9 - Antioxidant Mechanisms

Abstract

This chapter discusses the defense mechanisms of parasites against reactive oxygen species. Parasites are sensitive to oxidative stress as they live in oxygen-poor environments. Glutathione peroxidase activities are very low or absent, and in some cases, such as in trypanosomatids, replaced by trypanothione-dependent peroxidase activities. In any case, peroxidase activities are quite low when compared with the glutathione peroxidase activities in mammalian organs. Superoxide dismutases are present in most parasites, and these are features typical of facultative aerobes. Since superoxide dismutase catalyzes the dismutation of superoxide anion to form hydrogen peroxide and there is a deficiency in hydrogen peroxide-metabolizing systems, this explains the toxicity of superoxide anion to parasites. The chapter explains that parasites may tolerate a slow endogenous rate of hydrogen peroxide generation in their natural habitats but they are sensitive to an increased steady-state concentration of hydrogen peroxide because of drug metabolism or phagocytic cell attack. Oxidative damage caused by reactive oxygen species is called “oxidative stress.” Biological systems contain powerful enzymatic and non-enzymatic anti-oxidant mechanisms, and oxidative stress denotes a shift in the pro-oxidant/anti-oxidant balance in favor of the former. Reactive oxygen species are important mediators of several forms of cell damage in parasites and the chapter focuses on the defense mechanisms of parasites against these species.