Pb and Cd uptake in rice roots

Pb and Cd are heavy metal pollutants that inhibit plant growth. Using a cultivated rice variety (Dongjin, Oryza sativa L.), we studied how the transport and toxicity of Pb²⁺ and Cd²⁺ are affected by the presence of K⁺, Ca²⁺ or Mg²⁺. K⁺ had a little effect on uptake or toxicity of Pb²⁺ and Cd²⁺. Ca²⁺ or Mg²⁺ blocked both Cd²⁺ transport into rice roots and Cd²⁺ toxicity on root growth, which suggested that their detoxification effect is directly related to their blocking of entry of the heavy metals. Similarly, Ca²⁺ blocked both Pb²⁺ transport into the root and Pb²⁺ toxicity on root growth. The protective effect of Ca²⁺ on Pb²⁺ toxicity may be related to its inhibition of the heavy metal accumulation in the root tip, a potential target site of Pb²⁺ toxicity. Mg²⁺ did not ameliorate the Pb²⁺ toxicity on root growth as much as Ca²⁺ did, although it decreased Pb²⁺ uptake into roots similarly as Ca²⁺ did. These results suggest that the protective effect of Ca²⁺ on Pb²⁺ toxicity may involve multiple mechanisms including competition at the entry level, and that Pb²⁺ and Cd²⁺ may compete with divalent cations for transport into roots of rice plants.     

Effect of metallic cations and pH on reassembly of glial fibrillary acidic protein

Glial fibrillary acidic protein (GFAP), which was purified from acetone powder of the bovine spinal cord, was reassembled in 0.1 M imidazole HCl buffer containing metallic cations, Ca²⁺, Mg²⁺, Na⁺ or K⁺ at physiological or more acidic pH. An electron microscopy revealed reassembled glial filaments at pH 6.8 without any cations but amorphous aggregates at pH 6.3 which were readily observed as a white precipitate by the naked eye. Under more alkaline pH (pH 7.4) only rod-shaped short filaments were formed. In the presence of mM concentrations of Ca²⁺ or Mg²⁺, thick bundles of glial filaments, detectable by light microscopy, were formed at acidic pH. At pH 7.4 long reassembled filaments could be formed in the buffer containing divalent cations. Na⁺ (0.1 M) made filament-like structures of GFAP but they are rather random compared to the filaments promoted by the divalent cations. K⁺ made only amorphous aggregation of the short filaments. These findings indicate that the reassembly of GFAP at physiological pH requires essentially divalent cations but not ionic strength.     

Studies on the exchange of G-actin-bound calcium with bivalent cations

1. The possibility of the replacement of G-actin-bound calcium by various bivalent cations has been investigated. After the reaction with all cations studied, with the exception of Cu²⁺, action remains active, i.e., contains bound ATP and polymerizes in 0.1 M KCl.

2. The amount of G-actin-bound calcium, as well as the sum of bivalent cation after replacement, not removable by short-time Dowex-50 treatment, accounts to about 1 mole per 50000 g of G-actin.

3. The rate of exchange is of the same order for bivalent cations studied, including calcium.

4. G-actin-bound Ca²⁺ is fully replaced, besides free Ca²⁺, by free Mn²⁺ and Cd²⁺. The replacement with Mg²⁺, Co²⁺, Ni²⁺ and Zn²⁺ is not complete, and there is practically no reaction with Ba²⁺ and Sr²⁺.

5. Assuming the affinity constant of Ca²⁺ as 1, the following affinity constants for other bivalent cations were obtained: Mn²⁺, 0.90; Cd²⁺, 1.07; Mg²⁺, 0.27; Zn²⁺, 0.22; Co²⁺, 0.18; Ni²⁺, 0.08.

6. The results obtained show that there exists a close correlation between the ionic radius of a particular bivalent cation, and its ability to replace bound Ca²⁺.     

Divalent cation inhibition of barley root plasma membrane-bound Ca2+-ATPase activity and its reversal by monovalent cations

The inhibitory action of divalent cations on the Ca²⁺-ATPase activity of a plasma membrane-rich microsome fraction isolated from the roots of barley ( Hordeum vulgare L. cv. Conquest) was investigated. Using electron paramagnetic resonance spectroscopy to measure cation-induced changes in membrane lipid properties, it was demonstrated that certain divalent cations (Ca²⁺, Cd²⁺, UO²⁺₂) inhibit the Ca²⁺ ATP-ase by restriction of lipid polar head group mobility and not by alteration of membrane surface potential. Monovalent cations which stimulate the Ca²⁺-ATPase of barley roots (Na⁺, K⁺, ethanolamine HCl) can also reverse the Ca²⁺-ATPase inhibition by Cd²⁺. The degree of Na⁺ reversal of Cd²⁺-induced Ca²⁺-ATPase inhibition was influenced by the nature of the anion.     

