Binding of adriamycin to sulphated mucopolysaccharides

Two distinct, Mn²⁺- and Mg²⁺-dependent enzymes with ribonuclease H (RNase H) activities, which degrade RNA-DNA hybrid have been purified from rat liver nuclei. These two enzymes were eluted at 0.16 M and 0.37 M of potassium chloride in phosphocellulose chromatography, respectively, and further purified by blue Sepharose. They are distinguished from one another by their ionic requirement, molecular weight, sedimentation coefficient, optimal pH, sensitivity to the -SH reagent and mode of cleavage.     

Modification by bovine growth hormone of liver plasma membrane enzymes, phospholipids, and circular dichroism

Liver plasma membrane phospholipid distribution, protein conformation, and 5′-nucleotidase, Mg²⁺-adenosine triphosphatase and (Na⁺ + K⁺)-adenosine triphosphatase specific activities, were shown to depend on pituitary status and treatment with bovine growth hormone.In whole liver homogenates, hypophysectomy produced a decrease in the proportion of phosphatidyl serine, lysophosphatidyl choline, and phosphatidic acid and diphosphatidyl glycerol and an increased proportion of phosphatidyl ethanolamine. The phospholipid distribution in liver plasma membranes was the same for normal and hypophysectomized rats. Plasma membranes obtained from bovine growth hormone-treated hypophysectomized rats had approximately 50%, more phosphatidyl serine than membranes obtained from untreated hypophysectomized or normal rats.Plasma membranes from hypophysectomized rats had 75% of the 5′-nucleotidase, the same level of (Na⁺ + K⁺)-adenosine triphosphatase, and twice the Mg²⁺-adenosine triphosphatase of membranes from normal rats. Twelve hours after administration of bovine growth hormone to hypophysectomized rats, (Na⁺ + K⁺)-adenosine triphosphatase had almost doubled and Mg²⁺-adenosine triphosphatase decreased by 50%. 5′-Nucleotidase remained unchanged. Twenty-four hours after bovine growth hormone administration, both (Na⁺ + K⁺)-adenosine triphosphatase and 5′-nucleotidase had increased. Mg²⁺-adenosine triphosphatase was 23% of the baseline level of untreated hypophysectomized rats. Treatment for 3 days or 5 days increased the 5′-nucleotidase 2-fold.Circular dichroism spectra of liver plasma membranes isolated from hypophysectomized rats consistently showed greater negative ellipticity in the far ultraviolet range (250-190 nm) than those from normal rats or rats treated with bovine growth hormone.     

Androgen treatment protects mouse liver chromatin from cleavage by endogenous nucleases during aging

We have examined the endogenous nuclease activity of the liver of intact, castrated and testosterone-treated mice of different ages. Both Mg²⁺- and Ca²⁺-dependent endogenous nuclease activities decline in old age. Withdrawal of the hormone increases nuclease activity in the immature and young. However, testosterone administration prevents the digestion of nuclei to different extents in all ages. These findings suggest a possible protective role of testosterone in the cleavage of liver chromatin by endogenous nucleases during the aging of mice.     

Nuclear DNA Ligase and Its Action on Chromatin DNA in Neuronal, Glial and Liver Nuclei Isolated from Adult Guinea Pigs

