Inhibition of terminal deoxynucleotidyl transferase by adenine dinucleotides. Unique inhibitory action of Ap5A.

Terminal deoxynucleotidyltransferase (TdT) exhibits strong sensitivity to ATP and its dinucleotide analogues, Ap2A, Ap3A, Ap4A, Ap5A and Ap6A. Similar to ATP, all of the dinucleotides appear to be competitive inhibitors of TdT catalysis with respect to substrate deoxynucleoside triphosphates and effectively block the UV-mediated substrate cross-linking to TdT. Among the various dinucleotides, Ap5A and Ap6A (diadenosine 5'-5' penta- and hexaphosphate, respectively) are significantly more effective than dinucleotides containing 2, 3 or 4 phosphate backbones. Furthermore, Ap5A is found to be the only dinucleotide which has reactivity at both substrate- and primer-binding domains in TdT.

Role of phenolic O-H and methylene hydrogen on the free radical reactions and antioxidant activity of curcumin.

To understand the relative importance of phenolic O-H and the CH-H hydrogen on the antioxidant activity and the free radical reactions of Curcumin, (1,7-bis[4-hydroxy-3-methoxyphenyl]-1,6-heptadiene-3,5-dione), biochemical, physicochemical, and density functional theory (DFT) studies were carried out with curcumin and dimethoxy curcumin (1,7-bis[3, 4-dimethoxy phenyl]-1,6-heptadiene-3,5-dione). The antioxidant activity of these compounds was tested by following radiation-induced lipid peroxidation in rat liver microsomes, and the results suggested that at equal concentration, the efficiency to inhibit lipid peroxidation is changed from 82% with curcumin to 24% with dimethoxy curcumin. Kinetics of reaction of (2,2'-diphenyl-1-picrylhydrazyl) DPPH, a stable hydrogen abstracting free radical was tested with these two compounds using stopped-flow spectrometer and steady state spectrophotometer. The bimolecular rate constant for curcumin was found to be approximately 1800 times greater than that for the dimethoxy derivative. Cyclic voltammetry studies of these two systems indicated two closely lying oxidation peaks at 0.84 and 1.0 V vs. SCE for curcumin, while only one peak at 1.0 V vs. SCE was observed for dimethoxy curcumin. Pulse radiolysis induced one-electron oxidation of curcumin and dimethoxy curcumin was studied at neutral pH using (*)N(3) radicals. This reaction with curcumin produced phenoxyl radicals absorbing at 500 nm, while in the case of dimethoxy curcumin a very weak signal in the UV region was observed. These results suggest that, although the energetics to remove hydrogen from both phenolic OH and the CH(2) group of the beta-diketo structure are very close, the phenolic OH is essential for both antioxidant activity and free radical kinetics. This is further confirmed by DFT calculations where it is shown that the -OH hydrogen is more labile for abstraction compared to the -CH(2) hydrogen in curcumin. Based on various experimental and theoretical results it is definitely concluded that the phenolic OH plays a major role in the activity of curcumin.

Does beta-carotene protect membrane lipids from nitrogen dioxide?

Lipid peroxidation in rat liver microsomes, induced by nitrogen dioxide (NO2), a free radical toxin, was examined in the absence and in the presence of varying concentrations of beta-carotene. The extent of peroxidation was assayed by determining the malonaldehyde formed as thiobarbituric acid reactive substances (TBARS). When the concentration of beta-carotene was 13.8 and 43.1 nmol/mg protein, no protection was seen, rather an increase of 10% and 30%, respectively, in TBARS was observed as compared with the normal microsomes containing no beta-carotene. However, at beta-carotene concentrations of 66.5 and 89.4 nmol/mg protein, only a marginal increase of 9% and 4% in TBARS, respectively, was observed. The amount of beta-carotene consumed during peroxidation, determined by following the absorbance at 450 nm, was found to increase linearly with increased exposure to NO2. The direct reaction of NO2 with beta-carotene was studied in an inert organic solvent, acetonitrile, by following the absorption spectrum of beta-carotene in the wavelength region 220-600 nm. The rate of loss of beta-carotene was found to be much faster than that in microsomes. The results suggest that in in vitro systems, the reaction of secondary lipid-derived radicals with beta-carotene and their relative competition for NO2 plays an important role in the actual function of beta-carotene as a prooxidant or an antioxidant. Another lipid soluble antioxidant, alpha-tocopherol (vitamin-E), showed significant protection against NO2-induced lipid peroxidation at a concentration of 45 nmol/mg protein under these conditions.

Impaired mitochondrial oxidative energy metabolism following paracetamol-induced hepatotoxicity in the rat.

