Energy-dependent calcium sequestration activity in rat liver microsomes

Stimulation of the renal endoplasmic reticulum calcium pump: a possible biomarker for platinate toxicity

Antitumor platinum compounds such as cisplatin are frequently nephrotoxic. The mechanism of nephrotoxicity has not been determined. It has been proposed that some toxicants may act by interfering with the mechanisms that control cellular Ca2+ homeostasis. An important factor in the regulation of cytosolic Ca2+ is the endoplasmic reticulum (ER) calcium pump. The activity of this pump was determined by measuring ATP-dependent microsomal sequestration of 45Ca. Administration of nephrotoxic doses of platinum compounds to rats was associated with an increase in renal ER calcium pump activity. This was the earliest response observed after cisplatin treatment (it occurred within 4 hr) and preceded increases in blood urea nitrogen and serum creatinine by at least 1 day. The dose-response curve for the increase in renal ER calcium pump activity was similar to the increase in the number and size of smooth ER aggregates observed in the S3 segment of the proximal tubule 24 hr following cisplatin administration. Only minor morphological changes were observed at this time. There was a significant increase in calcium content of kidneys of rats 24 hr after treatment with a dose of cisplatin that caused a maximal increase in ER calcium pump activity. This indicates that a disruption of normal calcium homeostasis may occur before histological evidence of nephrotoxicity. Platinates that were not toxic to the kidney did not elevate renal ER calcium pump activity. It is suggested that the activity of the ER calcium pump may be a useful biomarker for cellular toxicity and may be a factor in the mechanism of toxicity.

Identification and characterization of a Mg2+-dependent and an independent Ca+2-ATPase in microsomal membranes of rat testis

Rat testicular microsomal membrane fraction contains both Mg+2-dependent and Mg+2-independent Ca+2-ATPase activity. The latter activity is about two times higher than the former. Calcium ion required for maximum activation of Mg+2-independent Ca+2-ATPase in 3.0 mM, whereas for the dependent one it is 2.5 mM. Both the enzymes are resistant to cold shock upto seven days. Histidine and imidazole buffers are found to be the most suitable for dependent and independent enzyme activities, respectively. The pH optima for dependent one is 7.5, whereas for the independent one it is 8.5. Temperature optima for the former is 37 degrees C and for latter one it is 40 degrees C. Among all the nucleotides tested, ATP is found to be the best substrate for both the enzymes. The optimum concentration of ATP for dependent and independent enzyme activities are 3.0 mM and 1.5 mM respectively. Divalent metal ions like Zn+2, Ba+2 and Mn+2 have been found to inhibit Mg+2-dependent Ca+2-ATPase activity whereas Mg+2-independent Ca+2-ATPase activity is inhibited by the divalent ions except zinc which is found to stimulate the enzyme activity. Both the enzymes are inhibited by vanadate, EDTA and EGTA. I50, for vanadate is 0.05 and 0.125 mM for dependent and independent activities, respectively. Sulfhydryl groups modifying agents e.g., NEM, DTNB and chlorpromazine are found to affect the enzyme activities in different ways. Thus NEM and chlorpromazine are found to inhibit and DTNB stimulate the enzyme activities in both the cases.

Acrylonitrile depletes glutathione without changing calcium sequestration in hepatic microsomes and mitochondria

Acrylonitrile administered either in vivo or in vitro reduced the level of non-protein thiols, GSH and GSSG in rat liver (in vivo) and liver microsomes (in vitro). It neither influenced protein thiols nor calcium sequestration in the microsomes and mitochondria. The fact that the GSSG level was not increased indicates that a mere unoxidative depletion of GSH does not lead to impaired hepatocyte Ca homeostasis, which has been associated with decreased GSH:GSSG ratio. An opposite effect was caused by CCl4 which did not considerably change the protein and non-protein SH, but strongly decreased microsomal calcium sequestration.

2,5-Di(tert-butyl)-1,4-benzohydroquinone--a novel inhibitor of liver microsomal Ca2+ sequestration

Treatment of rat liver microsomes with 2,5-di(tert-butyl)-1,4-benzohydroquinone caused a dose-related inhibition (Ki congruent to 1 microM) of ATP-dependent Ca2+ sequestration. This was paralleled by a similar impairment of the microsomal Ca2+-stimulated ATPase activity. In contrast, the hydroquinose failed to induce Ca2+ release from Ca2+-loaded liver mitochondria (supplied with ATP), and inhibited neither the mitochondrial F1F0-ATPase nor the Ca2+-stimulated ATPase activity of the hepatic plasma membrane fraction. The inhibition of microsomal Ca2+ sequestration was not associated with any apparent alteration of membrane permeability or loss of other microsomal enzyme activities or modification of microsomal protein thiols. These findings suggest that 2,5-di(tert-butyl)-1,4-benzohydroquinone is a potent and selective inhibitor of liver microsomal Ca2+ sequestration which may be a useful tool in studies of Ca2+ fluxes in intact cells and tissues.