Abstract: DNA ligase activities were measured in neuron-rich and glial nuclear preparations and liver nuclei isolated from adult guinea pigs. The enzymatic properties of cerebral and liver nuclear DNA ligases were studied with isolated nuclei and nuclear extracts. ATP (K_m= 46–48 μM) and bivalent cation (Mg²⁺ or Mn²⁺) were required for the maximal activities in cerebral and liver nuclei. β-Mercaptoethanol did not affect the activities, but N-ethylmaleimide and p-chloromercuribenzoate completely inhibited the activities. Deoxyadenosine-5′-triphosphate partially inhibited the activities in both cerebral and liver nuclei. An interdependent effect of Na⁺ and Mg²⁺ on the enzyme activities was observed. A high concentration (200 mM) of Na⁺ activated both enzymes and shifted to the acid side the optimal pH for both enzymes. DNA ligase was more easily extracted with lower concentrations of NaCl from liver nuclei than from cerebral nuclei, but the extraction curves from both nuclear species reached a plateau level (92% of total activities of nuclear enzymes) at 200 mM-NaCl. Apparent K_m for the substrate [³²P]phosphoryl DNA was determined according to a modification of the Michaelis-Menten equation, which was applied for the case where an unknown amount of substrate nicks in chromatin DNA coexisted with the nicks in exogenous substrate DNA. Neuronal and glial nuclear enzymes had similar K_m values (about 20 μg of [³²P]phosphoryl DNA/ml), but the liver nuclear enzyme had a higher K_m value (54 μg of [³²P]phosphoryl DNA/ml). The modified Michaelis-Menten equation provided the amounts of nicks available as substrate in chromatin DNA of isolated nuclei. Neuronal and glial nuclei contained 1.5 and 0.29 pmol of nicks/μg of nuclear DNA, respectively, in contrast to an intermediate amount of nicks in liver nuclei (0.63 pmol/μg of nuclear DNA). DNA ligase activity in neuronal nuclei [312 units (fmol of 5′-phosphomonoester converted into a phosphatase-resistant form per min at 37°C) per μg of nuclear DNA] was 11-fold higher than that in glial nuclei [28.7 units/μg of nuclear DNA]. Liver nuclei contained an intermediate activity [54.7 units/μg of nuclear DNA].     

Effects of delta9-tetrahydrocannabinol on ATPases in mouse brain subcellular fractions.

The effect of (?)-Δ⁹-THC on the activities of Mg²⁺?, Na⁺?K⁺? and Mg²⁺Ca²⁺-ATPases were studied in mouse brain subcellular fractions. $\text{In vitro}$treatment with Δ⁹-THC produced a dose dependent stimulation of Mg²⁺ ATPase in the crude mitochondrial fraction and its subfractions and a dose-related inhibition of this activity in the microsomal fraction. Na⁺-K⁺- and Mg²⁺-Ca²⁺-ATPase activities were inhibited in a dose-related manner in all subcellular fractions studied.     

Differential effects of ca2+ and Mg2+ on endonuclease activation in isolated promyelocytic HL-60 cell nuclei

In autodigestion assays, endonucleaw activity in non-apoptotic HL-60 promydocytic leukemia cell nuclei cleaved the chromatin of he autologous cells to an oligonucleosomal length pattern. Both EGTA and EDTA inhibited the activation of endonuclease activity in isolated HL-60 cell nuclei. The inhibition by EDTA could be reversed by exogenous Ca²⁺. but not by exogenous Mg²⁺. In Ca²⁺/Mg²⁺-free nuclei digation buffer, addition of Ca^2→ (1-10 mmol/L) induced endonuclease activity in the isolated nuclei, while addition of Mg²⁺ had no effect. In the presence of Ca²⁺(0.1 mmol/L), endonuclease activity was enhanced by exogenous Mg²⁺ (0.1-10mmol/L). These results suggest that the endonuclease responsible for internucleosomal DNA fragmentation in HL-60 cells during apoptosis is activated by Ca²⁺ and further modulated by Mg²⁺ in the presence of ca²⁺.     