Mechanism Underlying the ATP-Induced Increase in the Cytosolic Ca2+ Concentration in Chick Ciliary Ganglion Neurons

Abstract: We examined the mechanism underlying the ATP-induced increase in the cytosolic Ca²⁺ concentration ([Ca]_in) in acutely isolated chick ciliary ganglion neurons, using fura-2 microfluorometry. The ATP-induced increase in [Ca]_in was dependent on external Ca²⁺, was blocked in a dose-dependent manner by reactive blue 2, and was substantially inhibited by both L- and N-type Ca²⁺ channel blockers. ATP was effective in increasing [Ca]_in in the presence of a desensitizing concentration of nicotine (100 µ M ), and simultaneous addition of maximal doses of ATP and nicotine caused an additive increase in [Ca]_in, suggesting that ATP acts on a site distinct from nicotinic acetylcholine receptors. ATP also increased the cytosolic Na⁺ concentration as determined by sodium-binding benzofuran isophthalate microfluorometry. These results suggest that ATP increases Na⁺ influx through P₂ purinoceptor-associated channels resulting in membrane depolarization, which in turn increases Ca²⁺ influx through voltage-dependent Ca²⁺ channels. However, ATP still caused a small increase in [Ca]_in under Na⁺-free conditions, and this [Ca]_in increase was little affected by Ca²⁺ channel blockers. ATP also increased Mn²⁺ influx under Na⁺-free conditions, as indicated by quenching of fura-2 fluorescence. These results suggest that nonselective cationic channels activated by ATP are permeable not only to Ca²⁺ but also to Mn²⁺, in addition to monovalent cations.     

Requirement of Extracellular Calcium Ions for the Early Fertilization Events in the Medaka Egg

The requirement for calcium and the change in calcium content in eggs of Oryzias iatipes during the cortical reaction and sperm penetration were examined. Naked eggs failed to exhibit the cortical reaction upon insemination under Ca Mg-free conditions. These eggs exhibited the cortical reaction by reinsemination in the presence of extracellular Ca²⁺. The effect of extracellular Ca²⁺ on sperm penetration could be replaced by one of several divalent cations in the external medium. Unlike the cortical reaction, sperm penetration failed to be induced by microinjection to increase intracellular Ca²⁺. Verapamil significantly reduced the action of extracellular Ca²⁺ or Ba²⁺ of divalent cations examined in fertilization, while TEA and TTX had no effect on fertilization in the presence of these cations. No ⁴⁵Ca uptake into the egg proper was recognized before completion of the cortical reaction. These observations suggest that extracellular divalent cations are indispensable for sperm stimulation of the egg and its penetration into the egg, for which an influx of Ca²⁺ from the external medium is not required.     

Improvement of the binding capacity of metal cations by sugar-beet pulp. 2. Binding of divalent metal cations by modified sugar-beet pulp

Binding of some divalent cations (Ca²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Pb²⁺ and Zn²⁺) in aqueous solution by saponified and cross-linked sugar-beet pulp was investigated. Saponification doubled the cation-exchange capacity, while cross-linking decreased specific surface area and hydration properties to low and stable values independent of pH and ionic strength conditions. The sorption isotherms indicated a high metal-binding capacity which increased with sorbent concentration, and followed a clear order of selectivity: Cu²⁺˜Pb²⁺ Zn²⁺˜Cd²⁺ > Ni²⁺ > Ca²⁺. The sorption data were better represented by the Langmuir isotherm than by the Freundlich one, suggesting that the monolayer sorption, mainly due to ion-exchange, would not be disturbed by lateral interactions between cations sorbed with similar sorption energies. The same order of selectivity could be drawn from the Langmuir parameters, sorption equilibrium constants (K_L) and maximum binding capacities (Me_Amax). Whatever the cation, K_L decreased with increasing sorbent concentration, while Me_bmax increased. Higher quantities of Cu²⁺ and Pb²⁺ than predicted by the one divalent cation to two carboxyl functions ratio were bound. This was attributed to the partial contribution to the sorption phenomenon of hydroxyl functions close to ionic sites, explaining the higher affinity of such cations for substrates. Cross-linked pulp exhibited higher metalbinding capacity per volume unit than the raw pulp.     