1. Effects of paracetamol treatment in vivo at subtoxic (375 mg kg-1 body weight) and toxic (750 mg kg-1 body weight) doses on energy metabolism in rat liver mitochondria were examined. 2. Paracetamol treatment resulted in a significant loss in body weights without affecting the liver protein contents. Toxic doses, however, resulted in 21% decrease in the yield of mitochondrial proteins. 3. Subtoxic doses of paracetamol did not, in general, affect the respiratory parameters in the liver mitochondria except in the case of succinate where both the state 3 respiration and the ADP-phosphorylation rates increased by 28%. 4. Toxic doses of paracetamol caused 25 to 47% decrease in the state 3 respiration rates depending on the substrate used. ADP/O ratios also decreased significantly with pyruvate + malate and succinate as the substrates. Consequently, ADP-phosphorylation was impaired significantly from 20 to 63%. 5. Subtoxic doses of paracetamol resulted in increased contents of cytochrome c + c1 while the toxic doses caused lowering of the cytochromes aa3 and b contents. 6. Glutamate and succinate dehydrogenase activities decreased in both the experimental groups while Mg2+-ATPase activity was impaired only after toxic dose-treatment. 7. The results show that toxic doses of paracetamol result in impaired energy coupling in the liver mitochondria. Effects of subtoxic doses were also demonstrable in terms of impaired dehydrogenases activities.

Effect of experimental thyrotoxicosis on oxidative phosphorylation in rat liver, kidney and brain mitochondria.

Coupled phosphorylation was examined in liver, kidney and brain mitochondria from rats made thyrotoxic by injecting repeated doses of triiodothyronine. Liver and kidney mitochondria were maximally affected under these conditions, whereas effects on brain mitochondria were marginal. State-3 respiration rates with succinate decreased considerably in all the tissues, whereas glutamate oxidation increased in liver, but decreased in kidney and brain mitochondria. Oxidation rates of beta-hydroxybutyrate decreased in kidney and brain mitochondria but were not significantly affected in liver mitochondria. Oxidation of ascorbate + TMPD was not affected. State-4 respiration rates increased in general with all the substrates resulting in lowering of the RCI. The ADP/O ratios decreased in a site-specific manner in the mitochondria from the three tissues. The content of cytochrome b decreased in all three tissues, whereas the content of cytochrome c + c1 increased in liver and kidney but decreased in brain. The content of cytochrome a, however, was not significantly affected. Basal and Mg2+-stimulated ATPase activities increased in mitochondria of liver and kidney but not in those of brain; total ATPase activities, however, were not altered. The results imply that excessive levels of thyroid hormones over normal in the serum can lead to impairment of mitochondrial energy metabolism in a tissue-specific manner.

Comparative antioxidant activity of individual herbal components used in Ayurvedic medicine.

Four aqueous extracts from different parts of medicinal plants used in Ayurveda (an ancient Indian Medicine) viz., Momardica charantia Linn (AP1), Glycyrrhiza glabra (AP2), Acacia catechu (AP3), and Terminalia chebula (AP4) were examined for their potential as antioxidants. The antioxidant activity of these extracts was tested by studying the inhibition of radiation induced lipid peroxidation in rat liver microsomes at different doses in the range of 100-600 Gy as estimated by thiobarbituric acid reactive substances (TBARS). Of all these extracts, AP4 showed maximum inhibition in the TBARS formation and hence is considered the best antioxidant among these four extracts. The extracts were found to restore antioxidant enzyme superoxide dismutase (SOD) from the radiation induced damage. The antioxidant capacities were also evaluated in terms of ascorbate equivalents by different methods such as cyclic voltammetry, decay of ABTS(.-) radical by pulse radiolysis and decrease in the absorbance of DPPH radicals. The results were found to be in agreement with the lipid peroxidation data and AP4 showed maximum value of ascorbate equivalents. Therefore AP4, with high antioxidant activity, is considered as the best among these four extracts.

Effect of propranolol on rat brain synaptosomal Na(+)-K(+)-ATPase, Mg(2+)-ATPase and Ca(2+)-ATPase.

The beta blocker drug propranolol (PPL) significantly inhibited Na(+)-K(+)-ATPase, Mg(2+)-ATPase and Ca(2+)-ATPase activities in a concentration dependent manner in rat brain synaptosomes. The concentrations required for 50% inhibition (IC50) in the activity of these enzymes were 1.5-1.8 mM. The double-reciprocal plot of ATP-stimulated Na(+)-K(+)-ATPase activity in the presence of PPL showed apparent decrease in K(m) and Vmax and the inhibition was of an uncompetitive type with respect to ATP. The nature of inhibition by PPL of Na(+)-activated Na(+)-K(+)-ATPase activity was of a mixed type showing an increase in Km and decrease in Vmax. Potassium activation kinetics of Na(+)-K(+)-ATPase displayed uncompetitive type of inhibition with PPL since Km and Vmax were decreased. Magnesium activation of Mg(2+)-ATPase showed decrease in Vmax with no apparent change in Km in the presence of PPL. The drug inhibited synaptosomal Ca(2+)-ATPase in an uncompetitive manner. The observed inhibition of synaptosomal ATPases indicates possible alterations in the synaptic transmission by the beta blocker drug PPL.

