Molecular inhibitory mechanisms of antioxidant enzymes in rat liver and kidney by cadmium

Catalase, Mn-superoxide dismutase (MnSOD) and Cu,Zn-superoxide dismutase (CuZnSOD) activities were studied in rat liver and kidney 6-48 h after CdCl(2) intraperitoneal administration or 10-30 days daily oral CdCl(2) intake in drinking water. This approach provided some indications as to the sensitivity of each enzyme to cadmium toxicity. These experiments showed that the formation of thiobarbituric acid reactive substance (TBARS) did not strictly depend on how well the antioxidant enzyme worked. From in vitro experiments it appeared that TBARS removal by vitamin E did not restore the three enzyme activities at all. As for cadmium's inhibitory mechanism on catalase activity, our data, obtained in the pH range 6.0-8.0, are a preliminary indication that the negative effect of this metal is probably due to imidazole residue binding of His-74 which is essential in the decomposition of hydrogen peroxide. Cadmium inhibition of liver mitochondrial MnSOD activity was completely removed by Mn(2+) ions, suggesting that the reducing effect on this enzyme is probably due to the substitution of cadmium for manganese. We also observed the antioxidant capacity of Mn(2+) ions, since they were able to normalize the increased TBARS levels occurring when liver mitochondria were exposed to cadmium. The reduced activity of CuZnSOD does not seem to be due to the replacement of Zn by Cd, nor to the peroxides formed. As this enzyme activity was almost completely recovered after 48 h, we hypothesize that the momentary inhibition is imputable to a cadmium/enzyme interaction. This causes some perturbation in the enzyme topography which is critical for its catalytic activity. The pathological implications linked to antioxidant enzyme disorders induced by cadmium toxicity are discussed.

Enzyme activity alteration by cadmium administration to rats: the possibility of iron involvement in lipid peroxidation

The specific activities of D-3-hydroxybutyrate dehydrogenase (BDH) and glutamate dehydrogenase (GDH) are reduced in the liver and kidney of rats intoxicated with 2.5 mg Cd/kg body wt and sacrificed after 24 h; conversely ketone-body concentration is strongly increased in both of these organs and blood. In the same animals a great stimulation of antioxidant enzymes glutathione reductase and glutathione peroxidase occurs. The prooxidant state induced by cadmium in liver mitochondria and microsomes is unaffected by superoxide dismutase, catalase, or mannitol, whereas it is completely blocked by vitamin E thus excluding the involvement of reactive oxygen species in this process. The mechanism by which cadmium induces lipid peroxidation has been investigated by measuring the effect of this metal on liposomes. Ninety-minute treatment of liposomes with CdCl2 does not induce any lipid peroxidation. In contrast, Fe2+ ions under the same conditions cause strong liposome peroxidation. It has also been observed that cadmium promotes a time-dependent iron release from biological membranes. When lipid peroxidation is induced by a low concentration (5 microM) of FeCl2, in place of CdCl2, the characteristics of this process and the sensitivity to the various antioxidants used are similar to those observed with Cd. From these results we conclude that the prooxidative effect of cadmium is an indirect one since it is mediated by iron. With regard to the inhibitory effect on BDH and GDH following cadmium intoxication, it does not appear to be imputable to lipid peroxidation since in vitro investigations indicate that the presence of vitamin E does not remove the inhibition at all.

Lipid composition of liver mitochondria and microsomes in hyperthyroid rats

Triiodothyronine-induced alteration of the lipid pattern in rat-liver mitochondria and microsomes has been investigated. In mitochondria, a 25% total cholesterol decrease and a 14% phospholipid increase have been detected. In these hyperthyroid rat liver organelles, a strong decrease in the total cholesterol/phospholipid molar ratio occurs. On the contrary, in microsomes from the same animals, a decrease of about 23% has been measured for both total cholesterol and phospholipids; hence, in this fraction, the total cholesterol/phospholipid molar ratio is unaffected by hyperthyroidism. The liver mitochondrial phospholipid composition, unlike the microsomal composition, is altered significantly in hyperthyroid rats; a 7.4% phosphatidylcholine decrease is accompanied by a similar additive percentage increase of both phosphatidylethanolamine and cardiolipin. In regard to total phospholipid fatty acid composition in liver microsomes from hyperthyroid rats, no variation has been observed compared with the control rats, whereas in mitochondria from the same animals, a meaningful linoleic acid decrease with a similar arachidonic acid increase has been found. In addition to fatty acid alteration, the separated mitochondrial phospholipid classes also exhibit some increase in stearic acid. Among phospholipids, cardiolipin changes the most of the esterified fatty acids in hyperthyroid rat liver. In this compound, a strong increase in the percentage of both palmitic and stearic acid and a 32.4% decrease of linoleic acid have been found.

