of 50, 100, and 200 mg 1-1 and Cd2+ at concentrations of 1, 5, 10 and 50 mg 1-1. After five days of incubation, both heavy metals were determined by means of AAS following wet mineralisation of roots and
shoots. The results obtained indicate that Pb2+ were transported to shoots from roots at a lower rate than Cd2+.
Phosphoenolpyruvate carboxylase (PEPC) isolated from germinating maize seeds was inhibited to a comparable degree by solutions
containing 0.001 mmol 1-1 Pb2+, 0.01 mmol 1-1 Cd2+, and 0.005 mmol 1-1 Cu2+. The enzyme was protected against this inhibition by the addition of mercaptoethanol, the substrate (PEP), or the cofactor
(Mg2+). The inhibition increased during a 20 min incubation of the enzyme with salts of the metals. Mn2+, Ni2+, and Co2+ ions could partially substitute for the metal cofactor Mg2+. Km values for these metal ions were as follows: for Mg2+ 0.07 mmol 1-1 in the range from 0 to 0.30 mmol 1-1 Mg2+; 0.71 mmol 1-1 for 0.30 to 2.50 mmol 1-1 Mg2+; for Mn2+ 0.36 mmol 1-1; for Ni2+ 0.34 mmol 1{-1}; and for Co2+ 0.20 mmol 1-1. The activity of the enzyme reached with Mn2+ 85 %, with Ni2+ 65 %, and with Co2+ 55 % of the activity recorded with Mg2+. 相似文献
19.
Characterization of the Effects of Divalent Cations on the Coupled Activities of the H-ATPase in Tonoplast Vesicles 总被引:4,自引:2,他引:2
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The substrate requirement of the H+-ATPase in purified corn root tonoplast vesicles was investigated. The coupled activities, ATP hydrolysis and proton pumping, were simultaneously supported only by Mg2+ or Mn2+. The presence of Ca2+ or Ba2+ did not significantly affect the coupled activities. The addition of Cd2+, Co2+, Cu2+, and Zn2+ inhibited both the hydrolysis of Mg-ATP and the proton transport. However, the inhibition of proton pumping was more pronounced. Based on equilibrium analysis, both ATP-complexed and free forms of these cations were inhibitory. Inhibition of the hydrolysis of Mg-ATP could be correlated to the concentrations of the ATP-complex of Zn. On the other hand, the free Cu2+ and Co2+ were effective in inhibiting hydrolysis. For proton pumping, the ATP complexes of Co2+, Cu2+, and Zn2+ were effective inhibitors. However, this inhibition could be further modulated by free Co2+, Cu2+, and Zn2+. While the equilibrium concentrations of Cd-ATP and free Cd2+ were not estimated, the total concentration of this cation needed to inhibit the coupled activities of the H+-ATPase was found to be in the range of 10 to 100 micromolars. The presence of free divalent cations also affected the structure of the lipid phase in tonoplast membrane as demonstrated by the changes of emission intensity and polarization of incorporated 1,6-diphenyl-1,3,5-hexatriene. The differential inhibition caused by these cations could be interpreted by interactions with the protogenic domain of the membrane as previously proposed in “indirect-link” mechanism. 相似文献
20.
Effects of heavy metals on pollen tube growth and ultrastructure 总被引:3,自引:0,他引:3
Summary The influence of different concentrations of the heavy metals cadmium (Cd2+), cobalt (Co2+), copper (Cu2+), iron (Fe2+ and Fe3+), mercury (Hg2+), manganese (Mn2+), and zinc (Zn2+), plus aluminium (Al3+) (a toxic metal in polluted areas), on pollen germination and tube growth ofLilium longiflorum was investigated using light microscopy. Effects could be observed with 3 M and 100 M of heavy metal, added as chloride salts to the medium. Cd2+, Cu2+, and Hg2+, showed the greatest toxicity, whereas germination and growth rate was less affected by Mn2+. Affected tubes showed swelling of the tip region. Tubes treated with Cd2+, Co2+, Fe2+, Fe3+, Hg2+, and Mn2+ were also prepared for ultrastructural studies. In all cases, the main effect was abnormal cell wall organization, mostly at the tip, where round, fibrillar aggregates, the shape and size of secretory Golgi vesicles were formed. They built up a loose network which could be up to 10 m thick compared to untreated tubes where the cell wall was composed of thin layers of long fibrils and about 100 nm thick. Cd2+ was the only metal which produced effects at the intracellular level: organelle distribution within the tip region appeared disorganized. A general mechanism of heavy metal action on pollen tube growth is discussed. 相似文献