Activation of Na+-K+-adenosine triphosphatase by spermine.

Spermine activated Na⁺-K⁺-ATPase when the concentrations of K⁺ and ATP were low, whereas it inhibited K⁺-dependent and ouabain-inhibitable monophosphatase. The activating effect of sperimine was not due to the substitution for K⁺ or Na⁺. Excess K⁺ inhibited Na⁺-K⁺-ATPase partially, and reduced the spermine activation. When 1 mM ATP was used, spermine at higher concentrations inhibited Na⁺-K⁺-ATPase, and did not activate at all. It is suggested that the K⁺-sites essential to Na⁺-K⁺-ATPase and the K⁺-phosphatase co-exist at different places of the enzyme.     

Irreversible zinc ion inhibition of (Na+ -K+)-adenosinetriphosphatase, Na+ -phosphorylation, and K+-p-nitrophenylphosphatase of Electrophorus electricus electroplax.

Zinc ion in micromolar concentrations is an irreversible inhibitor of Electrophorus electricus electroplax microsomal (Na⁺-K⁺)-ATPase. The rate of inhibition is dependent on [ZnCl₂] and the extent of inhibition varies with the ratio of ZnCl₂ to microsomal protein. The same kinetics are observed for inhibition of K⁺ -p-nitrophenylphosphatase and steady-state levels of Na⁺ -dependent enzyme phosphorylation. The observations suggest that a Zn²⁺ -sensitive conformational restraint is important to both kinase and phosphatase activities. The fact that inhibition is irreversible has implications for models seeking to relate zinc effects in tissue to inhibition of (Na⁺-K⁺)-ATPase.     

Effect of Hypoxia on Na+-K+-Cl− Cotransport in Cultured Brain Capillary Endothelial Cells of the Rat

Abstract: The effect of hypoxia on Na⁺,K⁺-ATPase and Na⁺-K⁺-Cl^? cotransport activity in cultured rat brain capillary endothelial cells (RBECs) was investigated by measuring ⁸⁶Rb⁺ uptake as a tracer for K⁺. RBECs expressed both Na⁺,K⁺-ATPase and Na⁺-K⁺-Cl^? cotransport activity (4.6 and 5.5 nmol/mg of protein/min, respectively). Hypoxia (24 h) decreased cellular ATP content by 43.5% and reduced Na⁺,K⁺-ATPase activity by 38.9%, whereas it significantly increased Na⁺-K⁺-Cl^? cotransport activity by 49.1% in RBECs. To clarify further the mechanism responsible for these observations, the effect of oligomycin-induced ATP depletion on these ion transport systems was examined. Exposure of RBECs to oligomycin led to a time-dependent decrease of cellular ATP content (by ～65%) along with a complete inhibition of Na⁺,K⁺-ATPase and a coordinated increase of Na⁺-K⁺-Cl^? cotransport activity (up to 100% above control values). Oligomycin augmentation of Na⁺-K⁺-Cl^? cotransport activity was not observed in the presence of 2-deoxy-d -glucose (a competitive inhibitor of glucose transport and glycolysis) or in the absence of glucose. These results strongly suggest that under hypoxic conditions when Na⁺,K⁺-ATPase activity is reduced, RBECs have the ability to increase K⁺ uptake through Na⁺-K⁺-Cl^? cotransport.     

Carotenoid oxidative degradation products inhibit Na+-K+-ATPase

This study investigates the biological significance of carotenoid oxidation products using inhibition of Na⁺-K⁺-ATPase activity as an index. β-Carotene was completely oxidized by hypochlorous acid and the oxidation products were analyzed by capillary gasliquid chromatography and high performance liquid chromatography. The Na⁺-K⁺-ATPase activity was assayed in the presence of these oxidized carotenoids and was rapidly and potently inhibited. This was demonstrated for a mixture of β-carotene oxidative breakdown products, β-Apo-10′-carotenal and retinal. Most of the β-carotene oxidation products were identified as aldehydic. The concentration of the oxidized carotenoid mixture that inhibited Na⁺-K⁺-ATPase activity by 50% (IC₅₀) was equivalent to 10μM non-degraded β-carotene, whereas the IC₅₀ for 4-hydroxy-2-nonenal, a major lipid peroxidation product, was 120 μM. Carotenoid oxidation products are more potent inhibitors of Na⁺-K⁺-ATPase than 4-hydroxy-2-nonenal. Enzyme activity was only partially restored with hydroxylamine and/or β-mercaptoethanol. Thus, in vitro binding of carotenoid oxidation products results in strong enzyme inhibition. These data indicate the potential toxicity of oxidative carotenoid metabolites and their activity on key enzyme regulators and signal modulators.     