Glutathione depleting agents and lipid peroxidation

The mechanisms by which glutathione (GSH) depleting agents produce cellular injury, particularly liver cell injury have been reviewed. Among the model molecules most thoroughly investigated are bromobenzene and acetaminophen. The metabolism of these compounds leads to the formation of electrophilic reactants that easily conjugate with GSH. After substantial depletion of GSH, covalent binding of reactive metabolites to cellular macromolecules occurs. When the hepatic GSH depletion reaches a threshold level, lipid peroxidation develops and severe cellular damage is produced. According to experimental evidence, the cell death seems to be more strictly related to lipid peroxidation rather than to covalent binding. Loss of protein sulfhydryl groups may be an important factor in the disturbance of calcium homeostasis which, according to several authors, leads to irreversible cell injury. In the bromobenzene-induced liver injury loss of protein thiols as well as impairment of mitochondrial and microsomal Ca2+ sequestration activities are related to lipid peroxidation. However, some redox active compounds such as menadione and t-butylhydroperoxide produce direct oxidation of protein thiols.

Lipid peroxidation, protein thiols and calcium homeostasis in bromobenzene-induced liver damage

The mechanisms of bromobenzene hepatotoxicity in vivo were studied in mice. The relationships among glutathione (GSH) depletion, lipid peroxidation, loss of protein thiols, disturbed calcium homeostasis and liver necrosis were investigated. Liver necrosis (as estimated by the serum glutamate-pyruvate transaminase (SGPT) level) appeared between 9 and 12 hr and increased at 18 hr. Lipid peroxidation which was already detectable at 6 hr in some animals, increased thereafter showing a good correlation with the severity of liver necrosis. Despite a quite fast depletion of hepatic GSH, a significant decrease in protein thiols could be observed at 12-18 hr only. Loss of protein thiols in both whole liver and subcellular fractions (microsomes and mitochondria) was correlated with lipid peroxidation. Also a good inverse correlation was seen between lipid peroxidation and the calcium sequestration activity of liver microsomes and mitochondria. The treatment of mice with desferrioxamine (DFO) after bromobenzene-intoxication completely prevented lipid peroxidation, loss of protein thiols and liver necrosis in the animals sacrificed 15 hr after poisoning. When, however, the animals were examined at 24 hr, although the general correlation between lipid peroxidation and liver necrosis was held, in some animals (about 30% of the survivors) elevation of SGPT was observed in the virtual absence of lipid peroxidation. It seems likely therefore that the liver damage seen during the first phase of bromobenzene-intoxication is strictly related to lipid peroxidation. It is, however, possible that in some animals in which for some reason lipid peroxidation does not develop, another mechanism of liver necrosis unrelated to lipid peroxidation occurs at later times.

Effect of W7 on Ca2+ uptake and Ca2+-ATPase activities of the endoplasmic reticulum of rat liver

In an initial attempt to use calmodulin antagonists as probes to study the role of calmodulin in the modulation of Ca2+ uptake activity in the endoplasmic reticulum of rat liver, we noticed that W7 had a differential effect on the Ca2+ uptake and Ca2+-ATPase activities. To test the specificity of this effect and explore the underlying mechanism, we examined the effects of W7 on Ca2+ accumulation and release by endoplasmic reticulum in both permeabilized hepatocytes and a subcellular membrane fraction (microsomes) enriched in endoplasmic reticulum. W7 reduced the steady-state Ca2+ accumulation in both preparations in a dose-dependent fashion but the half-maximal inhibitory concentrations were different for Ca2+ accumulation (90 microM) and Ca2+-ATPase activity (500 microM). Kinetic analysis indicated that the inhibition of both Ca2+ uptake and Ca2+-ATPase activity by W7 was noncompetitive with respect to Ca2+ and ATP. Addition of W7 did not enhance the rate of Ca2+ efflux from microsomes after Ca2+ influx had been terminated. The effect of W7 was apparently not related to its calmodulin antagonist properties as the phenomenon could not be demonstrated with the other more specific calmodulin antagonists, calmidazolium or compound 48/80. A similar observation with W7 has also been reported with the endoplasmic reticulum of pancreatic islets (B. A. Wolf, J. R. Colca, and M. L. McDaniel (1986) Biochem. Biophys. Res. Commun. 141, 418-425). We concluded that the effects of W7 on microsomal Ca2+ handling were not the result of increased membrane permeability to Ca2+ but rather were due to dissociation of Ca2+ uptake from Ca2+-ATPase activity.

Fast-twitch and slow-twitch/cardiac Ca2+ ATPase genes map to human chromosomes 16 and 12

The fast-twitch and slow-twitch/cardiac Ca2+ ATPase genes have been assigned to human chromosomes 16 and 12, respectively, using rodent-human somatic cell hybrids and filter hybridization analysis of cell hybrid DNA. A rabbit cDNA for the fast-twitch ATPase hybridizes to a prominent single fragment in human genomic DNA digested with the restriction enzyme BamHI. By correlating the presence of this fragment in somatic cell hybrid DNA with the human chromosome content of the hybrids, the fast-twitch ATPase gene can be assigned to human chromosome 16. A slow-twitch/cardiac ATPase cDNA clone was isolated from a human muscle cDNA library and used to detect human fragments in EcoRI-digested somatic cell hybrid DNA. By correlating the presence of these fragments with the human chromosome content of the hybrids, the slow-twitch/cardiac ATPase gene can be assigned to human chromosome 12. Thus, the two ATPase genes, which are probably related to each other by an ancient duplication event, are not syntenic in the human genome.

