ABSTRACT
The discovery of the physiological and pathophysiological roles of nitric oxide (NO) in the 1980s became one of the most remarkable events in biology (Furchgott and Zavadski 1980; Palmer et al. 1987). From the chemical point of view, NO is an uncharged paramagnetic molecule. Its chemical and physiological properties are a result of its tendency to stabilize an unpaired electron (Stamler 1994; Kerwin et al. 1995). In contrast to oxygen radicals, the half lifetime of NO reaches several seconds depending on the type of tissues and physiological conditions (Kikuchi et al. 1993) as a result of which NO molecules can easily penetrate through biological membranes and interact with intracellular and extracellular structures that are located relatively far from the place where these molecules were produced, and readily react with other substances as well (Moncada et al. 1991). Nitric oxide plays a dual role in an organism: On the one hand, it diffuses into parasite cells and inhibits the key enzymes necessary to those cells, thereby exhibiting a protective effect against the parasite cells by destroying them; on the other hand, NO produced in an excess amount acts as a strong cytostatic, which causes appreciable harm to the organism itself under conditions of oxidative stress and production of active oxygen forms, mainly peroxynitrites (Schmidt and Walter 1994), and eventually takes part in the development of inammatory processes.