Effect of ethanol on adenosine triphosphatase and enolase activities in rat brain and in cultured nerve cells

The effect of alcohol on enzymes involved in energy metabolism of nervous tissue were analyzed, in vivo after acute and chronic ethanol administration to rats and in vitro by addition of 50 mM and 100 mM ethanol to the medium of cultured nerve cells: chick neurons, chick glial cells, a neuronal cell line (MT17) and a glial tumoral cell line (C6). The parameters we measured were (Na⁺,K⁺), Mg²⁺ and ecto Ca²⁺,Mg²⁺ ATPase activities involved in transport phenomena and enolase activities (non neuronal NNE and neuron specific enolase NSE) as markers of nerve cell maturation. In vivo, after chronic ethanol administration (Na⁺,K⁺) ATPase activity was increased while Mg²⁺ dependent activity was not affected. Enolase activity was decreased. Acute ethanol administration decreased (Na⁺,K⁺) ATPase activity, while Mg²⁺ dependent activity was not affected. In cultured nerve cells ethanol effect was dose, time and cell type dependent; alterations of the cell membrane by trypsinization of the tissue before seeding modifies the effect of ethanol on the enzymes we analyzed. Our results suggest that alcohol effect on nerve cells depends mainly on the lipoprotein structure of the cell membranes which may have different properties from one cell type to another.     

Zn2+-Dependent Acid Inorganic Pyrophosphatase Activity as Related to Other Pyrophosphatases in Oryza sativa

Acid inorganic pyrophosphatase on the one hand, and Mg²⁺-dependent alkaline inorganic pyrophosphatase and Zn²⁺-dependent acid inorganic pyrophosphatase on the other hand showed opposite trends in their activities in rice (Oryza sativa L. cv. Ratna) seedlings grown in dark and sun. The opposite trends in their activities were also noted in rice seedlings grown from gamma-irradiated seeds and in detached rice leaves floated on water in dark. The ratios of Mg²⁺ dependent alkaline inorganic pyrophosphatase/acid inorganic pyrophosphatase and Zn²⁺-dependent acid inorganic pyrophosphatase/acid inorganic pyrophosphatase changed significantly in response to the above physical treatments, but the ratio of Mg²⁺ dependent alkaline inorganic pyrophosphatase/Zn²⁺ dependent acid inorganic pyrophosphatase remained relatively stable. The conclusion is that Zn²⁺-dependent acid inorganic pyrophosphatase activity is the same as that of Mg²⁺-dependent alkaline inorganic pyrophosphatase and is different from that of acid inorganic pyrophosphatase, which requires no metal ion for activity. The acid and alkaline inorganic pyrophosphatase activities are due to separate enzyme proteins.     

Enzymic transfer of glucose and xylose from uridine diphosphate glucose and uridine diphosphate xylose to bilirubin by untreated and digitonin-activated preparations from rat liver

1. Digitonin-treated and untreated homogenates, cell extracts and washed microsomal preparations from liver of Wistar R rats are capable of transferring sugar from UDP-glucose or UDP-xylose to bilirubin. No formation of bilirubin glycosides occurred with UDP-galactose or d-glucose, d-xylose or d-glucuronic acid as the sources of sugar. 2. Procedures to assay digitonin-activated and unactivated bilirubin UDP-glucosyltransferase and bilirubin UDP-xylosyltransferase were developed. 3. In digitonin-activated microsomal preparations the transferring enzymes had the following properties. Both enzyme activities were increased 2.5-fold by pretreatment with digitonin. They were optimum at pH6.6–7.2. Michaelis–Menten kinetics were followed with respect to UDP-glucose. In contrast, double-reciprocal plots of enzyme activity against the concentration of UDP-xylose showed two intersecting straight-line sections corresponding to concentration ranges where either bilirubin monoxyloside was formed (at low UDP-xylose concentrations) or where mixtures of both the mono- and di-xyloside were synthesized (at high UDP-xylose concentrations). Both enzyme activities were stimulated by Mg²⁺; Ca²⁺ was slightly less, and Mn²⁺ slightly more, stimulatory than Mg²⁺. Of the activities found in standard assay systems containing Mg²⁺, 58–78% (substrate UDP-glucose) and 0–38% (substrate UDP-xylose) were independent of added bivalent metal ion. Double-reciprocal plots of the Mg²⁺-dependent activities against the concentration of added Mg²⁺ were linear. 4. In comparative experiments the relative activities of liver homogenates obtained with UDP-glucuronic acid, UDP-glucose and UDP-xylose were 1:1.5:2.7 for untreated preparations and 1:0.29:0.44 after activation with digitonin. 5. Bilirubin UDP-glucuronyltransferase was protected against denaturation by human serum albumin, whereas bilirubin UDP-xylosyltransferase was not. 6. Digitonin-treated and untreated liver homogenates from Gunn rats were inactive in transferring sugar to bilirubin from UDP-glucuronic acid (in agreement with the work of others), UDP-glucose or UDP-xylose.     