Stimulatory effect of glucose 6-phosphate on the non-mitochondrial Ca2+ uptake in permeabilized hepatocytes and Ca2+ release by inositol trisphosphate

The relationships between Ca2+ transport and glucose-6-phosphatase activity, previously studied in isolated liver microsomes, were investigated in permeabilized hepatocytes in the presence of mitochondrial inhibitors. It was found that the addition of glucose 6-phosphate to the cells markedly stimulates the MgATP-dependent Ca2+ uptake. A progressive increase in the stimulation of Ca2+ uptake was seen with increasing amounts of glucose 6-phosphate up to 5 mM concentrations. Vanadate, when added in adequate concentrations (20-40 microM) to the hepatocytes inhibits both the glucose-6-phosphatase activity and the stimulation of Ca2+ uptake by glucose 6-phosphate, while not affecting the MgATP-dependent Ca2+ uptake. The addition of inositol 1,4,5-trisphosphate to permeabilized hepatocytes in which Ca2+ had been accumulated in the presence of MgATP and glucose 6-phosphate, results in a rapid release of Ca2+.

Inhibition of rat liver microsomal Ca2+-ATPase by fluorescein-5'-isothiocyanate

Rat liver microsomal fraction was incubated at pH 8.8 with fluorescein-5'-isothiocyanate in a Tris-buffered sucrose medium. This treatment completely inhibited ATP-dependent Ca2+ transport, Ca2+-ATPase activity, and Ca2+-ATPase phosphoenzyme intermediate formation. Inhibition of Ca2+ transport and phosphoenzyme intermediate formation by fluorescein-5'-isothiocyanate was partially prevented by including ATP in the treatment medium. These data taken together are consistent with the proposal that fluorescein-5'-isothiocyanate binds the Ca2+-ATPase ATP-binding site, suggesting the presence of a lysine residue in this domain. Fluorescein-5'-isothiocyanate labeling of microsomal proteins had no measurable effect on the basal, Mg2+-ATPase activity. Using fluorescein-5'-isothiocyanate-labeled microsomal fraction, we demonstrated that the Mg2+-ATPase activity was inhibited by Ca2+.

Mechanism of arachidonic acid-induced Ca2+ mobilization from rat liver microsomes

Recent evidence indicates that unesterified arachidonic acid functions as a mediator of intracellular Ca2+ mobilization by inducing Ca2+ release from the endoplasmic reticulum of pancreatic islet beta cells in a manner closely similar to that of inositol 1,4,5-trisphosphate. To test the generality and explore the mechanism of this phenomenon we have examined the effects of arachidonic acid on calcium accumulation and release by hepatocyte subcellular fractions enriched in endoplasmic reticulum (microsomes). At concentrations above 0.017 mumol/mg microsomal protein, arachidonate induced rapid (under 2 min) 45Ca2+ release from microsomes that had been preloaded with 45Ca2+. Arachidonate also suppressed microsomal 45Ca2+ accumulation when present during the loading period, as reflected by reduction both of 45Ca2+ accumulation at steady state and of the rate of uptake. Neither the cyclooxygenase inhibitor indomethacin nor the lipoxygenase/cyclooxygenase inhibitor BW755C suppressed arachidonate-induced 45Ca2+ release, indicating that this effect was not dependent upon oxygenation of the fatty acid to metabolites. The long-chain unsaturated fatty acids oleate and linoleate were less potent than arachidonate in inducing 45Ca2+ release, and the saturated fatty acid stearate did not exert this effect. Albumin prevented 45Ca2+ release by arachidonate, presumably by binding the fatty acid. As is the case for inositol 1,4,5-trisphosphate, the ability of arachidonate to induce 45Ca2+ release was dependent on the ambient free Ca2+ concentration. Arachidonate did not influence microsomal membrane permeability or Ca2+-ATPase activity and may exert its effects on microsomal Ca2+ handling by activation of a Ca2+ extrusion mechanism or by dissociating Ca2+ uptake from Ca2+-ATPase activity.

Cytosolic calcium after carbon tetrachloride, 1,1-dichloroethylene, and phenylephrine exposure. Studies in rat hepatocytes with phosphorylase a and quin2

Carbon tetrachloride (CCl4) and 1,1-dichloroethylene (DCE), both hepatotoxins, inhibit sequestration of Ca2+ by rat liver endoplasmic reticulum (ER) both in vivo and in vitro. It is possible that, as a result, cytosolic Ca2+ concentrations rise in liver cells. In experiments presented here, isolated hepatocytes were exposed to CCl4, DCE, and phenylephrine (PE), a non-hepatotoxic alpha 1-adrenergic agent that mobilizes Ca2+. Cytoplasmic Ca2+ concentrations were evaluated by two methods: indirectly by assaying the activity of glycogen phosphorylase a, and directly by monitoring the fluorescence of quin2. In primary hepatocyte cultures, CCl4, DCE, and PE exposure increased the activity of phosphorylase a at 5 min from 39 +/- 2 to 130 +/- 12, 80 +/- 13, and 97 +/- 10 nmoles PO4(3-)/mg protein/min respectively. In rat hepatocyte suspensions loaded with quin2 and exposed to CCl4, DCE, or PE, cytosolic Ca2+ concentrations were elevated within 20 sec to 0.83 +/- 0.13, 0.59 +/- 0.06 and 0.99 +/- 0.14 microM Ca2+ respectively. Basal Ca2+ levels in these cells averaged 0.25 +/- 0.03 microM. Thus, CCl4 and PE apparently increased cytosolic Ca2+ levels to approximately the same extent, whereas DCE was somewhat less effective. The durations of the effects of CCl4 and PE were examined further by determining their time courses of elevated phosphorylase a activity. In hepatocyte cultures, increased phosphorylase a activity persisted through at least 60 min following CCl4 exposure. In contrast, phosphorylase a activity returned to basal levels by 20 min after PE. Increases in cytoplasmic Ca2+ levels that are sustained rather than transient may distinguish these hepatotoxic chlorinated aliphatic hydrocarbons from non-toxic hormonal agents.