Enhancement of Tryptophan Uptake by Divalent Cations in the Absence of Sodium Ions

Abstract: Nations were found to inhibit the uptake of L-tryptophan into synaptosomes with a shallow dose-response curve. Almost maximal inhibition was obtained with 10 mM-Na⁺. The divalent cations Ca²⁺ and Mg²⁺ were shown to be responsible for the increased uptake of L-tryptophan in the absence of Na⁺ ions. Other divalent cations also promoted tryptophan uptake under this condition (Ca²⁺ < Mg²⁺ < Mn²⁺ < Fe²⁺ < Zn²⁺ < Cu²⁺). It was concluded that monovalent chelate complexes were responsible for this enhancing effect. The measured L-tryptophan uptake was the net product of membrane bound and unbound tryptophan. Both bound and unbound tryptophan were increased in the presence of divalent cations. If no divalent cations were added to the incubation medium, Na⁺ ions decreased the unbound tryptophan but were without effect on bound tryptophan. Under these circumstances D-tryptophan had no effect on binding of the L-isomer and affected the transport of 1.-tryptophan only at very high does (100 x conc. L-tryptophan). These results suggest that I -tryptophan binds to a stereospecific transport carrier located in the synaptosomal membrane and that Na⁺ ions prevent the translocation of this carrier amino acid complex from the outer to the inner site of the neuronal membrane.     

Inhibition of oxidative phosphorylation by Ca or Sr: A competition with Mg for the formation of adenine nucleotide complexes

Intramitochondrial Sr²⁺, similar to Ca²⁺, inhibits oxidative phosphorylation in intact rat-liver mitochondria. Both Ca²⁺ and Sr²⁺ also inhibit the hydrolytic activity of the ATPase in submitochondrial particles. Half-maximal inhibition of ATPase activity was attained at a concentration of 2.5 mM Ca²⁺ or 5.0 mM Sr²⁺ when the concentration of Mg²⁺ in the medium was 1.0 mM. The inhibition of ATPase activity by both cations was strongly decreased by increasing the Mg²⁺ concentration in the reaction medium. In addition, kinetical data and the determination of the concentration of MgATP, the substrate of the ATPase, in the presence of different concentrations of Ca²⁺ or Sr²⁺ strongly indicate that these cations inhibit ATP hydrolysis by competing with Mg²⁺ for the formation of MgATP. On the basis of a good agreement between these results with submitochondrial particles and the results of titrations of oxidative phosphorylation with carboxyatractyloside or oligomycin in mitochondria loaded with Sr²⁺ it can be concluded that intramitochondrial Ca²⁺ or Sr²⁺ inhibits oxidative phosphorylation in intact mitochondria by decreasing the availability of adenine nucleotides to both the ADP/ATP carrier and the ATP synthase.     

Cations Affect [3H]Mazindol and [3H]WIN 35,428 Binding to the Human Dopamine Transporter in a Similar Fashion

Abstract: The present study addresses the possibility that there are different cocaine-related and mazindol-related binding domains on the dopamine transporter (DAT) that show differential sensitivity to cations. The effects of Zn²⁺, Mg²⁺, Hg²⁺, Li⁺, K⁺, and Na⁺ were assessed on the binding of [³H]mazindol and [³H]WIN 35,428 to the human (h) DAT expressed in C6 glioma cells under identical conditions for intact cell and membrane assays. The latter were performed at both 0 and 21°C. Zn²⁺ (30–100 µ M ) stimulated binding of both radioligands to membranes, with a relatively smaller effect for [³H]mazindol; Mg²⁺ (0.1–100 µ M ) had no effect; Hg²⁺ at ∼3 µ M stimulated binding to membranes, with a relatively smaller effect for [³H]mazindol than [³H]WIN 35,428 at 0°C, and at 30–100 µ M inhibited both intact cell and membrane binding; Li⁺ and K⁺ substitution (30–100 m M ) inhibited binding to membranes more severely than to intact cells; and Na⁺ substitution was strongly stimulatory. With only a few exceptions, the patterns of ion effects were remarkably similar for both radioligands at both 0 and 21°C, suggesting the involvement of common binding domains on the hDAT impacted similarly by cations. Therefore, if there are different binding domains for WIN 35,428 and mazindol, these are not affected differentially by the cations studied in the present experiments, except for the stimulatory effect of Zn²⁺ at 0 and 21°C and Hg²⁺ at 0°C.     