Studies on the interaction of chick brain microsomal (Na+ + K+)-ATPase with copper

Chick brain microsomal ATPase was strongly inhibited by Cu²⁺. (Na⁺ + K⁺)-ATPase was more susceptible to low levels of Cu²⁺ than Mg²⁺-ATPase. The inhibition of (Na⁺ + K⁺)-ATPase could be partially protected from Cu²⁺ in the presence of ATP in the preincubation period. When Cu²⁺ (6 μM) was preincubated with the enzyme in the absence of ATP, only sulfhydryl-containing amino acids (d-penicillamine and l-cysteine) could reverse the inhibition. At lower concentrations of Cu²⁺ (< 1.4 μM), in the absence of ATP during preincubation, the inhibition could be completely reversed by the addition of 5 mM l-phenylalanine and l-histidine as well as d-penicillamine and l-cysteine.Kinetic analysis of action of Cu²⁺ (1.0 μM) on (Na⁺ + K⁺)-ATPase revealed that the inhibition was uncompetitive with respect to ATP. At a low concentration of K⁺ (5 mM), V with Na⁺ was markedly decreased in the presence of Cu²⁺ and K_m was about twice that of the control. However, at high K⁺ concentration (20 mM), the K_m for Na⁺ was not affected. At both low (25 mM) and high (100 mM) Na⁺, Cu²⁺ displayed non-competitive inhibition of the enzyme with respect to K⁺.On the basis of these data, we suggest that Cu²⁺ at higher concentrations (> 6 μM) inactivates the enzyme irreversibly, but that at lower concentrations (< 1.4 μM), Cu²⁺ interacts reversibly with the enzyme.     

Caffeine prevents high-intensity exercise-induced increase in enzymatic antioxidant and Na+-K+-ATPase activities and reduction of anxiolytic like-behaviour in rats

Objective: Here we investigated the impact of chronic high-intensity interval training (HIIT) and caffeine consumption on the activities of Na⁺-K⁺-ATPase and enzymes of the antioxidant system, as well as anxiolytic-like behaviour in the rat brain.

Methods: Animals were divided into groups: control, caffeine (4?mg/kg), caffeine (8?mg/kg), HIIT, HIIT plus caffeine (4?mg/kg) and HIIT plus caffeine (8?mg/kg). Rats were trained three times per week for 6 weeks, and caffeine was administered 30 minutes before training. We assessed the anxiolytic-like behaviour, Na⁺-K⁺-ATPase, superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) activities, levels of reduced glutathione (GSH) and thiobarbituric acid reactive substances (TBARS) in the brain.

Results and discussion: HIIT-induced anxiolytic-like behaviour increased Na⁺-K⁺-ATPase and GPx activities and TBARS levels, altered the activities of SOD and CAT in different brain regions, and decreased GSH levels. Caffeine, however, elicited anxiogenic-like behaviour and blocked HIIT effects. The combination of caffeine and HIIT prevented the increase in SOD activity in the cerebral cortex and GPx activity in three brain regions. Our results show that caffeine promoted anxiogenic behaviour and prevented HIIT-induced changes in the antioxidant system and Na⁺-K⁺-ATPase activities.     