Cadmium-dependent enzyme activity alteration is not imputable to lipid peroxidation

The effect of cadmium on the liver-specific activities of NADPH-cytochrome P450 reductase (CPR), malic dehydrogenase (MDH), glyceraldehyde-3-phosphate dehydrogenase (GADPH), and sorbitol dehydrogenase (SDH) was assessed 6, 24, and 48 h after administration of the metal to rats (2.5 mg/kg of body weight, as CdCl2, single ip injection). CPR specific activity increased after 6 h and afterward decreased significantly, while MDH specific activity increased up to 24 h and then remained unchanged. Both SDH and GADPH specific activities reduced after 6 h, the former only a little but the latter much more, and after 24 and 48 h were strongly inhibited. In vitro experiments, by incubating rat liver microsomes, mitochondria, or cytosol with CdCl2 in the pH range 6.0-8.0, excluded cadmium-induced lipid peroxidation as the cause of the reduction in enzyme activity. In addition, from these experiments, we obtained indications on the type of interactions between cadmium and the enzymes studied. In the case of CPR, the inhibitory effect is probably due to Cd2+ binding to the histidine residue of the apoenzyme, which, at physiological pH, acts as a nucleophilic group. In vitro, mitochondrial MDH was not significantly affected by cadmium at any pH, indicating that this enzyme is probably not involved in the decrease in mitochondrial respiration caused by this metal. As for GADPH specific activity, its inhibition at pH 7.4 and above is imputable to the binding of cadmium to the SH groups present in the enzyme active site, since in the presence of dithiothreitol this inhibition was removed. SDH was subjected to a dual effect when cytosol was exposed to cadmium. At pH 6.0 and 6.5, its activity was strongly stimulated up to 75 microM CdCl2 while at higher metal concentrations it was reduced. At pH 7.4 and 8.0, a stimulation up to 50 microM CdCl2 occurred but above this concentration, a reduction was found. These data seem to indicate that cadmium can bind to different enzyme sites. One, at low cadmium concentration, stimulates the SDH activity while the other, at higher metal concentrations, substitutes for zinc, thus causing inhibition. This last possibility seems to occur in vivo essentially at least 24 h after intoxication. The cadmium-induced alterations of the investigated enzymes are discussed in terms of the metabolic disorders produced which are responsible for several pathological conditions.

Effect of thyroid hormones on microsomal fatty acid chain elongation synthesis in rat liver

Evidence is presented that rat liver microsomal fatty acid chain elongation synthesis and desaturation, as well as acetyl-CoA carboxylase and fatty acid synthetase, are strongly influenced by thyroid hormone level. Conversely, the fatty acid chain elongation system in mitochondria, unlike the oxidative capacity of palmitate, NADH, succinate and malate, does not seem significantly affected by the thyrotoxic state. In triiodothyronine-induced or thyroxine-induced hyperthyroidism, rat liver acetyl-CoA carboxylase, fatty acid synthetase and microsomal chain elongation and desaturation reactions are not greatly affected after the first 10 days of treatment, while after longer intervals a respective increase in these activities is shown of up to 87, 116 and 65% after 22 days. In propylthiouracil-induced hypothyroidism, all the above synthetic activities are strongly reduced immediately after three days of drug administration and diminished no further following longer periods. Although the pattern of synthesized fatty acids in the thyrotoxic state is similar to that obtained from normal subcellular rat fractions, the esterification process of fatty acids in microsomal lipids appears to be slightly inhibited in hypothyroid rats and increased following triiodothyronine or thyroxine administration. Finally, a reduction in the hepatic cyclic AMP level of about 41% is reported after 19 days of triiodothyronine-administration to rats. On the basis of the observed insensitivity of the mitochondrial fatty acid chain elongation system to the thyrotoxic state, a tentative interpretation of its role in the hepatic cell is postulated.