Effects of Microinjected Cations on the Early Event of Fertilization in the Medaka Egg

Effects of microinjected cations on the early events of fertilization were examined using eggs of Oryzias latipes . Microinjection of either Ca²⁺, Ba²⁺ or Sr²⁺ into the thin cortical cytoplasm induced breakdown of cortical alveoli (vesicles) (CABD) under Ca-Mg-free conditions, but microinjection of Mg²⁺, Mn²⁺ or Co²⁺ prevented CABD at the injected region when the eggs were inseminated in regular saline. Under Ca-Mg-free conditions, CABD could also be induced by microinjection of various solutions (NaCl, choline chloride, sucrose, pH buffer) without any divalent cations or ionophore A23187. Ca²⁺ microinjected into the cortical cytoplasm did not play a role in sperm penetration. Upon microinjection with either Ca²⁺, Mg²⁺ or K⁺, the resting membrane potential leakage was transiently observed. However, depolarization of the membrane followed by slow hyperpolarization was observed only upon microinjection of Ca²⁺. From these experiments, it was inferred that microinjected divalent cations such as Ca²⁺, Ba²⁺ or Sr²⁺ do not act directly upon the cortical alveolus membrane, but trigger the induction of CABD via depolarization of the membrne and increase in intracellular Ca²⁺.     

Enhancement by tetraphenylboron of inhibition of mitochondrial respiration induced by 1-methyl-4-phenylpyridinium ion (MPP)

The effect of tetraphenylboron (TPB⁻), an activator of a membrane transport of lipophilic cations, on the inhibition of mouse liver mitochondrial respiration induced by a neurotoxin, 1-methyl-4-phenylpyridinium ion (MPP⁺), and by some structurally related compounds was studied. Of the compounds tested, MPP⁺ and 4-phenylpyridine (4-PP) significantly inhibited the respiration in an ADP-activated oxidation of substrates (state 3). TPB⁻, dose-dependently, shortened the lag time of MPP⁺-induced inhibition and thus lowered the concentrations of MPP⁺ for the inhibition. However, TPB⁻, even at the high concentration (10 μM), did not significantly affect 4-PP-induced inhibition. Carbonyl-cyanide-m-chlorophenylhydrazone (CCCP) blocked the respiratory inhibition by MPP⁺, independent of K⁺ concentration in the medium, and valinomycin blocked the inhibition only in the medium containing high K⁺ concentration. Determination of the intramitochondrial MPP⁺ concentration revealed about 1000-fold concentrated MPP⁺ from that in the medium during the incubation with TPB⁻, indicative of potentiation of MPP⁺ transport into mitochondria by TPB⁻. This might account for the enhancement of respiratory inhibition by MPP⁺. In the case of 4-PP, it will penetrate the mitochondrial membrane and intrinsically inhibit the respiration, but cannot accumulate in mitochondria. The present results indicate that, although the inhibitory potency of MPP⁺ per se is similar to 4-PP, MPP⁺ will be highly concentrated within mitochondria by the membrane potential, as the drive force for its transport.     

Activation of Sea Urchin Eggs by Halothane and Its Inhibition by Dantrolene

Eggs of the sea urchin, Hemicentrotus pulcherrimus , were stimulated by halothane, known to induce Ca²⁺ release from sarcosome, to cause fertilization membrane formation in normal and Ca²⁺ free artificial sea water. In the absence of external Ca²⁺, halothane-induced formation of fertilization membrane was inhibited by dantrolene, an inhibitor of Ca²⁺ release from sarcosome, but was not blocked by nifedipine, a Ca²⁺ antagonist specific to Ca²⁺ channels in plasma membrane. Ca²⁺ release from sedimentable fraction isolated from eggs was induced by halothane and was inhibited by dantrolene, but was not blocked by nifedipine. In normal artificial sea water, halothane-caused egg activation was not inhibited either by dantrolene or by nifedipine, but was blocked in the presence of both compounds. ⁴⁵Ca²⁺ influx was substantially stimulated by halothane in eggs exposed to ⁴⁵CaCl₂. Halothane-induced ⁴⁵Ca²⁺ influx into eggs was inhibited by nifedipine but was not blocked by dantrolene. When Ca²⁺ release from intracellular organellae is blocked, Ca²⁺ transport through Ca²⁺ channels in plasma membrane probably acts as a "fail-safe" system to induce an increase in cytosolic Ca²⁺ level, resulting in egg activation.     