Additive effects of thyroid hormone,growth hormone and testosterone on deoxyribonucleic acid-dependent ribonucleic acid polymerase in rat-liver nuclei

1. The stimulations of DNA-dependent RNA polymerase in isolated rat-liver nuclei by thyroid hormone, human growth hormone and testosterone are compared. 2. Single or multiple administrations of growth-promoting doses of tri-iodo-l-thyronine, human growth hormone and testosterone stimulate the Mg²⁺-activated RNA-polymerase reaction in nuclei from thyroidectomized, hypophysectomized and castrated rats respectively. The magnitude of stimulation was proportional to the degree of enhancement of liver growth by each hormone. After a single injection, the latent period preceding the stimulation was 1, 2 and 10hr. for growth hormone, testosterone and tri-iodothyronine respectively. The time-course of stimulation of enzyme activity and the synthesis of rapidly labelled nuclear RNA in vivo were also different for each hormone. 3. Growth hormone administration failed to stimulate the Mn²⁺/ammonium sulphate-activated RNA-polymerase reaction. Thyroid hormone and testosterone, however, stimulated it but the effect was less pronounced and occurred several hours later than that observed for the Mg²⁺-activated RNA-polymerase reaction. 4. In combination experiments, hypophysectomized or the thyroidectomized rats were given growth hormone or tri-iodothyronine in a single or repeated doses at levels that produced the maximum stimulation of Mg²⁺-activated RNA-polymerase activity. Taking into account the different latent period for each hormone, a single administration of the second hormone caused an additional stimulation of the enzyme activity. Similar additive effects were observed in thyroidectomized–castrated rats after treatment with tri-iodothyronine and testosterone. The magnitude of the additional stimulation caused by the administration of the second hormone was compatible with the capacity of that hormone to promote liver growth in rats deprived of it. 5. It is concluded that, although these hormones have some similar effects, the regulation of nuclear RNA synthesis may be mediated via different routes for each hormone.     

In vitro formation of glucose-6-phosphate from glyceraldehyde-3-phosphate by liver cytosol from fed and starved rats. Effect of divalent cations on the conversion rate.

Liver cytosol preparations from fed rats are shown to form glucose-6-phosphate from glyceraldehyde-3-phosphate at a rate of 1.6 μmoles·min^?1·g liver wet weight^?1 in presence of 0.4 mM Mg²⁺. This rate is more than doubled by 30 μM EGTA and/or Mg²⁺-concentrations ≥2 mM. 48 hours starvation increases the rate of glucose-6-phosphate formation at 0.4 mM Mg²⁺ to 3.0 μmoles·min^?1·g liver wet weight^?1 and greatly diminishes the effect of EGTA and of higher Mg²⁺-concentrations. Inhibition of glucose-6-phosphate formation by Ca²⁺ and Zn²⁺ is shown to be more pronounced with cytosol from fed than from 48 hours starved rats.     