Changes in liver enzyme activity in the teleost Sparus aurata in response to cadmium intoxication

Enzyme activity modulation by cadmium in the liver of the teleost fish Sparus aurata was investigated in vivo following 3 and 6 days of CdCl2 administration (2.5 mg/kg body wt). The specific activities of the mitochondrial enzymes NAD-isocitrate dehydrogenase, succinate dehydrogenase, and malate dehydrogenase were stimulated by approximately 20% after 3 days administration and were further increased (by about 40%) after 6 days treatment. In comparison with these enzymes, the activities of glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT) in mitochondria were less stimulated after the two indicated intervals of treatment. Cadmium significantly reduced the activities of liver cytoplasmic GOT and GPT while a simultaneous increase occurred in the serum activities of these same enzymes. The activity of liver NADPH-cytochrome P450 reductase was stimulated by 25 and 40% after 3 and 6 days cadmium intoxication, respectively. Lastly, the antioxidant enzymes glutathione peroxidase and glutathione reductase in liver and catalase in both liver and blood were strongly reduced after 3 and 6 days cadmium administration. These data suggest that cadmium in fish hepatocytes alters cell membrane structure and concomitantly induces some perturbation in the integrity of the mitochondrial membrane.

The response of rat liver lipid peroxidation, antioxidant enzyme activities and glutathione concentration to the thyroid hormone

1. In liver microsomes from hyperthyroid rats NADPH-dependent lipid peroxidation induces a hydroperoxide formation 56% higher than that in euthyroid ones. 2. The addition of 5 microM Fe2+ (or Fe3+) strongly decreases the hydroperoxide level in favour of that of TBA-reactive substances. Higher iron concentrations (30 microM) have no significant effect. 3. In hepatocytes from hyperthyroid rats CCl4-induced lipid peroxidation produces an amount of TBA-reactive substances four times higher than that in those from euthyroid rats. 4. In the liver of hyperthyroid rats a GSH concentration decrease (by about 35%) is found while the opposite occurs in the blood of the same animals where GSH increases 2.5 times. 5. It is shown that in the liver of hyperthyroid rats, besides higher lipid peroxidation, a more active defense mechanism is operating since both glutathione peroxidase and glutathione reductase specific activities are higher than in euthyroid rats.

Turnover of fatty acids in rat liver cardiolipin: comparison with other mitochondrial phospholipids

Following intraperitoneal administration of 1-14C-linoleic acid or 2-3H-acetate to rats, the specific radioactivities of both liver cardiolipin and other mitochondrial phospholipids after different time intervals were measured. Comparison of the data obtained with those from another stock of rats treated with 32P-phosphate or 2-3H-glycerol showed that the fatty acids of cardiolipin, like those of other phospholipids, exhibit an independent turnover with respect to the remaining parts of the molecule. The half-life of acyl moieties of cardiolipin is ca. 20% higher than that of the same components of other mitochondrial phospholipids. Moreover, it appears that, in both cardiolipin and other phospholipids, linoleyl residues turn over faster than nonessential fatty acids. Discussion is made as to whether this characteristic can be related to the role of phospholipids in the functioning of some enzymes bound to the inner mitochondrial membrane.

The Nrf2 transcription factor contributes to the induction of alpha-class GST isoenzymes in liver of acute cadmium or manganese intoxicated rats: comparison with the toxic effect on NAD(P)H:quinone reductase

In rat liver, in addition to their intrinsic transferase activity, alpha-class GSTs have Se-independent glutathione peroxidase activity toward fatty acid hydroperoxides, cumene hydroperoxide and phospholipids hydroperoxides but not toward H(2)O(2.) We have previously shown that hepatic GST activity by these isoenzymes is significantly increased 24h after cadmium or manganese administration (Casalino et al., 2004). Here it is reported that Se-independent glutathione peroxidase activity by alpha-class GSTs is also stimulated in the liver of intoxicated rats. The stimulation is associated with a higher level of alpha-class GST proteins, whose induction is blocked by actinomycin D co-administration. The observed Se-independent glutathione peroxidase activity is due to alpha-class GST isoenzymes, as indicated by the studies with diethyldithiocarbamate which, at any concentration, equally inhibits both GST and Se-independent glutathione peroxidase and is an uncompetitive inhibitor of both enzymes. As for liver Se-GSPx, it is not at all affected under these toxic conditions. For comparison, we have evaluated the status of another important antioxidant enzyme, NAD(P)H:quinone reductase, 24h after cadmium or manganese administration. NQO1 too results strongly stimulated in the liver of the intoxicated rats. In these animals, a higher expression of Nrf2 protein is observed, actively translocated from the cytoplasm to the nucleus. The results with the transcription inhibitor, actinomycin D, and the effects on Nrf2 protein are the first clear indication that acute manganese intoxication, similarly to that of cadmium and other heavy metals, increases both the hepatic level of Nrf2 and its transfer from the cytoplasm to the nucleus where it actively regulates the induction of phase II enzymes.