Rat hypothalamic extract inhibits vasopressin-stimulated Na+-K+-ATPase activity in the rat renal medulla

Arginine vasopressin stimulates Na⁺-K⁺-ATPase activity located in the rat thick ascending limb of s'Henle loop. Mammalian hypothalamus appears to produce a factor capable of inhibiting Na⁺-K⁺-ATPase activity in a variety of tissues. The effect of a purified rat hypothalamic extract with and without AVP on rat renal Na⁺-K⁺-ATPase activity was evaluated by a cytochemical technique. The hypothalamic extract alone failed to affect basal Na⁺-K⁺-ATPase activity throughout renal segments after 10 min exposure. Na⁺-K⁺-ATPase activity stimulated by AVP (1–10 fmol l^?1) for 10 min was inhibited by rat hypothalamic extract over the concentration range 10^?7–10^?3 U ml^?1 in a dose-dependent manner. Complete inhibition of AVP-stimulated Na⁺-K⁺-ATPase activity occurred at a hypothalamic extract concentration of 10^?3 U ml^?1. Only Na⁺-K⁺-ATPase activity located in the renal medullary thick ascending limb was influenced by the rat hypothalamic extract.     

Development of (Na+ -K+)-ATPase in rat hindbrain: increments in parallel with Na+-dependent phosphorylation and K+-pnitrophenylphosphatase.

Rat hindbrain NaI-enriched microsomal (Na⁺-K⁺)-ATPase activity, K⁺-pNPPase activity, and Na⁺-dependent steady-state phosphorylation levels all increase approx 10-fold relative to microsomal protein between 5 days prenatally and 60 days postnatally. These activities, as well as the mean wet weight of the hindbrain, are at half of their 60 day values shortly after the 10th postnatal day. For all ages, these hindbrain activities average over twice those found in the forebrain in a companion paper (Bertoni & Siegel , 1978). Increases during development in the amount of the related phosphorylatable polypeptide, estimated by densitometry of stained polyacrylamide gels containing fixed amounts of microsomal protein dissolved in SDS, are in agreement with increases in steady state levels of Na⁺-dependent phosphorylation. The fraction of total phosphorylation that is Na⁺-dependent rises steadily during development consistent with, but not obligatorily due to, a conversion of some of the previously Na⁺-independent portion. Mg2⁺-ATPase and Mg²⁺ -pNPPase activities and steady-state Na⁺,-independent phosphorylation levels do not increase in parallel during development. These observations add further support to the proposed partial reaction scheme for (Na⁺- K⁺)-ATPase. The major increments in (Na ⁺-K⁺)-ATPase occur simultaneously with the deposition of specialized plasma membranes, particularly in the molecular layer of the cerebellum, as described in previous studies of rat hind brain.     

An endogenous factor which interacts with synaptosomal membrane Na+, K+-ATPase activation by K+

In previous papers, the isolation of brain soluble fractions able to modify neuronal Na⁺, K⁺-ATPase activity has been described. One of those fractions-peak I-stimulates membrane Na⁺, K⁺-ATPase while another-peak II-inhibits this enzyme activity, and has other ouabain-like properties. In the present study, synaptosomal membrane Na⁺, K⁺-ATPase was analyzed under several experimental conditions, using ATP orp-nitrophenylphosphate (p-NPP) as substrate, in the absence and presence of cerebral cortex peak II. Peak II inhibited K⁺-p-NPPase activity in a concentration dependent manner. Double reciprocal plots indicated that peak II uncompetitively inhibits K⁺-p-NPPase activity regarding substrate, Mg²⁺ and K⁺ concentration. Peak II failed to block the known K⁺-p-NPPase stimulation caused by ATP plus Na⁺. At various K⁺ concentrations, percentage K⁺-p-NPPase inhibition by peak II was similar regardless of the ATP plus Na⁺ presence, indicating lack of correlation with enzyme phosphorylation. Na⁺, K⁺-ATPase activity was decreased by peak II depending on K⁺ concentration. It is postulated that the inhibitory factor(s) present in peak II interfere(s) with enzyme activation by K⁺.     