Activity of some respiratory enzymes and cytochrome contents in rat hepatic mitochondria following aflatoxin B1 administration.

Effects of aflatoxin B1 (AFB1) administration (7 mg/kg body weight, i.p.) on rat hepatic mitochondrial respiratory components have been examined. Succinoxidase and cytochrome oxidase activities were decreased in liver mitochondria isolated from rats 12-24 h after AFB1 treatment. Both enzyme activities returned to normal levels after 48 h. Glutamate dehydrogenase and beta-hydroxybutyrate dehydrogenase activities did not show any alterations up to 24 h and thereafter increased at 48-72 h. Succinate dehydrogenase activity was impaired by 41% at 12 h and thereafter was found to be normal. The intramitochondrial cytochrome b content declined at 24-72 h, whereas cytochrome aa3 content was decreased maximally at 72 h after AFB1 administration. These observations on mitochondrial enzyme activities and cytochrome contents correlate well with our earlier observations made on hepatic mitochondrial respiratory rates after AFB1 treatment. The impairment of respiratory functions possibly results from membrane damage and selective modification of gene expression in mitochondria imparted by AFB1.

Respiratory functions in kidney mitochondria following paracetamol administration to young-adult & old rats.

Effects of paracetamol treatment in vivo to young-adult (2-3 months) and old (22-24 months) rats at subtoxic (375 mg/kg) and toxic (750 mg/kg) doses on kidney mitochondrial energy metabolism were examined. Administration of paracetamol (both doses) to young-adult animals did not, in general, affect the respiratory functions of kidney mitochondria. On the other hand, treatment of toxic doses to aged animals resulted in decrease in the state 3 respiration rates with glutamate, pyruvate + malate and succinate as substrates. Succinoxidase activity was also impaired in this experimental group. With subtoxic doses, state 3 respiration rate was decreased only with pyruvate + malate as substrate. Results indicate that the in vivo administration of paracetamol impaired kidney mitochondrial energy metabolism in aged animals.

Perturbations in phosphoinositide metabolism and protein kinase C activity in mouse liver following whole body irradiation.

The involvement of the signal transduction pathway in mouse liver following whole body irradition was investigated. Mice were exposed to 60Co gamma rays (3 Gy) and sacrificed after different time intervals. Various elements of phosphatidyl inositol signal transduction pathway were investigated. Alterations could be seen as early as 15 min of irradiation. These changes are reflected in elevation in DAG levels and increased activation of PKC, an enzyme which is involved in tumorigenesis. The chronological appearance of various transducers following whole body irradiation is of significance since these early effects may set the stage for radiation-induced tumorigenesis and hence may be used to manipulate tumor response to radiotherapy.

Effect of aflatoxin B in vitro on rat liver mitochondrial respiratory functions.

Isolated rat liver mitochondria were incubated with various concentrations of aflatoxin B (AFB) for different periods of time. Respiration rates were then measured with two substrates (succinate and glutamate). State 3 respiration rate (with added ADP) declined with increase in preincubation concentrations of AFB1 (0.15-0.50 mM). On the other hand, state 4 respiration rate (after depletion of added ADP) was found to increase with increased pretreatment concentration of AFB. As a consequence, respiratory control index was reduced attaining minimum value with 0.25 mM AFB and preincubation time of 10 min. The induced anomaly in mitochondrial respiratory functions appear to be due to membrane damage caused by interaction of reactive AFB1 metabolite generated by mitochondrial cytochrome P-450 enzymes with mitochondrial components.

Effect of streptozotocin-induced diabetes on oxidative energy metabolism in rat kidney mitochondria. A comparative study of early and late effects.

AIM: The effects of streptozotocin (STZ)-induced diabetes on oxidative energy metabolism of rat kidney mitochondria were examined at the end of 1 week and 1 month of STZ treatment. METHODS: At the end of 1 week of induction of diabetes, respiration rates with pyruvate + malate and succinate as the substrates increased while those with beta-hydroxybutyrate and ascorbate + TMPD decreased. Respiration with glutamate was not affected. Insulin treatment had no alleviating effect. The changes persisted through 1 month of induction of diabetes and were not corrected by insulin treatment even at this stage. beta-hydroxybutyrate dehydrogenase activity registered significant decrease while the succinate dehydrogenase activity increased in diabetic and insulin-treated diabetic animals whereas only marginal changes were evident in the composition of the cytochromes. RESULTS: The ATPase activity tended to be high in the diabetic groups and was restored by insulin treatment. At both the stages, i.e. early and late stages of diabetes the mitochondria were tightly coupled and the ADP/O ratios were in normal expected ranges. CONCLUSION: Taken together, the results suggest that kidney is the major target tissue to suffer impairment of mitochondrial function with the onset of the disease which persists throughout and that insulin treatment is ineffective in restoring the normal state.

