Describe the Hemoglobin Digestive Process
P. falciparum, is the parasite responsible for the malaria, a disease that causes approximately one million deaths annually. It undergoes the process of erythrocytic schizogony which is responsible for the clinical manifestations of malaria. The trophozoite ingests and degrades approximately 80% of the hemoglobin in the host cell (Brayl 2005). As the trophozoite continues to develop, its endolysosomal system takes up the cytoplasm of the host cell using cytosomes which contains host hemoglobin. Cytosomes are formed by an invagination formed from the PVM (parasitophorous vacuolar membrane) and PPM (parasite plasma membrane) from which transport vesicles are pinched off. These vesicles fuse with the digestive vacuole (DV) and releases into contents into the DV.
Researchers postulate that the initiation of the hemoglobin digestion begins in the transport vesicles and is concluded in the DV (Brayl 2005). During digestion, the protein component of the hemoglobin is broken down into peptides due to enzymatic action by a metalloprotease and cysteine and aspartic proteases. Once the hemoglobin is broken into small peptides, they are removed from the endolysosomal system into the cytoplasm of the parasite. Tilley et al., (2010) posit that once in the cytoplasm, cytosolic aminopepetidase hydrolyses the small peptides into liberated amino acids. This digestion process consumes a large amount of the parasite’s energy, due to the recycling of the PVM and PPM and the creation of a suitable proton gradient for the optimum functioning of the protease enzymes (Tilley et al. 2010). A byproduct produced by this process is known as heamatin. Haematin is detoxified by its crystallization into haemozoin and then preserved in the digestive vacuole (Tilley et al. 2010).
Mode of Action of Quinoline
Drugs containing quinoline have been used in the treatment of malaria since the separation of quinine from the Cinchona tree bark in the early nineteenth century. Since the 1920s, synthetic quinolines have been used in therapy. Chloroquine, CQ, has been widely used as a synthetic quinoline. CQ is thought to bind to Ferriprotoporphyrin, (FP- remaining undigested heme), and is taken up highly selectively and concentrates in the infected erythrocytes (Brayl 2005). This can be attributed to the proton-trapping system because CQ is a weak base, thus, when it is not charged diffuses easily into the acidic sections. In these sections, the CQ it attaches itself to the protons and is trapped. CQ is absorbed by FP 20 times faster than mammalian cells which contain acidic lysosomes (Brayl 2005). The complex formed by CQ and FP and its subsequent accumulation in the parasitic cells is responsible for its death. The CQ-FP complex causes the parasitic membranes to lyse through a mechanism of lipid peroxidation (Brayl 2005).
P. falciparum Drug Resistance and the Role of PfCRT
The development of resistance to P. falciparum infection has greatly hindered treatment by anti-malarial drugs. Studies have identified that the P. falciparum chloroquine resistance transporter (PfCRT) and PfMDR1 (multidrug resistance) proteins as the targets for potential mutations (Valderramos 2006). PfCRT is an important membrane protein located on the parasite’s digestive vacuole (Rowena, 2009). Valderramos, (2006), associates mutations in PfCRT with reduced efficacy of CQ treatment and PfMDR1 with mefloquine treatment. A resistant parasite is unresponsive to treatment by these agents or less responsive than those parasites which are not resistant. It seems fitting in deed that the parasite would develop an adaptation in the transporter protein that is in charge of transporting the therapeutic agent that would kill it. Research studies by various scientists agree that PfCRT is the transporter protein responsible for CQ uptake into the digestive vacuole (Valderramos 2006; Brayl 2005; Rowena 2009)
There are three models which have been proposed to describe CQ resistance. These are: efflux of chloroquine; CQ leak out of DV membrane; and changes in pH at DV membrane (Valderramos 2006). Cecilia, (2010), describes the models above as the partitioning model, the channel model, and the carrier model respectively. The partitioning model suggests that the fast efflux of CQ by CQ resistant (CQR) parasites possess a pathway for CQ that is enhanced across the DV. The channel model postulates that mutated PfCRT contains a leak which allows protonated CQ to passively diffuse out of the DV, thus no accumulation of the drug. The carrier model presents PfCRT as a carrier molecule, subject to the demands of kinetics. Should there be a mutation in PfCRT, the molecule would not be able to carry CQ efficiently. Valderramos (2006), suggests that both the partitioning model and the channel model could work together with the carrier models to result in CQ resistance of P. falciparum.
The P. falciparum parasite mutates in the transporter protein in the DV of the parasite so as to be able to resist the action of quinoline drugs on the parasite. This mutation has been vital to its survival because the quinoline drugs form complexes with the undigested heme (FB), which then lyses the parasitic membranes, destroying it. Much research has been dedicated to find new ways to combat the mutating parasite in its bid for survival. This includes: finding new ways to use older drugs; redesigning currently existing drugs; and finding new drugs due to better understanding of the biological structure of the parasite (Biagini, et al. 2003). The use of 4-aminoquinoline drugs, for example, quinine, mefloquine, and amodiaquine has long been employed in malaria therapy. Their efficacy has been hindered by CQ resistance which has evolved in the parasites over time as an adaptation. New studies have demonstrated that the parasite experiences difficulty in developing a CQ resistance to drugs which bind to FP (Biargini, et al. 2003). Research therefore focuses on the creation of novel derivatives of 4-aminoquinolines which will then be able to combat malaria effectively.
For example, Amodiaquine (AQ) is effective against CQR-falciparum, but its use is not recommended due to resulting agranulocytosis and hepatoxicity on human subjects when used. However, Brayl, (2005) points out that studies have resulted in fluoromodiaquine, an alternative to AQ which is safer (Brayl 2005; Biargini et al. 2003).
Conclusion
The digestion of haemoglobin in the trophozoite of P. falciparum takes place in its digestive vacuole. Earlier therapy with antimalarials specifically targeted the PfCRT in the DV, inhbit this process, and thus, effectively destroy the parasite. As a result, the parasite has evolved to preserve this function, mainly believed to be a function of protein tansporter protein in the DV’s mainly PfCRT and PfMDR1. Research is now focused on finding new chemotherapeutic agents to combat the parasite. Understanding the mode of CQ resistance gives better insight into developing older classic 4-aminoquinolines. New derivatives of these drugs are believed to be able to combat CQR parasites.
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