Changes in Na+-K+-ATPase activity influence cell attachment to fibronectin

Most vital cellular functions aredependent on a fine-tuned regulation of intracellular ion homeostasis.Here we have demonstrated, using COS cells that were untransfected ortransfected with wild-type rat ouabain-resistantNa⁺-K⁺-ATPase, that partial inhibition ofNa⁺-K⁺-ATPase has a dramatic influence oncell attachment to fibronectin. Ouabain dose-dependently decreasedattachment in untransfected cells and in cells expressing wild-typeNa⁺-K⁺-ATPase, but not in cells expressingouabain-insensitive Na⁺-K⁺-ATPase, whereasinhibition of Na⁺-K⁺-ATPase by loweringextracellular K⁺ concentration decreased attachment in allthree cell types. Thirty percent inhibition ofNa⁺-K⁺-ATPase significantly attenuatedattachment. Na⁺-K⁺-ATPase inhibition caused asustained increase in the intracellular Ca²⁺ concentrationthat obscured Ca²⁺ transients observed in untreated cellsduring attachment. Inhibitors of Ca²⁺ transporterssignificantly decreased attachment, but inhibition ofNa⁺/H⁺ exchanger did not. Ouabain reduced focaladhesion kinase autophosphorylation but had no effect on cell surfaceintegrin expression. These results suggest that the level ofNa⁺-K⁺-ATPase activity strongly influences cellattachment, possibly by an effect on intracellular Ca²⁺.

     

Studies on (Na+-K+)-activated ATPase XXXIV. Phosphatidylserine not essential for (Na+-K+)-ATPase activity

An (Na⁺-K⁺)-ATPase preparation, consisting of NaI-treated microsomes from cattle brain, was incubated with a phosphatidylserine decarboxylase preparation from Escherichia coli. This led to a reduction in the phosphatidylserine content from 10.1 % to less than 0.1%, accompanied by an equimolar formation of phosphatidylethanolamine. Since the (Na⁺-K⁺)-ATPase activity was not reduced, it can be concluded that phosphatidylserine is not essential for the Na⁺-K⁺)-ATPase activity.     

Endothelin 1 Stimulates Na+,K+-ATPase and Na+-K+-Cl− Cotransport Through ETA Receptors and Protein Kinase C-Dependent Pathway in Cerebral Capillary Endothelium

Abstract: The effect of endothelins (ET-1 and ET-3) on ⁸⁶Rb⁺ uptake as a measure of K⁺ uptake was investigated in cultured rat brain capillary endothelium. ET-1 or ET-3 dose-dependently enhanced K⁺ uptake (EC₅₀ = 0.60 ± 0.15 and 21.5 ± 4.1 nM, respectively), which was inhibited by the selective ET_A receptor antagonist BQ 123 (cyclo-d -Trp-d -Asp-Pro-d -Val-Leu). Neither the selective ET_B agonists IRL 1620 [N-succinyl-(Glu⁹,-Ala^11,15)-ET-1] and sarafotoxin S6c, nor the ET_B receptor antagonist IRL 1038 [(Cys¹¹,Cys¹⁵)-ET-1] had any effect on K⁺ uptake. Ouabain (inhibitor of Na⁺,K⁺-ATPase) and bumetanide (inhibitor of Na⁺-K⁺-Cl^? cotransport) reduced (up to 40% and up to 70%, respectively) the ET-1-stimulated K⁺ uptake. Complete inhibition was seen with both agents. Phorbol 12-myristate 13-acetate (PMA), activator of protein kinase C (PKC), stimulated Na⁺,K⁺-ATPase and Na⁺-K⁺-Cl^? cotransport. ET-1-but not PMA-stimulated K⁺ uptake was inhibited by 5-(N-ethyl-N-isopropyl)amiloride (inhibitor of Na⁺/H⁺ exchange system), suggesting a linkage of Na⁺/H⁺ exchange with ET-1-stimulated Na⁺,K⁺-ATPase and Na⁺-K⁺-Cl^? cotransport activity that is not mediated by PKC.     