Rapid Communication: The Role of P-Type Calcium Channels in the Depolarization-Induced Activation of Nitric Oxide Synthase in Frontal Cortex

Abstract: In this study we demonstrate that 50 mRS K⁺ stimulates the conversion of L-[³H] arginine to L-[³H] citrulline and that this effect is blocked by 10 μ M AT-nitro- l -arginine, a nitric oxide synthase inhibitor, and Ca²⁺-free conditions. Amiloride (1 m M ) and low Na⁺ conditions were used to test the possible involvement of the Na⁺-Ca²⁺ exchanger. These treatments were without effect. The calcium channel blockers 10 mRS Mg²⁺, 100 μ M Cd²⁺, and 10 mRS Co²⁺ also blocked the K⁺ response, suggesting the involvement of voltage-dependent calcium channels (VDCCs). The specific VDCC involved seems to be the P type, as funnel-web spider toxin blocked the response whereas 200 μ M Ni²⁺, 10 μ M nifedipine, and 100 n M ω-conotoxin did not.     

Opiates Inhibit Acetylcholine Release from Torpedo Nerve Terminals by Blocking Ca2+ Influx

Abstract: In the present communication we report that Ca²⁺-dependent acetylcholine release from K⁺-depolarized Torpedo electric organ synaptosomes is inhibited by morphine, and that this effect is blocked by the opiate antagonist naloxone. This finding suggests that the purely cholinergic Torpedo electric organ neurons contain pre-synaptic opiate receptors whose activation inhibits acetylcholine release. The mechanisms underlying this opiate inhibition were investigated by comparing the effects of morphine on acetylcholine release induced by K⁺ depolarization and by the Ca²⁺ ionophore A23187 and by examining the effect of morphine on ⁴⁵Ca²⁺ influx into Torpedo nerve terminals. These experiments revealed that morphine inhibits ⁴⁵Ca²⁺ influx into K⁺-depolarized Torpedo synaptosomes and that this effect is blocked by naloxone. The effects of morphine on K⁺ depolarization-mediated ⁴⁵Ca²⁺ influx and on acetylcholine release have similar dose dependencies (half-maximal inhibition at 0.5–1 μ M ), suggesting that opiate inhibition of release is due to blockage of the presynaptic voltage-dependent Ca²⁺ channel. This conclusion is supported by the finding that morphine does not inhibit acetylcholine release when the Ca²⁺ channel is bypassed by introducing Ca²⁺ into the Torpedo nerve terminals via the Ca²⁺ ionophore.     

Uptake of metals by bacterial polysaccharides

J.L. GEDDIE AND I.W. SUTHERLAND. 1993. The binding of cations by a range of bacterial polysaccharides was examined. Comparison of native and deacetylated polymers indicated the influence of polysaccharide acetylation on ion uptake and selectivity. The effects of temperature and pH on ion uptake were also examined. Metal ion uptake was carried out by dialysis and samples were analysed using ion chromatography. The native acetylated polymers showed a selectivity for Ca²⁺ > Mg²⁺ > monovalent cations, whereas samples lacking acetyl groups showed a selectivity for monovalent cations > Mg²⁺ > Ca²⁺. Increased temperatures reduced the capacity for several of the polymers to bind the cations; The Zoogloea ramigera polymer appeared least affected. The pH value also affected uptake.     