Rat liver glutathione S-transferase activity stimulation following acute cadmium or manganese intoxication

The effect of cadmium or manganese administration on rat liver glutathione S-transferase (GST) has been investigated. The activity of this enzyme in liver cytosol, where almost all the cellular activity is present, had increased by more than 36% 24 h after a single i.p. injection of CdCl(2) (2.5 mg kg(-1) b.w.) or MnCl(2) (2.0 mg kg(-1) b.w.). After shorter and longer time intervals, a lower enzyme activity stimulation was observed in both cases. When liver cytosol was incubated for 10 min with 75 microM CdCl(2) or 40 microM MnCl(2), no effect was observed on enzyme activity. The increase in GST following cadmium or manganese administration was blocked by prior administration of actinomycin D, indicative of a possible transcription-dependent response. The liver soluble GST from both control and metal-treated rats was not at all affected by Vitamin E, in the range of 20-300 microM. By contrast, hematin was seen to be a competitive inhibitor of this liver enzyme from both types of rats by using CDNB as substrate and the K(i) value was equal to 0.22 microM. The possibility that under the conditions used class alpha GST isoenzymes are affected by cadmium or manganese is discussed.

Antioxidant effect of hydroxytyrosol (DPE) and Mn2+ in liver of cadmium-intoxicated rats

Liver TBARS formation in cadmium-intoxicated rats was completely reduced by administering a low amount of MnCl(2) (2 mg/kg b.w.) 1 h before intoxication. A similar antioxidant effect was first shown by hydroxytyrosol (2-(3,4-dihydroxyphenyl)ethanol, (DPE), a phenolic compound present in olive oil, given twice to rats (9 mg/kg b.w.) after cadmium administration. The antioxidant properties shown in vivo by both Mn(2+) and DPE were also active in vitro when rat liver microsomes were subjected to lipid peroxidation by cadmium or other prooxidant systems. The increase in liver glutathione concentrations occurring in cadmium-intoxicated rats, was also found, for the first time, 24 h after MnCl(2) administration. Unlike cadmium intoxication, which caused a higher formation of both glutathione and TBARS, Mn(2+) induced glutathione synthesis without any TBARS formation. The same situation was also observed when cadmium plus Mn(2+) or cadmium plus DPE was given to rats. Our data show that: (a). both DPE and low Mn(2+) concentrations may have an antioxidant effect in the livers of cadmium-intoxicated rats and (b). Mn(2+), like cadmium, induces liver glutathione synthesis and this effect is probably independent of TBARS formation.

Acute cadmium intoxication induces alpha-class glutathione S-transferase protein synthesis and enzyme activity in rat liver

Acute cadmium intoxication affects glutathione S-transferase (GST) in rat liver. It has been found that 24h after i.p. cadmium administration to rats, at a dose of 2.5 mg CdCl(2)kg(-1) body weight, the activity of this enzyme in liver cytosol increased by 40%. A less stimulatory effect persisted till 48 h and thereafter the enzyme activity normalized. Since, GST isoenzymes belong to different classes in mammalian tissues, we used quantitative immunoassays to verify which family of GST isoenzymes is influenced by this intoxication. Only alpha-class glutathione S-transferase (alpha-GST) proteins were detected in rat liver cytosol and their level increased by about 25%, 24h after cadmium treatment. No pi-GST isoforms were found in liver cytosol from either normal or cadmium-treated rats. Co-administration of actinomycin D with cadmium normalized both the protein level and the activity of alpha-GST, suggesting that some effect occurs on enzyme transcription of these isoenzymes by this metal. On the other hand, it seems unlikely that the stimulatory effect is due to the high level of peroxides caused by lipid peroxidation, since Vitamin E administration strongly reduced the TBARS level, but did not cause any GST activity decrease.

