Oxidative stress in malaria parasite-infected erythrocytes: host–parasite interactions

Abstract

Experimenta naturae, like the glucose-6-phosphate dehydrogenase deficiency, indicate that malaria parasites are highly susceptible to alterations in the redox equilibrium. This offers a great potential for the development of urgently required novel chemotherapeutic strategies. However, the relationship between the redox status of malarial parasites and that of their host is complex. In this review article we summarise the presently available knowledge on sources and detoxification pathways of reactive oxygen species in malaria parasite-infected red cells, on clinical aspects of redox metabolism and redox-related mechanisms of drug action as well as future prospects for drug development. As delineated below, alterations in redox status contribute to disease manifestation including sequestration, cerebral pathology, anaemia, respiratory distress, and placental malaria. Studying haemoglobinopathies, like thalassemias and sickle cell disease, and other red cell defects that provide protection against malaria allows insights into this fine balance of redox interactions. The host immune response to malaria involves phagocytosis as well as the production of nitric oxide and oxygen radicals that form part of the host defence system and also contribute to the pathology of the disease. Haemoglobin degradation by the malarial parasite produces the redox active by-products, free haem and H₂O₂, conferring oxidative insult on the host cell. However, the parasite also supplies antioxidant moieties to the host and possesses an efficient enzymatic antioxidant defence system including glutathione- and thioredoxin-dependent proteins. Mechanistic and structural work on these enzymes might provide a basis for targeting the parasite. Indeed, a number of currently used drugs, especially the endoperoxide antimalarials, appear to act by increasing oxidant stress, and novel drugs such as peroxidic compounds and anthroquinones are being developed.

Introduction

Over 500 million episodes of clinical malaria occur annually, with up to 2.7 million deaths (Murphy and Breman, 2001), most of them in African children under the age of 5 years. In children, cerebral malaria, severe anaemia and respiratory distress due to systemic acidaemia are principal causes of death (Marsh et al., 1995). In Asia, deaths in adults are relatively more prevalent, and frequently result from renal failure or acute respiratory distress syndrome, with local lung pathology (White and Ho, 1992). Up to 10,000 pregnant women (Guyatt and Snow, 2001) and 200,000 newborns (Steketee et al., 2001) also die as a consequence of malaria in pregnancy each year. Oxidative stress might play a key role in many of these fatal endpoints and—at the same time—oxidative stress represents a most promising rationale for antimalarial chemotherapy.

The detoxification of reactive oxygen species (ROS) is a challenge for erythrocytes infected with Plasmodium. As a result of the high metabolic rate of the rapidly growing and multiplying parasite, large quantities of toxic redox-active by-products are generated. Central to the generation of oxidative stress is the degradation of host haemoglobin by the parasite. Haemoglobin represents the major source of amino acids for Plasmodium, but its degradation in an acidic food vacuole results in the production of toxic free haem (ferri/ferroprotoporphyrin IX; FP) and ROS (see Tilley et al., 2001, for review). Most of the FP is sequestered into a crystalline form, known as haemozoin or malaria pigment (Egan et al., 2002, Slater and Cerami, 1992). Alternative detoxication pathways, including FP degradation (Zhang et al., 1992, Loria et al., 1999), reaction with glutathione (Ginsburg et al., 1998), and the binding to FP-binding proteins (Harwaldt et al., 2002, Campanale et al., 2003), may also contribute to FP detoxification. However, if even a small amount (e.g. 0.5%) of the FP escapes the neutralisation processes, it could cause redox damage to host proteins and membranes, inhibit parasite enzymes and lyse erythrocytes (see Tilley et al., 2001, for review). Apart from this metabolically derived oxidative stress, the production of ROS by the host immune system adds to the overall oxidative burden of the parasitised cell.

In order to maintain a redox equilibrium, malaria parasites are equipped with a range of low molecular weight antioxidants—the most prominent being the tripeptide glutathione (GSH)—as well as with antioxidant enzymes (Fig. 1; see also Ginsburg, 2002, malaria parasite metabolic maps at http://www.sites.huji.ac.il/malaria). The latter include glutathione- and thioredoxin-dependent proteins (Rahlfs et al., 2002, Becker et al., 2003a) as well as superoxide dismutase. Notably, Plasmodium possesses neither a classical catalase nor a classical glutathione peroxidase (Sztajer et al., 2001). Some of the enzymes present have been studied in functional and structural detail over the last years and represent promising targets for the development of novel antimalarial drugs.

In this regard it is interesting to note that a number of drugs currently in clinical use exert their activities, at least in part, by increasing oxidative stress in the parasitised erythrocyte. For example, chloroquine functions by preventing FP detoxification and its activity can be enhanced by depletion of GSH. The redox cycling of the metabolites of primaquine exerts a substantial oxidative stress, and artemisinin is thought to react with haem moieties forming cytotoxic radicals. Potential new drugs that act by interfering with the redox metabolism of malarial parasites include peroxidic antimalarials, which act by alkylation of haem or proteins, inhibitors of antioxidant enzymes such as glutathione reductase and glutathione S-transferase, and also redox-active anthroquinones and xanthones, which are likely to interfere with haemozoin formation.

Based on the mounting evidence that oxidative stress is an important clinical and pathobiochemical factor as well as an effective therapeutic principle in malaria, we have attempted to summarise the presently available knowledge on redox reactions in Plasmodium-infected red blood cells (IRBCs) and the redox aspects of the host response to malaria parasite infection.