Impaired calcium sequestration activity in liver microsomes from fasted rats

Calcium uptake activity was assayed in liver microsomal vesicles from fed and fasted rats. This activity required ATP and was stimulated by the calcium trapping agent oxalate. The most striking feature was the low rate of calcium accumulation in liver microsomes from fasted rats. Maximal rate was inhibited up to 66 and 82% after 1 and 3 days starvation, respectively. This defective microsomal calcium handling suggests its possible involvement in the massive glycogen breakdown during starvation.

Ionic and substrate requirements of the high affinity calcium pumping ATPase in endoplasmic reticulum of pancreas

1. Calcium transport and ATPase activities were determined in microsomal vesicles from pancreatic tissue enriched in endoplasmic reticulum membranes. 2. Calcium transport and ATPase share the following properties: (i) magnesium was required with a K0.5 of 0.7 mM and maximal pumping ATPase activity at 5 mM Mg-ATP; (ii) at saturating magnesium concentrations, calcium increased ATP splitting activity up to three times with an apparent K0.5 close to 0.3 microM calcium; (iii) potassium stimulated the high calcium affinity Mg2+-dependent ATPase and calcium transport. 3. The properties of the calcium pumping system fulfil the cationic and substrate requirements from a physiological point of view.

Inhibition of liver endoplasmic reticulum calcium pump by CCl4 and release of a sequestered calcium pool

One of the earliest effects observed in rat liver after CCl4 administration is inhibition of an ATP-dependent calcium pump found at the endoplasmic reticulum. This report confirms that the amount of calcium associated with the microsomal fraction is reduced after CCl4 administration and, for the first time, demonstrates time-, dose-, and metabolism-dependent relationships between inhibition of the liver microsomal calcium pump and the amount of calcium found in the microsomal fraction. Furthermore, release of calcium from the endoplasmic reticulum is shown to cause activation of a cytoplasmic enzyme that responds to increases of ionized calcium, glycogen phosphorylase. This suggests that the endoplasmic reticulum calcium pump sequesters an intracellular pool of calcium within the endoplasmic reticulum. This pool of calcium may be released into the cytoplasm as a consequence of inhibition of the calcium pump by CCl4.

A direct colorimetric assay for Ca2+ -stimulated ATPase activity

A simple and rapid colorimetric assay for measuring the high affinity Ca2+-ATPase activity in subcellular fractions is presented. With this method a one-step addition of a malachite green/molybdate/polyvinyl alcohol reagent to the assay mixture at the end of the incubation period is all that is required for the spectrophotometric quantification of the phosphomolybdate-malachite green complex. The presence of polyvinyl alcohol allows the quantification of released phosphate without having to separate it from protein. We have validated this assay by characterizing the high affinity Ca2+-ATPase activity in isolated rat liver microsomes. Comparable Ca2+-ATPase activities in rat liver microsomes and adipocyte plasma membranes were found when measured with this colorimetric assay and an isotopic assay. This method is applicable to the measurement of other types of ATPase activities.

Benoxaprofen induced toxicity in isolated rat hepatocytes

The toxicity of benoxaprofen, a non-steroidal anti-inflammatory compound was investigated using rat hepatic microsomal and isolated hepatocyte suspensions. In microsomes, benoxaprofen produced a Type I binding spectra and competitively inhibited (ki 380 microM) the oxidative metabolism of aminopyrine. Marked toxicity was observed following incubation of benoxaprofen with isolated hepatocytes from either untreated, phenobarbitone (PB) or 3-methylcholanthrene (3-MC) pretreated male rats. In untreated hepatocytes increases in the intracellular lactate/pyruvate (L/P) ratio and alanine aminotransferase (ALT) release were related to the benoxaprofen concentration and duration of incubation. Alterations in L/P ratio preceded the release of cytosolic ALT and at 4 h a well defined dose-response relationship existed between the benoxaprofen concentration and the observed increases in the L/P ratio and ALT release. Pretreatment of animals with either PB or 3-MC did not affect the temporal nature nor the magnitude of the hepatocyte response to benoxaprofen. In addition, inhibitors of cytochrome P-450 isozymes (SKF-525A, metyrapone and alpha-napthoflavone) were ineffective with regard to modifying the observed toxicity. The results of this study suggest that hepatic cytochrome P-450 mediated metabolism may not be implicated in the toxicity of benoxaprofen in isolated hepatocytes. However, alterations in the cellular redox state and evidence of plasma membrane bleb formation suggest that benoxaprofen may uncouple oxidative phosphorylation and disturb intracellular calcium ion homeostasis.