The modulating effects of pH and benzyladenine on the Ca2+– and Mg2+-ATPases in the plasmalemma of wheat root cells

Plant cells frequently and rapidly have to respond to environmental changes for survival. Regulation of transport and other energy-requiring processes in the plasmalemma of root cells is therefore one important aspect of the ecological adaptation of plants. Wheat (Triticum aestivum L. cv. Drabant) was grown hydroponically, with or without 50 nM benzyladenine in the medium, and plasma membranes from root cells of 8-day-old plants were prepared by aqueous polymer two-phase partitioning. The influence of Ca²⁺ and Mg²⁺ on the plasmalemma ATPase activities was investigated. The presence of benzyladenine during growth increased the ATPase activity, that dependent upon Ca²⁺ more than that elicited by Mg²⁺. As a general characteristic, ATP was the preferred substrate, but all nucleotide tri- and diphosphates could be accepted with activities in plasma membranes from control plants of 7-36% (Mg²⁺) and 40-86% (Ca²⁺) and in plasma membranes from benzyladenine-treated plants of 12-47% (Mg²⁺) and 53-102% (Ca²⁺) as compared with activities obtained with ATP. Nucleotidemonophosphates were not hydrolyzed by the preparations. In preparations from benzyladenine-treated plants one peak of Ca²⁺-ATPase at pH 5.2–5.6, with a tail from pH 6 and upwards, and one peak of Mg²⁺-ATPase at pH 6.0–6.5 were observed in the presence of EDTA in the assay media. In preparations from control plants, the addition of EDTA to the assays resulted in a wide optimum between pH 6 and 7 for Mg²⁺-ATPase and low Ca²⁺-ATPase activity with no influence of pH in the range 4.5 to 8. Analysis of the pH dependence in the presence of both Ca²⁺ and Mg²⁺ indicates that the control plants mainly contain Mg²⁺-ATPase corresponding to the proton pump. Preparations from benzyladenine-treated wheat roots show, in addition, activation by Ca²⁺, which, in the slightly alkaline pH range may correspond to a Ca²⁺-extruding (Ca²⁺+ Mg²⁺)-ATPase. In the acidic range, the responses are more complicated: the Mg²⁺-ATPase is inhibited by vanadate, while the Ca²⁺-ATPase is insensitive, and benzyladenine added during growth influences the interaction between Ca²⁺ and Mg²⁺ in a way that parallels the effect of high salt medium.     

Membrane-bound ATPase in chloroplasts of Euglena gracilis

Membrane-bound ATPase activities in chloroplasts of Euglena were examined. Ca²⁺- and Mg²⁺-dependent activities were relatively high in membrane preparations and could not be further activated by a number of procedures. The enzyme was found to be highly specific for purine nucleotides and was inhibited by the usual inhibitors of photophosphorylation. K_m values of Ca²⁺ and Mg²⁺ ATPase for ATP were 2.5 and 2.1 mM, respectively. Both activities were competitively inhibited by ADP and inorganic phosphate. A relationship was found between Ca²⁺- or Mg²⁺-dependent ATPase activities and chloroplast completeness. The possibilities that these activities result from one enzyme depending on Ca²⁺ or Mg²⁺ or from two different enzymes are discussed.     

Binding of ricin A chain to rat liver ribosomes: Relationship to ribosome inactivation

Ricin A chain was radioactively labeled using reductive alkylation, lactoperoxidase catalyzed iodination, and reaction with iodoacetamide or N-ethylmaleimide (NEM). The inhibition of cell-free rat liver protein synthesis by the modified A chains and the ribosome binding characteristics of each of the labeled derivatives was examined. [³H] NEM was found to quantitatively react with the A chain sulfhydryl group normally involved in a disulfide bond with the B chain in intact ricin. Labeling the protein with [³H] NEM had no effect on the in vitro inhibition of protein synthesis by the A chain. [³H] NEM-labeled A chain binds to rat liver ribosomes in a manner which is dependent on the concentrations of NaCl and Mg²⁺. At optimal Mg²⁺ concentration (5.5 mM), A chain binding to ribosomes is saturable and fully reversible either by dilution of the reaction mixture or by addition of unlabeled A chain. At 5.5 mM Mg²⁺, A chain was found to bind to a single site on rat liver ribosomes with a dissociation constant of 6.2 X 10^?8 M. [³H] NEM-labeled A chain did not bind to isolated 40S ribosomal subunits and bound to 60S ribosomal subunits with a 1 : 1 molar stoichiometry and a dissociation constant of 2.2 X 10^?7 M. The relationship between ribosome binding and A chain inhibition of eucaryotic protein synthesis is discussed.     