Radiation protection of DNA and membrane in vitro by extract of Hemidesmus indicus.

Radioprotective effect of H. indicus root extract on lipid peroxidation in rat liver microsomes and plasmid DNA was examined. Hemidesmus indicus (HI) root extract was found to protect microsomal membranes as evident from reduction in lipid peroxidation values. The extract could also protect DNA from radiation induced strand breaks.

Thyroid hormones and cathepsin D activity in the rat liver, kidney and brain.

The effect of thyroidectomy and subsequent treatment with tri-iodothyronine (T3), as well as that of thyrotoxicosis, was examined on cathepsin D activity in the rat liver, kidney and brain. Thyrotoxicosis resulted in a generalized increase in the enzyme activity in the 3 tissues; the effect of other thyroidal states was diverse and tissue-specific.

Insulin-dependent changes in lysosomal cathepsin D activity in rat liver, kidney, brain and heart.

Streptozotocin-induced diabetes resulted in a decrease in the cathepsin D activity (free and total) in rat liver, kidney, brain and heart with a concomitant increase in tissue protein content and amino acids pool size. Treatment with insulin brought about the restoration of the cathepsin D activity to normal or greater than normal levels; tissue protein content and amino acids pool size also returned to normal values.

Influence of insulin status on extra-mitochondrial oxygen metabolism in the rat.

The effects of alloxan diabetes and subsequent treatment with insulin on extra-mitochondrial oxygen metabolism in terms of D-amino acid oxidase (DAAO), xanthine oxidase and catalase were examined. The DAAO activity in the liver with D-alanine and D-serine decreased by 33-62% in the diabetic group while the decrease in the kidneys was 61-74%. Insulin treatment resulted in overstimulation of DAAO activity in the liver but not in the kidneys. Tissue glycogen content was lowered in the diabetic animals but was restored by insulin treatment. Tissue glycogen content and DAAO activity showed an inverse relationship. The xanthine oxidase activity in the two tissues decreased from 40-55%; the catalase activity decreased from 34-54%. Insulin treatment was unable to restore the xanthine oxidase and catalase activities in both the tissues.

Photoaffinity labeling of human c-myc protein with deoxythymidine triphosphate.

The recombinant human c-myc protein expressed in Escherichia coli can be efficiently labeled by ultraviolet-mediated cross-linking to dTTP and to a lesser extent to other nucleoside diphosphates and triphosphates, but not to nucleoside monophosphates. Specificity of nucleoside phosphate binding is suggested by (a) concentration-dependent competition by some nucleoside phosphates but not by others and (b) by the observation that the denatured myc protein does not bind the nucleotides. Competition experiments also indicate that the affinity of c-myc protein for nucleoside diphosphates and triphosphates is approximately the same irrespective of the nature of the base, or of the pentose sugar, but the thymine base permits the most efficient photoactivated cross-linking. The ultraviolet-mediated photoactivated cross-linking of deoxythymidine triphosphate has been used to identify the c-myc protein in extracts of cells which overexpress c-myc, and to identify the intermediates in myc oncoprotein degradation.

Alteration of energy-linked functions in rat hepatic mitochondria following aflatoxin B1 administration.

Respiratory activity in hepatic mitochondria have been examined following administration of the carcinogen aflatoxin, (AFB1) to rats. Measurement in isolated mitochondria of respiration rates in presence of ADP (state 3) and after its depletion (state 4) revealed that these rates were not significantly altered in livers of rats obtained 4-8 hours after single injection of AFB1 (7 mg/kg of body weight). After 12-24 hours, however, a generalized inhibition in state 3 respiration rate and ADP phosphorylation rate had been evident with several FAD- and NAD-linked oxidizing substrates. But the ADP:0 ratio did not show any alteration. State 4 respiration rates, on the other hand, were increased remarkably (38-94% depending on substrate used), thereby recording in each case a decrease in respiratory control ratio (state 3:state 4 ratio), indicating probable damage to mitochondrial membrane as a result of AFB1 ingestion. This was also evident from greater basal ATPase and Mg(2+)-ATPase activities and low total ATPase activity. After 48-72 hours of AFB1 treatment, the respiratory rates as well as the ATPase activities returned to normal levels, suggesting probable recovery of mitochondrial functions from the toxic effects of AFB1.