The effect of cytochrome P-450 and reduced glutathione on the ATP-dependent calcium pump of hepatic microsomes from male rats

In the presence of ATP hepatic microsomes sequester calcium. This sequestration is thought to be important in the modulation of free cytosolic calcium concentration. We find that on the addition of NADPH the uptake of calcium by the hepatic microsomes is inhibited 27-85%. This inhibition is reversed by the addition of 1 mM reduced glutathione (85-91% of control), incubation under a nitrogen atmosphere (112% of control), or incubation in a 80% carbon monoxide/20% oxygen atmosphere (75% of control). Superoxide dismutase had no effect on the inhibition, while catalase reversed the inhibition by 35%. The addition of 1 mM reduced glutathione at 2 and 5 min after the addition of NADPH led to uptakes of calcium which paralleled the uptake seen when the reduced glutathione was added at the beginning of the incubation. The effect of reduced glutathione showed saturation kinetics with a Km of 10 microM. Together these data suggest that cytochrome P-450 reduces the activity of the microsomal ATP-dependent calcium pump both by the production of hydrogen peroxide and by the direct oxidation of the protein thiols. The reversal of this effect by reduced glutathione appears to be enzymatically catalyzed.

In vivo indexes of platelet and vascular function during fish-oil administration in patients with atherosclerosis

Populations that consume a diet rich in marine lipids may have a lower risk of atherosclerotic disease. Fish oil contains the N-3 polyunsaturated fatty acid eicosapentaenoate, and the biosynthesis of thromboxanes and prostacyclins from eicosapentaenoate (thromboxane A3 and prostaglandin I3), rather than from the usual precursor arachidonate (thromboxane A2 and prostaglandin I2), may help to reduce the risk. To examine this hypothesis, we studied the effect of eicosapentaenoate supplementation (10 g per day) for one month on the synthesis of thromboxanes and prostacyclins, as assessed by urinary metabolite excretion, in six patients with peripheral vascular disease and seven normal controls. Supplementation markedly increased the eicosapentaenoate content of phospholipids from red cells and platelets. Synthesis of the platelet agonist thromboxane A2, which was elevated in the patients at base line, declined by 58 percent during supplementation but did not reach normal values. The decline in thromboxane A2, which is synthesized from arachidonate, coincided with the formation of the inactive thromboxane A3, which is synthesized from eicosapentaenoate. A lower dose of eicosapentaenoate (1 g per day) was not sufficient to maintain the changes in thromboxane A2 synthesis. Platelet function was only moderately inhibited during eicosapentaenoate supplementation, consistent with incomplete suppression of thromboxane A2 synthesis. These studies show that a high dose of eicosapentaenoate alters the pattern of synthesis of thromboxanes and prostacyclins. However, effects comparable to those of aspirin require long-term administration in high doses. Whether other properties of fish oil might render it a more attractive antithrombotic therapy remains to be determined.

Ca2+ homeostasis and cytotoxicity in isolated hepatocytes: studies with extracellular adenosine 5'-triphosphate

The incubation of isolated rat hepatocytes with extracellular adenosine 5'-triphosphate (ATP) resulted in an inhibition of Ca2+ efflux. The ATP-induced Ca2+ accumulation as determined by the increase in phosphorylase a activity and the Ca2+-sensitive fluorescent indicator (2-[(2-bis-[carboxymethyl]-amino-5-methylphenoxy)-methyl]-6-methoxy-8- bis-[carboxymethyl]aminoquinoline-tetrakis-[acetoxymethyl]ester) (Quin 2-AM) was associated with both the hydrolysis of ATP and the phosphorylation of a 110 kDa protein. No significant alteration in the intracellular ATP level was observed. The appearance of surface blebs and cytotoxicity followed the rise in cytosolic Ca2+, suggesting that the increased free Ca2+ may be responsible for the loss of viability. When a calmodulin inhibitor, 1-[bis(4-chlorophenyl)methyl]-3-[ 2-(2,4-dichlorophenyl)-2-[(2,4-dichlorophenyl)methoxy] ethyl]-1H- imidazolium chloride (calmidazolium), was included in the medium prior to ATP addition, bleb formation was reduced and the loss of viability was completely prevented, indicating that a Ca2+-calmodulin process may be involved in the initiation of cytotoxicity.