Enhanced glucose 6-phosphatase activity in liver of rats exposed to Mg-deficient diet

Total hepatic Mg²⁺ content decreases by >25% in animals maintained for 2 weeks on Mg²⁺ deficient diet, and results in a >25% increase in glucose 6-phosphatase (G6Pase) activity in isolated liver microsomes in the absence of significant changed in enzyme expression. Incubation of Mg²⁺-deficient microsomes in the presence of 1 mM external Mg²⁺ returned G6Pase activity to levels measured in microsomes from animals on normal Mg²⁺ diet. EDTA addition dynamically reversed the Mg²⁺ effect. The effect of Mg²⁺ or EDTA persisted in taurocholic acid permeabilized microsomes. An increase in G6Pase activity was also observed in liver microsomes from rats starved overnight, which presented a ∼15% decrease in hepatic Mg²⁺ content. In this model, G6Pase activity increased to a lesser extent than in Mg²⁺-deficient microsomes, but it could still be dynamically modulated by addition of Mg²⁺ or EDTA. Our results indicate that (1) hepatic Mg²⁺ content rapidly decreases following starvation or exposure to deficient diet, and (2) the loss of Mg²⁺ stimulates G6P transport and hydrolysis as a possible compensatory mechanism to enhance intrahepatic glucose availability. The Mg²⁺ effect appears to take place at the level of the substrate binding site of the G6Pase enzymatic complex or the surrounding phospholipid environment.     

The involvement of nuclear nucleases in rat thymocyte DNA degradation after γ-irradiation

Possible mechanisms of internucleosomal DNA fragmentation in thymocytes of irradiated rats were studied. It was shown that thymocyte nuclei contain at least two nucleases that cleave DNA between nucleosomes — a Ca²⁺/Mg²⁺-dependent nuclease and an acidic one which does not depend on bivalent ions. 2 and 3 h after irradiation at a dose of 10 Gy the initial rate of DNA cleavage by Ca²⁺/Mg²⁺-dependent nuclease in isolated nuclei increased three and seven times, respectively, but the kinetics of DNA digestion by acidic nuclease did not change. The experiments with cycloheximide indicated that Ca²⁺/Mg²⁺-dependent endonuclease turns over at a high rate. The activity of the cytoplasmic acidic and Mg²⁺-dependent nucleases was shown to increase (by 40 and 50%, respectively) 3 h after irradiation. The effect is caused by the de novo synthesis of the nucleases. At the same time the activity of nuclear nucleases did not essentially change. The chromatin isolated from rat thymocytes 3 h after irradiation did not differ in its sensitivity to some exogenic nucleases (DNAase I, micrococcal nuclease and nuclease from Serratia marcescens) from the control. Thus, Ca²⁺/Mg²⁺-dependent endonuclease seems to be responsible for the postirradiation internucleosomal DNA fragmentation in dying thymocytes.     

Activation of the template activity of isolated rat liver nuclei for DNA synthesis and its inhibition by NAD

The template activity of isolated rat liver nuclei for DNA synthesis assayed with $\text{E.}$$\text{coli}$ DNA polymerase was found to be dependent upon the presence of Ca²⁺ or Mg²⁺ in the incubation medium. DNA was prepared from isolated nuclei subjected to conditions which activated the template and centrifuged in an alkaline sucrose gradient. The distribution profile showed that smaller fragments were formed, suggesting enhancement of endonucleolytic activity. When isolated nuclei were incubated with NAD to induce poly(adenosine diphosphate ribose) formation and were subjected to the activation conditions, the template for DNA synthesis remained unchanged. The distribution profile in an alkaline sucrose gradient of DNA prepared from these nuclei and control nuclei was identical. The present findings suggest that the template-activating system for DNA synthesis was blocked when isolated nuclei were treated with NAD $\text{in}$$\text{vitro}$.     

