| Oxidative stress can be defined as a level of reactive oxygen species (ROS) formation that overcomes the threshold of cellular detoxification processes, which results in cellular damage. In chapter 1 of this study, we first examined intracellular sources (organelles) for "natural" formation of ROS, by using chemicals that are endogenously found. We determined that substrates metabolized by enzymes associated with mitochondria, peroxisomes, and endoplasmic reticula, formed ROS that lead to cytotoxicity, which became enhanced after catalase inhibition. A correlation between ROS formation and cytotoxicity was found, which led us to study this correlation using xenobiotics. In chapter 2, we examined ROS formation and cytotoxicity induced by a congeneric family of quinones and used quantitative structure-toxicity relationships (QSTRs) to analyze these results for both isolated rat hepatocytes and rat PC12 cells. A two dimensional (2D) QSTR or Hansch analysis was used to derive (multiple) linear regression equations for the cytotoxicity towards both cell types. This analysis identified electron affinity (or electrophilicity) of the quinone as the critical parameter responsible for the observed cytotoxicity and ROS formation. In chapter 3 we determined the effect of inhibiting NAD(P)H:Quinone Oxidoreductase (NQO), involved in a 2e- quinone reduction, with dicumarol on quinone cytotoxicity and ROS formation. Interestingly, apparent NQO inhibition affected hepatocyte susceptibility to quinones much more than PC 12 cells, which had little NQO activity relative to hepatocytes. Some quinones formed more ROS and were more cytotoxic to NQO inhibited hepatocytes, whilst other quinones were not affected. We derived 2D and three-dimensional (3D) QSTRs and found that both approaches yielded statistically significant models; but the 3D QSTR models were much more accurate. In the last chapter, we identified the bicarbonate anion (HCO3-) as an important modulator of ROS formation and cytotoxicity. The effect of HCO3 - was titratable, and increased the cytotoxicity of redox cycling quinones, but not alkylating quinones. A carbonate radical mediated cytotoxic mechanism is proposed. |
