Modulation of taurine levels in the rat liver alters methylene dianiline hepatotoxicity

Protective effect of taurine against potassium bromate-induced hemoglobin oxidation, oxidative stress, and impairment of antioxidant defense system in blood

Potassium bromate (KBrO3 ) is widely used as a food-additive and is a major water disinfection by-product. KBrO3 causes severe toxicity in humans and experimental animals. Bromate is considered a probable human carcinogen and a complete carcinogen in animals. We have investigated the potential role of taurine in protecting against KBrO3 -induced oxidative stress in rat blood. Animals were given taurine for 5 days prior to KBrO3 and then sacrificed. Blood was collected and used to prepare hemolysates and plasma, which were then used for the analysis of several biochemical parameters. Administration of single oral dose of KBrO3 alone induced hepato- and nephro-toxicity as evident by elevated marker levels in plasma. Lipid peroxidation and protein oxidation were increased both in plasma and erythrocytes, suggesting the induction of oxidative stress. KBrO3 increased methemoglobin, nitric oxide, and hydrogen peroxide levels. It also altered the activities of the major antioxidant enzymes and lowered the antioxidant power of blood. Administration of taurine, prior to treatment with KBrO3 , resulted in significant attenuation in all these parameters but the administration of taurine alone had no effect. These results show that taurine is effective in mitigating the oxidative insult induced in rat blood by KBrO3 .

Radiation protection following nuclear power accidents: a survey of putative mechanisms involved in the radioprotective actions of taurine during and after radiation exposure

There are several animal experiments showing that high doses of ionizing radiation lead to strongly enhanced leakage of taurine from damaged cells into the extracellular fluid, followed by enhanced urinary excretion. This radiation-induced taurine depletion can itself have various harmful effects (as will also be the case when taurine depletion is due to other causes, such as alcohol abuse or cancer therapy with cytotoxic drugs), but taurine supplementation has been shown to have radioprotective effects apparently going beyond what might be expected just as a consequence of correcting the harmful consequences of taurine deficiency per se. The mechanisms accounting for the radioprotective effects of taurine are, however, very incompletely understood. In this article an attempt is made to survey various mechanisms that potentially might be involved as parts of the explanation for the overall beneficial effect of high levels of taurine that has been found in experiments with animals or isolated cells exposed to high doses of ionizing radiation. It is proposed that taurine may have radioprotective effects by a combination of several mechanisms: (1) during the exposure to ionizing radiation by functioning as an antioxidant, but perhaps more because it counteracts the prooxidant catalytic effect of iron rather than functioning as an important scavenger of harmful molecules itself, (2) after the ionizing radiation exposure by helping to reduce the intensity of the post-traumatic inflammatory response, and thus reducing the extent of tissue damage that develops because of severe inflammation rather than as a direct effect of the ionizing radiation per se, (3) by functioning as a growth factor helping to enhance the growth rate of leukocytes and leukocyte progenitor cells and perhaps also of other rapidly proliferating cell types, such as enterocyte progenitor cells, which may be important for immunological recovery and perhaps also for rapid repair of various damaged tissues, especially in the intestines, and (4) by functioning as an antifibrogenic agent. A detailed discussion is given of possible mechanisms involved both in the antioxidant effects of taurine, in its anti-inflammatory effects and in its role as a growth factor for leukocytes and nerve cells, which might be closely related to its role as an osmolyte important for cellular volume regulation because of the close connection between cell volume regulation and the regulation of protein synthesis as well as cellular protein degradation. While taurine supplementation alone would be expected to exert a therapeutic effect far better than negligible in patients that have been exposed to high doses of ionizing radiation, it may on theoretical grounds be expected that much better results may be obtained by using taurine as part of a multifactorial treatment strategy, where it may interact synergistically with several other nutrients, hormones or other drugs for optimizing antioxidant protection and minimizing harmful posttraumatic inflammatory reactions, while using other nutrients to optimize DNA and tissue repair processes, and using a combination of good diet, immunostimulatory hormones and perhaps other nontoxic immunostimulants (such as beta-glucans) for optimizing the recovery of antiviral and antibacterial immune functions. Similar multifactorial treatment strategies may presumably be helpful in several other disease situations (including severe infectious diseases and severe asthma) as well as for treatment of acute intoxications or acute injuries (both mechanical ones and severe burns) where severely enhanced oxidative and/or nitrative stress and/or too much secretion of vasodilatory neuropeptides from C-fibres are important parts of the pathogenetic mechanisms that may lead to the death of the patient. Some case histories (with discussion of some of those mechanisms that may have been responsible for the observed therapeutic outcome) are given for illustration of the likely validity of these concepts and their relevance both for treatment of severe infections and non-infectious inflammatory diseases such as asthma and rheumatoid arthritis.