The effect of Mg2+ on hepatic microsomal Ca2+ and Sr2+ transport

The ATP-dependent uptake of Ca2+ by rat liver microsomal fraction is dependent upon Mg2+. Studies of the Mg2+ requirement of the underlying microsomal Ca2+-ATPase have been hampered by the presence of a large basal Mg2+-ATPase activity. We have examined the effect of various Mg2+ concentrations on Mg2+-ATPase activity, Ca2+ uptake, Ca2+-ATPase activity and microsomal phosphoprotein formation. Both Mg2+-ATPase activity and Ca2+ uptake were markedly stimulated by increasing Mg2+ concentration. However, the Ca2+-ATPase activity, measured concomitantly with Ca2+ uptake, was apparently unaffected by changes in the Mg2+ concentration. In order to examine the apparent paradox of Mg2+ stimulation of Ca2+ uptake but not of Ca2+-ATPase activity, we examined the formation of the Ca2+-ATPase phosphoenzyme intermediate and formation of a Mg2+-dependent phosphoprotein, which we have proposed to be an attribute of the Mg2+-ATPase activity. We found that Ca2+ apparently inhibited formation of the Mg2+-dependent phosphoprotein both in the absence and presence of exogenous Mg2+. This suggests that Ca2+ may inhibit (at least partially) the Mg2+-ATPase activity. However, inclusion of the Ca2+ inhibition of Mg2+-ATPase activity in the calculation of Ca2+-ATPase activity reveals that this effect is insufficient to totally account for the stimulation of Ca2+ uptake by Mg2+. This suggests that Mg2+, in addition to stimulation of Ca2+-ATPase activity, may have a direct stimulatory effect on Ca2+ uptake in an as yet undefined fashion. In an effort to further examine the effect of Mg2+ on the microsomal Ca2+ transport system of rat liver, the interaction of this system with Sr2+ was examined. Sr2+ was sequestered into an A23187-releasable space in an ATP-dependent manner by rat liver microsomal fraction. The uptake of Sr2+ was similar to that of Ca2+ in terms of both rate and extent. A Sr2+-dependent ATPase activity was associated with the Sr2+ uptake. Sr2+ promoted formation of a phosphoprotein which was hydroxylamine-labile and base-labile. This phosphoprotein was indistinguishable from the Ca2+-dependent ATPase phosphoenzyme intermediate. Sr2+ uptake was markedly stimulated by exogenous Mg2+, but the Sr2+-dependent ATPase activity was unaffected by increasing Mg2+ concentrations. Sr2+ uptake and Sr2+-dependent ATPase activity were concomitantly inhibited by sodium vanadate. In contrast to Ca2+, Sr2+ had no effect on Mg2+-dependent phosphoprotein formation. Taken together, these data indicate that Mg2+ stimulated Ca2+ and Sr2+ transport by increasing the Ca2+ (Sr2+)/ATP ratio.(ABSTRACT TRUNCATED AT 400 WORDS)

The kinetics of calcium influx into rat liver slices

The unidirectional influx of calcium across rat liver slices is a carrier-mediated process which displays saturation kinetics. The presence of Mg2+ in the incubation medium competitively inhibits calcium influx into rat liver slices. Metabolic inhibitors such as ouabain and 2,4-dinitrophenol at a concentration of 1 X 10(-4) and 2.5 X 10(-4) M respectively, inhibited significantly (P less than 0.001) calcium influx across the liver slices. Calcium influx is dependent on the presence of sodium in the extracellular medium, and is significantly reduced (P less than 0.01) when sodium concentration in the preincubation solution is reduced to zero.

On the role of thiol groups in the inhibition of liver microsomal Ca2+ sequestration by toxic agents

ATP-dependent Ca2+ sequestration by rat liver microsomes was assayed using three different methods, and characterized with regard to the effect of various inhibitors. When glucose and hexokinase were added in combination to deplete ATP in the incubation, Ca2+ uptake was followed by rapid release of Ca2+ from the microsomes. Ca2+ sequestration was inhibited by reagents that cause alkylation (e.g. p-chloromercuribenzoate) or oxidation (e.g. diamide) of protein sulfhydryl groups. Moreover, pretreatment of the microsomes with cystamine, which causes formation of mixed disulfides with protein thiols, also resulted in the inhibition of Ca2+ sequestration. It is concluded that microsomal Ca2+ sequestration is critically dependent on protein sulfhydryl groups, and that modification of protein thiols may be an important mechanism for the inhibition of microsomal Ca2+ sequestration by a variety of toxic agents.

The mechanism of calcium uptake by liver microsomes: effect of anions and ionophores

The mechanism of calcium uptake by liver microsomes was investigated using various anions and ionophores. Calcium uptake was shown to be specific to microsomes and unlikely to be due to contamination by plasma membranes by correlation of calcium uptake to the marker enzymes specific for these two fractions. Under the conditions employed, phosphates, sulfate, chloride, acetate, nitrate, thiocyanate, maleate, succinate and oxalate all stimulated calcium uptake by microsomes, but to different degrees. The greatest effect (4-6-fold) was observed with phosphate. On the contrary, phosphate is the only anion that stimulates the plasma membrane calcium uptake to any significant degree. Treatment of isolated microsomes with 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene (DIDS) resulted in inhibition of ATP- and anion-dependent calcium uptake. A lipid-permeable organic acid such as maleate retained its ability to promote calcium uptake in DIDS-treated microsomes. However, a lipophilic anion, such as nitrate, stimulated calcium uptake only in the presence of the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP). In addition, 2 microM valinomycin, when added in the absence or presence of 10 to 100 mM K+, had no stimulatory effect on calcium uptake. These results appear to be consistent with a model in which the active uptake of calcium into microsomes involves electroneutral Ca2+-nH+ exchange.

Thiols and pancreatic beta-cell function: a review

In pancreatic islets insulin secretion in response to a variety of stimulators is sensitive to the redox state of extracellular and intracellular thiols. In this connection variations of plasma glutathione (GSH) may also be of importance. In the process of stimulus-secretion coupling, membrane thiols play an important role. One major localization of critical thiols appears to be related to the influx of calcium through the voltage-dependent channel. Other transmembranal ion movements and the cAMP system seem to be less sensitive to thiol oxidation than calcium influx via voltage-dependent Ca channels.

