Purpose: To determine the extent to which the results of in vitro tests for antioxidant activity for taurine and a group of structurally-related compounds correlate with their antioxidant and cellular protective actions in the liver of rats exposed to acetaminophen (APAP).
Methods: The compounds used in the study were N-acetyltaurine (ATAU), hypotaurine (HYTAU), N-methyltaurine (MTAU), pantoyltaurine (PTAU), taurine (TAU) and thiotaurine (TTAU). In vitro antioxidant-antiradical activity was investigated using a battery of seven tests (AAPH, DPPH, deoxyribose, Fe(III)-dopamine, sodium nitroprusside-dopamine, unsaturated fatty-acid/Fe(III)-dopamine, superoxide anion) and N-acetylcysteine (NAC) as a reference compound. In vivo studies were carried out with male Sprague-Dawley rats, 225-250 g, in groups of 6. All treatments were by the intraperitoneal route. The dose of APAP (in 50% PEG 40) was 800 mg/kg. The treatment compounds were given 30 min before APAP at a dose of 2.4 mM/kg. Control animals received only 50% PEG 400. The rats were sacrificed by decapitation at 6 hr after receiving APAP, and their blood processed for the plasma fractions, which were used to measure alanine transaminase (ALT), aspartate transaminase (AST) and lactate dehydrogenase (LDH) activities. The livers, isolated by the freeze clamping technique, were used two prepare either of two homogenates, type A, made in 10 mL of Tris buffer pH 7.0 containing 1 mg of phenylmethylsulfonyl fluoride (0.5 g/10 mL ratio) and type B, made in 25% metaphosphoric acid plus PBS pH 8.0 (0.25 g/1 mL/3.75 mL ratio). In each case the suspension was centrifuged at 4oC to remove insoluble matter. Homogenate A was used to measure malondialdehyde (MDA) and homogenate B was used to measure reduced (GSH) and oxidized (GSSG) glutathione
Results: In the vitro tests, the reference compound NAC was invariably the most potent in decreasing free radical-lipid peroxidation (by 89-95%). Among the test compounds, PTAU (60-91% decreases) and TTAU (68-84% decreases) were the most potent ones, followed by ATAU (53-60% decreases), with TAU (18-66% decreases) and MTAU (14-28% decreases) appearing as the weakest. HYTAU, a sulfinic compound, was about equipotent with ATAU (53-69% decreases) except for a weaker effect in the superoxide anion test (38% decrease). In the rat, APAP was found to increase the liver MDA (by 134%) and to lower GSH (by 76%), GSSG (by 47% and the GSH/GSSG ratio (by 54%). Furthermore, relative to control values the plasma ALT, AST and LDH activities, serving as indices of hepatocellular damage, were markedly elevated (by 64%, 135% and 292%, respectively). Among the test compounds, PTAU, HYTAU and TTAU, in that order, were the most potent in lowering hepatic MDA formation (up by only 6%, 12% and 14%, respectively), followed by TAU (22%), with ATAU and MTAU providing little protection (68% and 80% increases, respectively).NAC was about equipotent with TTAU (only 16% increase). In terms of the GSH/GSSG ratio, only TTAU (+66%) and HYTAU (+56%) were able to reverse the effect of APAP, with PTAU (-12%), ATAU (-29%), TAU (-29%) and MTAU 9-50%) providing a significant although lesser protection. A pretreatment with NAC was also very protective (+46%). All of the treatment agents were able to lower the release of liver enzymes, with HYTAU providing the greatest effect, followed by PTAU, TAU and TTAU in that order. In contrast, ATAU and MTAU were the least potent and NAC the most.
Conclusion: The present results indicate that in vitro testing of TAU and structurally related compounds for antioxidant-antiradical activity may serve as a fast qualitative approach for predicting their potential antioxidant potencies and abilities to protect the liver integrity against APAP toxicity under in vivo conditions. In turn, these results point to the role played by structural features in the noted protection.
Cesar Lau-Cam– Professor of Pharmaceutical Sciences, St. John's University, Jamaica, New York