## 4 Specific Prenylquinones
### Tocopherol
#### 4.3.2. Tocopherol in Parasitic Protozoa  
Tocopherol biosynthesis was detected in asexual stages of _P. falciparum_ and in _L. amazonensis_ promastigotes. No specific enzymes involved intocopherol biosynthesis were predicted by bioinformatics approaches in either parasite. However, the incorporation of [3H]-GGPP in phytylated prerylquinone strongly suggests that both parasites possess an active pathway for phytol biosynthesis. Moreover, the incorporation of [3H] phytol intotocopherol in _Leishmania_ suggests an active phytol phosphorylation process for the salvage of phytol, which is typical for some photosynthetic organisms [123]. In this sense, prerylquinone biosynthesis is inhibited in both organisms by classic HPPD inhibitors: strongly by usnic acid in _P. falciparum_ and _L. amazonensis_ and weakly by nitisinone in _L. amazonensis_. Inhibition with usnic acid showed a dose-response effect on the growth of both parasites, causing an increase in ROS levels [64; 190]. It is not known whether the major lethal effects of usnic acid could be due to inhibition of the biosynthesis of the product derived from homogenistate or tyrosine, as occurs in other organisms [199]. The effect of usnic acid on parasite viability and oxidative stress was reversed by the addition of exogenous \(\alpha\)-tocopherol in _Plasmodium_ and _Leishmania_[64; 190]. Simultaneously, antiplasmodial oxidative stress-generating compounds such as chloroquine and cercosporin caused an increase in the \(\alpha\)-tocopherolquinone/\(\alpha\)-tocopherol ratio in _P. falciparum_, which is indicative of the antioxidant function [62]. However, notably, other authors have previously demonstrated that treatment with usnic acid generates oxidative stress in hepatic cells (which do not biosynthesize topcherol), an affect that can be reversed by exogenous addition oftocopherol [200]. Consequently, the origin of the oxidative stress of usnic acid treatment in _Plasmodium_ is unclear. Nevertheless, topcherol biosynthesis in _P. falciparum_ appears stimulated in vitro when the parasites are grown under 20% oxygen [190], which was also interpreted as a defense mechanism against oxidative stress. Although topcherol biosynthesis has been previously demonstrated in _P. falciparum_, there remain many uncertainties regarding the mechanism of the biosynthetic process. Specifically, there are no putative sequences for topcherol biosynthesis annotated in the _P. falciparum_ genome. Furthermore, metabolic labeling using isoprenic precursors only proved the polyprenylation process. The aromatic biosynthesis or external acquisition remains unexplored. Even before the characterization of topcherol biosynthesis in apicomplexan parasites, it was shown that the infected red blood cell membranes contained 5 fold more vitamin E and C than uninfected membranes. However, \(\alpha\)-tocopherolquinone was not detectable in infected red blood cell membranes. Thus, some authors suggested that a parasite-mediated vitamin redox modification had occurred [201]. In vivo experiments demonstrated that topcherol obtained from a host diet could be essential and stimulated parasitic development [201]. In this sense, vitamin E-deficient diets have been shown to be effective for disease control in vivo [202]. Studies focused on _T. cruzi_ and _P. berghei_ transmission have reported HPPD inhibition effects in hematophagous vectors [199; 203]. Usnic acid has been shown to inhibit vectorial stages of _Plasmodium_, whereas nitisinone has been shown to be extremely toxic for several hematophagous vectors, even at clinically achievable blood concentrations. Some authors have suggested HPPD inhibitors as selective insecticides, but these inhibitors should also be considered to control the spread of hematophagous vector-transmitted diseases [199; 203]. Tyrosine degradation has also been demonstrated to be essential in _Leishmania_ parasites as an excellent drug target for further exploration [89].