Calcium sequestration activity in rat liver microsomes. Evidence for a cooperation of calcium transport with glucose-6-phosphatase

Mechanisms regulating the energy-dependent calcium sequestering activity of liver microsomes were studied. The possibility for a physiologic mechanism capable of entrapping the transported Ca2+ was investigated. It was found that the addition of glucose 6-phosphate to the incubation system for MgATP-dependent microsomal calcium transport results in a marked stimulation of Ca2+ uptake. The uptake at 30 min is about 50% of that obtained with oxalate when the incubation is carried out at pH 6.8, which is the pH optimum for oxalate-stimulated calcium uptake. However, at physiological pH values (7.2-7.4), the glucose 6-phosphate-stimulated calcium uptake is maximal and equals that obtained with oxalate at pH 6.8. The Vmax of the glucose 6-phosphate-stimulated transport is 22.3 nmol of calcium/mg protein per min. The apparent Km for calcium calculated from total calcium concentrations is 31.9 microM. After the incubation of the system for MgATP-dependent microsomal calcium transport in the presence of glucose 6-phosphate, inorganic phosphorus and calcium are found in equal concentrations, on a molar base, in the recovered microsomal fraction. In the system for the glucose 6-phosphate-stimulated calcium uptake, glucose 6-phosphate is actively hydrolyzed by the glucose-6-phosphatase activity of liver microsomes. The latter activity is not influenced by concomitant calcium uptake. Calcium uptake is maximal when the concentration of glucose 6-phosphate in the system is 1-3 mM, which is much lower than that necessary to saturate glucose-6-phosphatase. These results are interpreted in the light of a possible cooperative activity between the energy-dependent calcium pump of liver microsomes and the glucose-6-phosphatase multicomponent system. The physiological implications of such a cooperation are discussed.

Inhibition of hepatocyte plasma membrane Ca2+-ATPase activity by menadione metabolism and its restoration by thiols

Incubation of isolated rat hepatocytes with cytotoxic concentrations of menadione resulted in inhibition of plasma membrane Ca2+-ATPase activity. This could be restored by subsequent treatment with either dithiothreitol or reduced glutathione, suggesting that the inhibition by menadione was due to oxidation of sulfhydryl groups critical for Ca2+-ATPase activity.

Electrogenic calcium transport in plasma membrane of rat pancreatic acinar cells

ATP-dependent 45Ca2+ uptake was investigated in purified plasma membranes from rat pancreatic acinar cells. Plasma membranes were purified by four subsequent precipitations with MgCl2 and characterized by marker enzyme distribution. When compared to the total homogenate, typical marker enzymes for the plasma membrane, (Na+,K+)-ATPase, basal adenylate cyclase and CCK-OP-stimulated adenylate cyclase were enriched by 43-fold, 44-fold, and 45-fold, respectively. The marker for the rough endoplasmic reticulum was decreased by fourfold compared to the total homogenate. Comparing plasma membranes with rough endoplasmic reticulum, Ca2+ uptake was maximal with 10 and 2 mumol/liter free Ca2+, and half-maximal with 0.9 and 0.5 mumol/liter free Ca2+. It was maximal at 3 and 0.2 mmol/liter free Mg2+ concentration, at an ATP concentration of 5 and 1 mmol/liter, respectively, and at pH 7 for both preparations. When Mg2+ was replaced by Mn2+ or Zn2+ ATP-dependent Ca2+ uptake was 63 and 11%, respectively, in plasma membranes; in rough endoplasmic reticulum only Mn2+ could replace Mg2+ for Ca2+ uptake by 20%. Other divalent cations such as Ba2+ and Sr2+ could not replace Mg2+ in Ca2+ uptake. Ca2+ uptake into plasma membranes was not enhanced by oxalate in contrast to Ca2+ uptake in rough endoplasmic reticulum which was stimulated by 7.3-fold. Both plasma membranes and rough endoplasmic reticulum showed cation and anion dependencies of Ca2+ uptake. The sequence was K+ greater than Rb+ greater than Na+ greater than Li+ greater than choline+ in plasma membranes and Rb+ greater than or equal to K+ greater than or equal to Na+ greater than Li+ greater than choline+ for rough endoplasmic reticulum. The anion sequence was Cl greater than or equal to Br greater than or equal to 1 greater than SCN greater than NO3 greater than isethionate greater than cyclamate greater than gluconate greater than SO2(4) greater than or equal to glutarate and Cl- greater than Br greater than gluconate greater than SO2(4) greater than NO3 greater than 1 greater than cyclamate greater than or equal to SCN, respectively. Ca2+ uptake into plasma membranes appeared to be electrogenic since it was stimulated by an inside-negative K+ and SCN diffusion potential and inhibited by an inside-positive diffusion potential. Ca2+ uptake into rough endoplasmic reticulum was not affected by diffusion potentials. We assume that the Ca2+ transport mechanism in plasma membranes as characterized in this study represents the extrusion system for Ca2+ from the cell that might be involved in the regulation of the cytosolic Ca2+ level.

Pathological mechanisms in carbon tetrachloride hepatotoxicity

Liver cell injury induced by carbon tetrachloride involves initially the metabolism of carbon tetrachloride to trichloromethyl free-radical by the mixed function oxidase system of the endoplasmic reticulum. It is postulated that secondary mechanisms link carbon tetrachloride metabolism to the widespread disturbances in hepatocyte function. These secondary mechanisms could involve the generation of toxic products arising directly from carbon tetrachloride metabolism or from peroxidative degeneration of membrane lipids. The possible involvement of radical species such as trichloromethyl (.CCl3), trichloromethylperoxy (.OOCCl3), and chlorine (.Cl) free radicals, as well as phosgene and aldehydic products of lipid peroxidation, as toxic intermediates is discussed. Data do not support the view that an increase in cytosolic free calcium is important in the toxic action of carbon tetrachloride or bromotrichloromethane. In addition, carbon tetrachloride-induced inhibition of very low density lipoprotein secretion by hepatocytes is not a result of elevated levels of cytosolic free calcium.