Actein inhibits the Na+-K+-ATPase and enhances the growth inhibitory effect of digitoxin on human breast cancer cells

The Na⁺-K⁺-ATPase is a known target of cardiac glycosides such as digitoxin and ouabain. We determined that the enzyme also is a target of the structurally-related triterpene glycoside actein, present in the herb black cohosh. Actein’s inhibition of Na⁺-K⁺-ATPase activity was less potent than that of digitoxin, but actein potentiated digitoxin’s inhibitory effect on Na⁺-K⁺-ATPase activity and MDA-MB-453 breast cancer cell growth. We observed different degrees of signal amplification for the two compounds. Actein’s inhibitory effect on ATPase activity was amplified 2-fold for cell growth inhibition, whereas digitoxin’s signal was amplified 20-fold. Actein induced a biphasic response in proteins downstream of ATPase: low dose and short duration of treatment upregulated NF-κB promoter activity, p-ERK, p-Akt and cyclin D1 protein levels, whereas higher doses and longer exposure inhibited these activities. Actein and digitoxin may be a useful synergistic combination for cancer chemoprevention and/or therapy.     

Sulfatide and Na<Superscript>+</Superscript>-K<Superscript>+</Superscript>-ATPase: A Salinity-sensitive Relationship in the Gill Basolateral Membrane of Rainbow Trout

We investigated the effect of salinity on the relationship between Na⁺-K⁺-ATPase and sulfogalactosyl ceramide (SGC) in the basolateral membrane of rainbow trout (Oncorhynchus mykiss) gill epithelium. SGC has been implicated as a cofactor in Na⁺-K⁺-ATPase activity, especially in Na⁺-K⁺-ATPase rich tissues. However, whole-tissue studies have questioned this role in the fish gill. We re-examined SGC cofactor function from a gill basolateral membrane perspective. Nine SGC fatty acid species were quantified by tandem mass spectrometry (MS/MS) and related to Na⁺-K⁺-ATPase activity in trout acclimated to freshwater or brackish water (20 ppt). While Na⁺-K⁺-ATPase activity increased, the total concentration and relative proportion of SGC isoforms remained constant between salinities. However, we noted a negative correlation between SGC concentration and Na⁺-K⁺-ATPase activity in fish exposed to brackish water, whereas no correlation existed in fish acclimated to freshwater. Differential Na⁺-K⁺-ATPase/SGC sensitivity is discussed in relation to enzyme isoform switching, the SGC cofactor site model and saltwater adaptation.This revised version was published online in June 2005 with a corrected cover date.     

cAMP and sodium transport in the freshwater crayfish, Cherax destructor

Crayfish in which sodium absorption was maximally stimulated had elevated levels of both cAMP and Na⁺-K⁺-ATPase activity in gill tissue. The concentration of cAMP and activity of Na⁺-K⁺-ATPase in gill tissue were monitored following transfer of crayfish from water containing 125 mmol.l⁻¹ Na to Na-free media. Both parameters were significantly elevated within 10 min of transfer to Na-free media and [cAMP] peaked between 1 and 2 h before falling transiently to the control level at 3 h. A second peak of [cAMP] and a further rise in Na⁺-K⁺-ATPase activity were evident 6 h after transfer and elevated levels were then maintained. The pattern observed was consistent with the existence of two separate mechanisms for the control of sodium absorption both of which stimulated the activity of Na⁺-K⁺-ATPase via elevation of the intracellular concentration of cAMP. The initial response was very rapid (<10 min) but of brief duration (1–2 h) and this mechanism appeared to be sensitive to changes in external ion levels. The second mechanism exhibited a much longer response time (3–6 h) and duration and was likely to be sensitive to changes in internal ion concentrations.     