The effects of magnesium and calcium ions on phosphatase activities at alkaline pH in the molar region of newborn mice

Summary The hydrolysis of ATP, AMP and glycerophosphate (GP) at alkaline pH in mineralizing bone and teeth of young mice has been studied histochemically. The substrates were visibly hydrolyzed to the same degree in osteoblasts, cells of stratum intermedium, odontoblasts and subodontoblasts at Ca²⁺ concentrations ranging from 10 mM to 600 mM. In the ameloblasts, however, only ATP was hydrolyzed. The ATPase activities gradually decreased at increasing Mg²⁺/Ca²⁺ ratios. The AMPase and GPase activities, on the other hand, were visibly unaffected. Marked cellular staining, including the nuclei was seen with AMP and GP as substrates when only Mg²⁺ ions were added. No ATPase activity at all could be recorded in media containing Mg²⁺ but no Ca²⁺ ions. The different phosphatase activities in cells involved in hard tissue formation were identically affected by preincubations with solutions containing various concentrations of Ca²⁺ or Mg²⁺ ions. The ATPase activity in striated muscle fibres and blood vessel walls, however, was affected differently by the same procedure.The results indicate that the phosphatase activities recorded in osteoblasts, cells of stratum intermedium, odontoblasts and subodontoblasts at alkaline pH belong to one single enzyme. The results also imply that CaATP is the preferred substrate in the enzymatic hydrolysis of ATP in hard-tissue-forming cells.     

Vitamin E up-regulates arachidonic acid release and phospholipase A2 in megakaryocytes

The alteration in calcium transport in the liver nuclei of rats orally administered carbon tetrachloride (CCl4) was investigated. Rats received a single oral administration of CCl₄(5, 10, and 25%, 1.0ml/100 g body weight), and 5, 24 and 48 h later the animals were sacrificed. The administration of CCl₄ (25%) caused a remarkable elevetion of calcium content in the liver tissues and the nuclei of rats. Liver nuclear Ca2+-ATPase activity was markedly decreased by CCl₄ (25%) administration. The presence of dibutyryl cyclic AMP(10^-4 and 10^-3 M) or inositol 1,4,5-trisphosphate (10^-6 and 10^-5 M) in the enzyme reaction mixture caused a significant decrease in Ca²⁺-ATPase activity in the liver nuclei obtained from normal rat, while the enzyme activity was significantly increased by calmodulin (1.0 and 2.0 g/ml). These signaling factor's effects were completely impaired in the liver nuclei obtained from CCl₄ (25%)-administered rats. DNA fragmentation in the liver nuclei obtained from CCl₄ -administered rats was significantly decreased by the presence of EGTA (2 mM) in the reaction mixture, suggesting that the endogenous calcium activates nuclear DNA fragmentation. The present study demonstrates that calcium transport system in the liver nuclei is impaired by liver injury with CCl₄ administration in rats.     

RNA polymerase activity in isolated nuclei ofNicotiana sanderae callus: Characteristics and modulation during differentiation

Isolated nuclei from differentiating cultures ofNicotiana sanderae showed increased levels of RNA polymerase activity as compared to the nuclei from callus cultures. The RNA synthetic activity was dependent on nucleotide triphosphates and Mg²⁺ and was destroyed by RNase. Maximum activity was obtained in the presence of 50 mM (NH₄)₂ SO₄ and α-amanitin inhibited 40% and 55% of the activity in the nuclei from callus and differentiating tissue respectively. The nuclei from differentiating tissue elicited a 3-fold increase in RNA polymerase I and a 4-fold augmentation in RNA polymerase II activities.