Enzymatic activation of chemicals to toxic metabolites

A variety of enzymes function in the oxygenation, oxidation-reduction, conjugation, and hydrolysis of drugs and other foreign chemicals. Often these enzymes detoxicate chemicals to prevent detrimental effects. In this review we will, however, concentrate on cases in which metabolism activates chemicals to reactive species which cause cellular damage. Particular attention will be given to mixed-function oxidases, which carry out a variety of oxygenations, as well as other reactions. (We will focus on cellular toxicity as opposed to initiation of tumorigenesis in this review.) In many cases, considerable circumstantial evidence exists linking these enzymes to enhanced toxicity of chemicals, although causal relationships have seldom been demonstrated. Further, in very few cases is the explicit cause of toxicity known. Modification of critical protein residues is suspected, although oxidative stress may also be involved in some cases. We discuss general aspects of mechanisms of toxic action, briefly list all cases in which metabolism is suspected to play a role in enhancing toxicity, and review a few examples in detail where substantial chemical and enzymatic information is available. The latter instances would involve knowledge of the enzymes involved, chemical evidence on the structures of the reactive metabolites, identification of adducts, and some inference into the biological processes which are effected to elicit toxicity. We consider, in this regard, vinyl halides (which have been a focus in our own laboratory), acetaminophen, pyrrolizidine alkaloids, and fluoroxene.

Investigation of the mechanism of hepatotoxicity of N-methylformamide in mice: effects on calcium sequestration in hepatic microsomes and mitochondria and on hepatic plasma membrane potential

N-Methylformamide is an antitumour drug with hepatotoxic properties. Three potential targets for hepatocellular toxic lesions caused by N-methylformamide were investigated: the mitochondrial and microsomal Ca2+ pumps and the functional integrity of the plasma membrane. The administration of N-methylformamide to mice caused a dramatic decrease in the ability of the liver mitochondria to sequester [45Ca2+]. This effect was dose-dependent and was not caused by dimethylformamide, N-hydroxymethylformamide or formamide. The microsomal Ca2+ pump was not affected by N-methylformamide. Incubations of isolated mitochondria with N-methylformamide for 1 hr also led to the inhibition of the Ca2+ sequestration. Incubation of isolated mouse hepatocytes with N-methylformamide did not cause changes in plasma membrane potential as measured using the lipophilic cation triphenylmethylphosphonium. Of the three targets studied, the mitochondrial Ca2+ pump may be the one through which N-methylformamide triggers the events leading ultimately to hepatic necrosis.

Menadione-induced cytotoxicity is associated with protein thiol oxidation and alteration in intracellular Ca2+ homeostasis

The toxicological implications of alterations in intracellular thiol homeostasis during menadione metabolism have been investigated using freshly isolated rat hepatocytes. A strict correlation between depletion of protein sulfhydryl groups and loss of cell viability was observed. Loss of protein thiols preceded cell death, and occurred more rapidly in cells with decreased levels of reduced glutathione. Depletion of protein thiols was also associated with inhibition of Ca2+ efflux from the cells and perturbation of intracellular Ca2+ homeostasis. It is proposed that the oxidative stress induced by menadione metabolism in isolated hepatocytes results in the depletion of both soluble and protein thiols, and that the latter effect is critically associated with a perturbation of Ca2+ homeostasis and loss of cell viability.

Monoclonal antibodies to the Ca2+ + Mg2+-dependent ATPase of sarcoplasmic reticulum identify polymorphic forms of the enzyme and indicate the presence in the enzyme of a classical high-affinity Ca2+ binding site

In order to determine whether polymorphic forms of the Ca2+ + Mg2+-dependent ATPase exist, we have examined the cross-reactivity of five monoclonal antibodies prepared against the rabbit skeletal muscle sarcoplasmic reticulum enzyme with proteins from microsomal fractions isolated from a variety of muscle and nonmuscle tissues. All of the monoclonal antibodies cross-reacted in immunoblots against rat skeletal muscle Ca2+ + Mg2+-dependent ATPase but they cross-reacted differentially with the enzyme from chicken skeletal muscle. No cross-reactivity was observed with the Ca2+ + Mg2+-dependent ATPase of lobster skeletal muscle. The pattern of antibody cross-reactivity with a 100,000 dalton protein from sarcoplasmic reticulum and microsomes isolated from various muscle and nonmuscle tissues of rabbit demonstrated the presence of common epitopes in multiple polymorphic forms of the Ca2+ + Mg2+-dependent ATPase. One of the monoclonal antibodies prepared against the purified Ca2+ + Mg2+-dependent ATPase of rabbit skeletal muscle sarcoplasmic reticulum was found to cross-react with calsequestrin and with a series of other Ca2+-binding proteins and their proteolytic fragments. Its cross-reactivity was enhanced in the presence of EGTA and diminished in the presence of Ca2+. Its lack of cross-reactivity with proteins that do not bind Ca2+ suggests that it has specificity for antigenic determinants that make up the Ca2+-binding sites in several Ca2+-binding proteins including the Ca2+ + Mg2+-dependent ATPase.