Characteristics of antibody inhibition of rat kidney (Na+−K+)-ATPase

Summary Antibodies which were raised against highly purified membrane-bound (Na⁺–K⁺)-ATPase from the outer medulla of rat kidneys inhibit the (Na⁺–K⁺)-ATPase activity up to 95%. The antibody inhibition is reversible. The time course of enzyme inhibition and reactivation is biphasic in semilogarithmic plots.In the purified membrane-bound (Na⁺–K⁺)-ATPase negative cooperativity was observed (a) for the ATP dependence of the (Na⁺–K⁺)-ATPase activity (n=0.86), (b) for the ATP binding to the enzyme (n=0.58), and (c) for the ouabain inhibition of the (Na⁺–K⁺)-ATPase activity (n=0.77). By measuring the Na⁺ dependence of the (Na⁺–K⁺-ATPase reaction, a positive homotropic cooperativity (n=1.67) was found.As reactivation of the antibody-inhibited enzyme proceeds very slowly (t _0.5=5.2hr), it was possible to measure characteristics of the antibody-(Na⁺–K⁺)-ATPase complex: The antibodies exerted similar effects on the ATP dependence of the (Na⁺–K⁺)-ATPase reaction and on the ATP binding of the enzyme.V _max of the (Na⁺–K⁺)-ATPase reaction and the number of ATP binding sites were reduced whileK _{0.5 ATP} for the (Na⁺–K⁺)-ATPase activity and for the ATP binding were increased by the antibodies. The Hill coefficients for the ATP binding and for the ATP dependence of the enzyme activity were not significantly altered by the antibodies. The antibodies increased theK _0.5 value for the Na⁺ stimulation of the (Na⁺–K⁺)-ATPase activity, but they did not alter the homotropic interactions between the Na⁺-binding sites. The negative cooperativity which was observed for the ouabain inhibition of the (Na⁺–K⁺)-ATPase activity was abolished by the antibodies.The data are tentatively explained by the following model: The antibodies bind to the (Na⁺–K⁺)-ATPase from the inner membrane side, reduce the ATP binding symmetrically at the ATP binding sites and reduce thereby also the (Na⁺–K⁺)-ATPase activity of the enzyme. The antibodies may inhibit the ATP binding by a direct interaction or by means of a conformational change at the ATP binding sites. This may possibly also lead to the alteration of the Na⁺ dependence of the (Na⁺–K⁺)-ATPase activity and to the observed alteration of the dose response to the ouabain inhibition.     

Modulation of embryonic NA+-K+-ATPase activity and mouse preimplantation development in vitro in media containing high concentrations of potassium

The effect of various potassium concentrations (ranging from 1.4 mM to 30 mM K⁺) in modified Tyrode's medium on the culture of mouse zygotes obtained after in vitro fertilization to the blastocyst stage was examined. A clear dose-dependent negative effect of increasing K⁺ concentrations on the preimplantation embryonic development in vitro was found. We have previously shown that significantly more two-cell embryos reach the blastocyst stage when cultured during the second day postinsemination in medium supplemented with taurine. Because taurine, an amino acid that abounds in the reproductive tract, has been reported to inhibit the enzyme Na⁺-K⁺-adenosine triphosphatase (Na⁺-K⁺-AT-Pase), we used two other conditions known to inhibit the Na⁺-K⁺-ATPase to study their effect on mouse embryo development. Culturing embryos during a short period (the second day postinsemination) in low extracellular K⁺ concentrations (1.4 mM) or in medium supplemented with ouabain (50 μM) showed positive effects similar to those of culturing in medium with taurine (10 mM). This beneficial effect of ouabain was found in various K⁺ concentrations tested, including the high concentrations present in the oviduct. Although the effects of low K⁺ and taurine can possibly be ascribed to their other cellular effects, the effect of ouabain shows that inhibition of the Na⁺-K⁺-ATPase during the two-cell stage in the mouse is beneficial for further embryonic development to the blastocyst stage. © 1993 Wiley-Liss, Inc.     

Moderate-to-severe ischemic conditions increase activity and phosphorylation of the cerebral microvascular endothelial cell Na+-K+-Cl- cotransporter