Effect of hyperthyroidism on lipogenesis in brown adipose tissue of young rats

Fatty acid synthetic capacity, investigated both in subcellular fractions and in vivo, is very active in brown adipose tissue of room temperature-acclimated rats. In hyperthyroid animals this tissue, analogously to the liver, exhibits an increased activity of acetyl-CoA carboxylase, fatty acid synthetase and microsomal fatty acid chain elongation, this last mechanism remaining unaffected in mitochondria. An enhancement of reducing capacities of a group of cytoplasmic NADP-dependent enzymes has also been observed in brown adipose tissue of hyperthyroid rats, probably due to a greater use of NADPH in lipogenesis under these conditions. An increase in palmitate oxidation and in polyenoic fatty acids was observed in mitochondria of brown adipose tissue from hyperthyroid animals. The latter increase is related to the importance of these compounds in the regulation of membrane fluidity and probably to an increased resistance to cold in the hyperthyroid state.

A possible mechanism for initiation of lipid peroxidation by ascorbate in rat liver microsomes

The mechanism by which lipid peroxidation progresses has been known for years, but there is disagreement regarding the mode of its initiation. The aim of this study was to examine: (a) the role of endogenous iron in the initiation of ascorbate-induced lipid peroxidation in microsomal and liposomal membranes; (b) the role of oxygen-free radicals in this process; and (c) the redox state of ascorbate during the course of lipid peroxidation. Ascorbate-induced lipid peroxidation was assessed by measuring hydroperoxide and thiobarbituric acid reactive substances (TBARS) formation in membranes after incubation in Tris-HCl buffer (pH 7.4) for 15 min. To confirm the role of endogenous iron and oxygen-free radicals, the effect of iron chelating agents (EDTA and thiourea) and radical scavengers (benzoate, mannitol, catalase and SOD) on lipid peroxidation was examined. Spectrophotometric measurements and ESR spectra have made it possible to determine ascorbate concentration and its redox state. Ascorbate promoted lipid peroxidation in both rat liver microsomes and liposomes without addition of exogenous iron. Iron chelating agents such as EDTA and thiourea inhibited lipid peroxidation, while SOD, catalase, mannitol and benzoate had no effect. The addition of 5 microM Fe2+ (or Fe3+) to the incubation mixture did not significantly alter hydroperoxide production, but that of TBARS was increased. Lipid peroxidation significantly altered the fatty acid profile in microsomes and liposomes, the most affected being the C20:4 and C22:6 species. Ascorbate in Tris-HCl buffer (pH 7.4) autoxidized very slowly. Its oxidation was catalyzed by Fe3+ ions at a rate determined by incubation time and iron concentration. In contrast, no ascorbate oxidation occurred in the presence of microsomes when lipid peroxidation was proceeding at a maximal rate. Under these conditions a typical ascorbyl radical ESR spectrum signal greater than that arising from ascorbate alone was obtained and the magnitude of this signal was unchanged by variations of microsome or ascorbate concentrations. A ferrous ion ascorbyl radical complex was responsible for this signal. These results suggest that an ascorbate-microsomal iron complex is responsible for the initiation of lipid peroxidation, and that during this process ascorbate remains in its reduced form.

Alteration of plasma and erythrocyte membrane lipid components in hyperthyroid rats

Cholesterol and phospholipid have been measured in plasma and erythrocyte membrane of hyperthyroid rats. It has been found that while the former is reduced by about 30% in plasma and increased by the same amount in erythrocyte membranes, on the contrary, the latter increases by 35% in both plasma and red cell membranes. It seems that when serum triiodothyronine increases, a major cholesterol transfer occurs from plasma to erythrocyte. In this way, by the concomitant phospholipid increase, it is possible to avoid an alteration of the cholesterol/phospholipid molar ratio in the red cells, thus preventing their abnormal function in hyperthyroid rats. The proposal is made that an additional reason for the plasma cholesterol decrease in hyperthyroid subjects can be attributable to a net transfer of this compound from plasma to erythrocyte.