Effect of β-alanine administration on carbon tetrachloride-induced acute hepatotoxicity

Mice were supplemented with beta-alanine (3%) in drinking water for one week. beta-Alanine intake reduced hepatic taurine levels, but elevated cysteine levels significantly. Hepatotoxicity of CCl4 in mice fed with beta-alanine was decreased as determined by changes in serum enzyme activities. Hepatic glutathione and taurine concentrations after CCl4 challenge were increased markedly by beta-alanine intake. The enhanced availability of cysteine for synthesis of glutathione and/or taurine appears to account for the hepatoprotective effects of beta-alanine against CCl4-induced liver injury.

N-oxidation of aromatic amines by intracellular oxidases

The introduction includes a literature review of DNA reactive species and DNA adduct formation that results from aromatic amine N-oxidation catalyzed by hepatic cytochrome P450 vs. that catalyzed by nonhepatic peroxidases. Experimental evidence is then described for a novel oxidative stress mechanism involving prooxidant N-cation radical formation by both oxidases, which is proposed as a contributing mechanism for aromatic amine induced cytotoxicity and carcinogenesis. Aromatic amine N-cation radicals formed by peroxidases were found to cooxidize GSH or NADH and form reactive oxygen species. The latter could explain the reported DNA oxidative damage found in vivo following methylaminoazobenzene administration [Hirano et al. Analyses of Oxidative DNA Damage and Its Repair Activity in the Livers of 3'-Methyl-4-dimethylaminoazobenzene-Treated Rodents. Jpn. J. Cancer Res. 2000, 91, 681-685]. It was also found that the prooxidant activity of the aromatic amine increased as its redox potential, i.e., ease of oxidation decreased with o-anisidine and aminofluorene being the most effective at forming reactive oxygen species. This suggests that the rate-limiting step in the cooxidation is the rate of arylamine oxidation by the peroxidase. Incubation of hepatocytes with aromatic amines caused a decrease in the mitochondrial membrane potential before cytotoxicity ensued. The CYP1A2-induced hepatocytes isolated from 3-methylcholanthrene administered rats were much more susceptible to some arylamines and were protected by CYP1A2 inhibitors. Hepatocyte GSH was also depleted by all arylamines tested and extensive GSH oxidation occurred with o-anisidine and aminofluorene, which was prevented by CYP1A2 inhibitors. This suggests that in intact hepatocytes CYP1A2 may also catalyze a one-electron oxidation of some arylamines to form prooxidant cation radicals, which cooxidize GSH to form the reactive oxygen species.

Taurine: protective properties against ethanol-induced hepatic steatosis and lipid peroxidation during chronic ethanol consumption in rats

Alcohol was administered chronically to female Sprague Dawley rats in a nutritionally adequate totally liquid diet for 28 days. This resulted in hepatic steatosis and lipid peroxidation. Taurine, when co-administered with alcohol, reduced the hepatic steatosis and completely prevented lipid peroxidation. The protective properties of taurine in preventing fatty liver were also demonstrated histologically. Although alcohol was found not to affect the urinary excretion of taurine (a non-invasive marker of liver damage), levels of serum and liver taurine were markedly raised in animals receiving alcohol + taurine compared to animals given taurine alone. The ethanol-inducible form of cytochrome P-450 (CYP2E1) was significantly induced by alcohol; the activity was significantly lower than controls and barely detectable in animals fed the liquid alcohol diet containing taurine. In addition, alcohol significantly increased homocysteine excretion into urine throughout the 28 day period of ethanol administration; however, taurine did not prevent this increase. There was evidence of slight cholestasis in animals treated with alcohol and alcohol + taurine, as indicated by raised serum bile acids and alkaline phosphatase (ALP). The protective effects of taurine were attributed to the potential of bile acids, especially taurine conjugated bile acids (taurocholic acid) to inhibit the activity of some microsomal enzymes (CYP2E1). These in vivo findings demonstrate for the first time that hepatic steatosis and lipid peroxidation, occurring as a result of chronic alcohol consumption, can be ameliorated by administration of taurine to rats.