Brain edema that forms during the early stages of stroke involves increased transport of Na⁺ and Cl^– across an intact blood-brain barrier (BBB). Our previous studies have shown that a luminal BBB Na⁺-K⁺-Cl^– cotransporter is stimulated by conditions present during ischemia and that inhibition of the cotransporter by intravenous bumetanide greatly reduces edema formation in the rat middle cerebral artery occlusion model of stroke. The present study focused on investigating the effects of hypoxia, which develops rapidly in the brain during ischemia, on the activity and expression of the BBB Na⁺-K⁺-Cl^– cotransporter, as well as on Na⁺-K⁺-ATPase activity, cell ATP content, and intracellular volume. Cerebral microvascular endothelial cells (CMECs) were assessed for Na⁺-K⁺-Cl^– cotransporter and Na⁺-K⁺-ATPase activities as bumetanide-sensitive and ouabain-sensitive ⁸⁶Rb influxes, respectively. ATP content was assessed by luciferase assay and intracellular volume by [³H]-3-O-methyl-D-glucose and [¹⁴C]-sucrose equilibration. We found that 30-min exposure of CMECs to hypoxia ranging from 7.5% to 0.5% O₂ (vs. 19% normoxic O₂) significantly increased cotransporter activity as did 7.5% or 2% O₂ for up to 2 h. This was not associated with reduction in Na⁺-K⁺-ATPase activity or ATP content. CMEC intracellular volume increased only after 4 to 5 h of hypoxia. Furthermore, glucose and pyruvate deprivation increased cotransporter activity under both normoxic and hypoxic conditions. Finally, we found that hypoxia increased phosphorylation but not abundance of the cotransporter protein. These findings support the hypothesis that hypoxia stimulation of the BBB Na⁺-K⁺-Cl^– cotransporter contributes to ischemia-induced brain edema formation. edema; blood-brain barrier; bumetanide; cell volume     

In vitro binding of inorganic mercury to the plasma membrane of rat platelet affects Na+-K+-ATPase activity and platelet aggregation

Hg²⁺ binding to ouabain-sensitive Na⁺-K⁺-ATPase of rat platelet membrane was specific with a K_a of 1.3×10⁹ moles and B_max of 3.8 nmoles/mg protein. The binding of mercury to Na⁺-K⁺-ATPase also inhibits the enzyme significantly (P<0.001), which is greater than its ouabain sensitivity. Further in the cytosol of washed platelets conjugation of reduced glutathione (GSH) to Hg²⁺ is correlated dose dependently (25, 50 and 100 pmoles) to enhanced GSH-S-transferase (GST) activity. It may be concluded from the present in vitro experiments that mercury binds specifically to thiol groups present in the platelet membrane Na⁺-K⁺-ATPase, inhibits the enzyme and induces changes in platelet function, namely, platelet aggregation by interfering with the sodium pump.     

九种常用杀虫剂对二化螟线粒体ATPase活力的抑制作用

研究了二化螟Chilo suppressalis线粒体Na⁺-K⁺-ATPase和Ca^2＋-Mg^2＋-ATPase的生物化学性质以及9种常用杀虫剂对这两种酶活性的影响。结果表明, 二化螟线粒体Na⁺-K⁺-ATPase和Ca^2＋-Mg^2＋-ATPase的最适反应条件为pH值7.4,温度37℃。 Na⁺-K⁺-ATPase的米氏常数（Km）为0.42 mmol/L,最大反应速度（Vmax）为302.47 nmol/(min·mg) 。Ca^2＋-Mg^2＋-ATPase的Km为0.40 mmol/L,Vmax为128.04 nmol/(min·mg)。药剂浓度为1×10^-4 mol/L时,5种菊酯类杀虫剂对离体ATPase活性抑制的顺序为：溴氰菊酯>联苯菊酯>百树菊酯>三氟氯氰菊酯和氟硅菊酯;对二化螟Na⁺-K⁺-ATPase的抑制率分别为40.12％、39.69％、27.27％、19.49％和18.71％;对Ca^2＋-Mg^2＋-ATPase的抑制率分别为29.27％、23.78％、19.88％、11.64%和14.34％。硫丹对二化螟Na⁺-K⁺-ATPase和Ca^2＋-Mg^2＋-ATPase的抑制率均为17.46％。甲胺磷和呋喃丹对Ca^2＋-Mg^2＋-ATPase的抑制率分别为27.16％和17.42％,对Na⁺-K⁺-ATPase则几乎没有抑制作用。实验结果还表明, 在1.6×10^-7～1×10^-4 mol/L的浓度范围内,上述9种杀虫剂对二化螟ATPase活性的抑制率存在明显的剂量-效应关系。