Vasopressin elevates cytosolic calcium in small cell lung cancer cells

The ability of vasopressin to elevate cytosolic Ca²⁺ in small cell lung cancer (SCLC) cells was investigated. Ten nanomolar vasopressin elevated the cytosolic Ca²⁺ in 6 of 8 SCLC cell lines that were loaded with Fura-2 AM. Using SCLC cell line NCI-H345, the effect of vasopressin was dose dependent, being maximal at 100 nM, where the cytosolic Ca²⁺ was elevated from 150 to 210 nM. Because addition of 1 mM EGTA had no effect on the vasopressin response, vasopressin released Ca²⁺ from intracellular pools. Also, oxytocin weakly elevated the cytosolic Ca²⁺. The response to vasopressin was strongly blocked by [(β-mercapto-β,β-cyclopentamethylene propionic acid)¹,O-MeTyr²,Arg⁸]vasopressin and weakly blocked by [(β-mercapto-β,β-cyclopentamethylene propionic acid)¹,O-MeTyr²,Orn⁸]vasotocin. These data suggest that V₁ vasopressin receptors are present on SCLC cells.     

Cation fluxes in the saps of Sinapis alba during the floral transition

Plants of Sinapis alba L. were induced to flower by either a single long day or a single displaced short day. The levels of three cations. Ca²⁺, Mg²⁺ and K⁺, were measured by atomic absorption spectrophotometry in exudates from roots, leaves and apical stem tips. The export of all three cations out of the root system (root exudate) was increased in induced as compared to non-induced plants. No changes were observed in cation export out of the mature leaves (leaf exudate). The supply of cations to the apical bud (apical exudate) did not originate from the phloem and, so, should mainly be of apoplastic origin. Only the supply of Ca²⁺ to the apical bud was increased, not the supply of Mg²⁺ or K⁺. The increase in Ca²⁺ supply was transient and occurred at about the same time as a conspicuous stimulation of cell division, previously detected in the apical bud.     

Characterization of mitochondrial glutamate dehydrogenase from dark‐grown soybean seedlings

Purified preparations of NAD(H)‐glutamate dehydrogenase (GDH, EC 1.4.1.2.) were assayed to determine the effects of mono‐ and divalent cations, nucleotides and select carbon compounds on NAD(H)‐dependent GDH activity. The amination reaction was stimulated 2‐ to 17‐fold by divalent cations (Ca²⁺ > Cd²⁺ > Co²⁺ > Mg²⁺ > Mn²⁺ > Zn²⁺ between 1 and 1000 µ M ), but the reaction was unaffected by monovalent cations (Na ⁺ and K ⁺). The amination reaction was most responsive to changes in Ca²⁺ in a NADH‐dependent manner. The addition of EDTA or EGTA nullified the stimulatory effects of Ca²⁺. Calmodulin alone or in combination with calmodulin antagonists did not affect the amination reaction. Divalent cations (at 1 m M ) inhibited the rate of the deamination reaction by 15 to 25%, while monovalent cations had no effect. ATP inhibited the amination reaction by 10 to 60%, while ADP had little or no effect. ATP or ADP decreased the rate of the deamination reaction 23 to 60 or 20 to 38%, respectively. Many tricarboxylic acid cycle intermediates inhibited the amination reaction, 20 to 50% of the inhibition could be attributed to the chelating capacity of intermediates. Conversely, most of the carbon sources tested did not affect the deamination reaction, the only appreciable differences were increases in activity with sucrose (21%) and glucose (41%) and a decrease in activity with pyruvate (34%). Inhibitors of sulfhydryl groups were used to examine the importance of reduced thiol groups in the amination or deamination reactions. The amination was not dependent on reduced thiol groups, whereas the deamination reaction was dependent on reduced thiol groups.     

DIVALENT CATION-ACTIVATED ECTO-NUCLEOSIDE TRIPHOSPHATASE ACTIVITY OF NERVOUS SYSTEM CELLS IN TISSUE CULTURE

Abstract— Intact neuroblastoma and glial cells in monolayer culture hydrolysed ATP added to their medium. Evidence is presented that ATP is cleaved outside of the permeability barrier of the plasma membrane and the product is liberated in the extracellular medium, i.e. the enzyme is an ecto-enzyme. Divalent cations such as Mg²⁺, Ca²⁺, Mn²⁺ and Co²⁺ activate the enzyme. In neuroblastoma cells, Ca²⁺ is the preferential cation for activation; Mg²⁺ in glial cells. Substrate specificity was very low when different nucleoside-5'-triphosphates were examined. Competition studies have revealed that all of the nucleoside triphosphates are hydrolysed by the same enzyme: divalent cation-activated ecto-nucleoside-5'-triphosphate phosphohydrolase.
Developmental pattern of the enzyme in several lines was established. The role of enzyme in the transport of divalent cations across the plasma membrane and/or in the physical properties of the membrane is suggested.