An ex vivo erythrocyte study : the protective effects of antioxidants against the toxicity of heavy metals in cigarette smoke.

Abstract

Besides environmental exposure to metals, cigarette smoke either primary or secondary also contributes to metal exposure. This exposure in South Africa as well as in the rest of the world is increasing. Exposure to heavy metals such as lead (Pb), cadmium (Cd), chromium (Cr) and aluminium (Al) found in cigarette smoke can cause heavy metal toxicity, which includes the inhibition of antioxidant pathways via the depletion of antioxidant elements such as glutathione (GSH) and inhibition of antioxidant enzymes resulting in improper eradication of reactive oxygen species (ROS) which leads to cellular damage. A diet which is low in endogenous antioxidants such as polyphenols exacerbates this toxic effect. Supplementation with dietary antioxidants, if positive would be a cost effective, relatively simple method in helping to decrease the toxic effects of heavy metal exposure. Therefore the aim of this study was twofold, namely to determine the toxicity of metals commonly found in cigarette smoke using an erythrocyte ex vivo model and then to determine if antioxidants that are bioavailable can reduce toxicity. The methods used by this study are haemolysis assay, Fenton reaction, dichlorodihydrofluorescein diacetate (DCFH-DA) assay, Trolox equivalent antioxidant capacity (TEAC) assay, total flavonoid content (TFC) assay, Glutathione (GSH) assay and Scanning Electron Microscopy (SEM). Pb was found to be the most toxic metal, causing 50% haemolysis (H50%) at a concentration of 16.00mM and toxicity was associated with echinocyte, type III formation. Pb was a poor catalyst of the Fenton reaction, but exposure of erythrocytes to Pb caused increased ROS formation. Pb did not bind GSH, however in erythrocytes it caused an increase in GSH levels. This implies that the oxidative effect of Pb, is not as a catalyst of the Fenton reaction or due to GSH binding. The observed effects may be due to the ability of Pb to inhibit antioxidant enzyme activity, resulting in an increase in GSH levels and subsequent accumulation of ROS. Catechin, gallic acid and 6-Hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) effectively reduced the oxidative effects of Pb. Catechin and gallic acid did not alter Pb induced increase in erythrocyte GSH levels. Catechin, gallic acid and Trolox bound GSH and reduced the amount of free GSH. Cd induced H50% at a concentration of 33.83mM and the formation of spherocytes. The latter was similar to the effect observed with the oxidant AAPH. Cd catalysed the Fenton reaction and binds GSH, however in erythrocytes Cd did not cause an increase in ROS or alter GSH levels. This implies that in the erythrocyte, Cd induced ROS formation, directly targets the cell membrane causing changes to membrane fluidity and morphology. Antioxidants did cause some inhibition of haemolysis and antioxidants quercetin and ascorbic acid inhibited the Fenton reaction and bound Cd. This metal antioxidant interaction caused a loss in the antioxidant activity of ascorbic acid but enhanced the activity of caffeic acid. Trolox reduced Cd – GSH binding. At a concentration of 47.83mM Cr caused H50% and induced echinocyte type III formation. Cr catalysed the Fenton reaction and bound GSH in a manner similar to Cd. Likewise Cr did not cause an increase in ROS formation but did cause changes in GSH levels, similar to those seen with Pb. The higher Cr concentrations required for haemolysis and echinocyte type III formation implies that Cr may be slightly less toxic than Cd and Pb. Catechin and gallic acid reduced Cr induced haemolysis. In the Fenton reaction, quercetin and ascorbic acid scavenged hydroxyl radicals and this caused a loss in the antioxidant activity of ascorbic acid while enhancing the antioxidant activity of quercetin. Catechin and gallic acid reduced the ability of Cr to bind GSH. Of all the metals investigated Al, was the least toxic, with H50% occurring at 81.26mM and was associated with echinocyte type I formation. Al was a poor catalyst of the Fenton reaction but did bind GSH. In ex vivo erythrocytes, Al did not induce ROS formation or changes in GSH levels. Catechin, gallic acid and Trolox reduced haemolysis. Trolox inhibited Al catalysis of the Fenton reaction. Al had no effect on the antioxidant activity of catechin, gallic acid and Trolox although metal antioxidant interactions enhanced the activity of quercetin and caused a loss in the antioxidant activity of ascorbic acid. Catechin and gallic acid caused an increase in erythrocyte GSH levels which was unaltered when erythrocytes were exposed to only Al. In summary, some antioxidants, especially catechin, gallic acid, Trolox and ascorbic acid prevented metal induced cellular damage. The observed effects may be related to direct radical scavenging, GSH protection against metal binding or may be due to unknown membrane effects especially considering the effect of Trolox. Of concern is the adverse effect of these metals on the bioactivity of ascorbic acid.