Besides the role of lipids in the proliferation and metabolism of the parasite, host lipids have also been implicated in the formation of haemozoin
in vivo[
108,
109]. Earlier, it has been shown that linoeic acid (a polyunsaturated fatty acid) may be necessary for the dimerization of ferriprotoporphyrin IX (a toxic compound released after the digestion of haemoglobin), the initial step in the production of haemozoin [
110]. Haemozoin is the end product of the plasmodial detoxification of free haem that is produced by haemoglobin degradation [
20]. Historically, it was thought that haemozoin was an inert waste product of the malaria parasite. However, recent research resulted in the recognition of the importance of haemozoin in different aspects of malaria [
20,
111,
112]. Haem crystalization is the target of the widely used anti-malarial aminoquinoline drugs [
113]. Moreover, not only does the haemozoin production require host lipids, but it appears also that the inhibition of host monocyte functions, one of the eminent immune-modulating haemozoin effects, is caused by hydroxyl fatty acids, generated by
Plasmodium spp. in large amounts in the human hosts. The lipid hypothesis postulates that haemozoin formation occurs most rapidly at lipid-water interfaces. In the past 3 three years, convincing evidence is emerging in favour of the lipid model. First, the lipid environments in a parasitized erythrocyte using Nile Red (a lipophilic stain), were characterized [
114]. Neutral lipids associated with the digestive vacuole of the parasite were observed. These were composed of di- and triacylgycerols (triglycerides); possibly storage organelles for lipid intermediates produced during the degradation of phospholipids in the food vacuole. Mono-, di- and triacylglycerol heterogeneous mixtures promote haemozoin formation, implying that these neutral lipids are involved in haem detoxification [
114]. It was demonstrated that triglycerides are a major lipid portion stored in lipid droplets in the late trophozoite and schizont stage of
P. falciparum. Besides haem detoxification, it may be utilized to store acryl groups for phospholipid synthesis, glycosyl phosphatidyl inositol (a glycolipid) synthesis, and possibly for beta-oxidation (the process by which fatty acid molecules are broken down in the mitochondria to generate acetyl-coA) [
115]. Second, another study demonstrated that in the unrelated haemozoin-forming organisms
Schistosoma mansoni, and in the kissing bug,
Rhodnius prolixus (triatomine vector of Chagas disease), haemozoin formation occurs inside lipid droplet-like particles or in close association to phospholipid membranes (both hydrophobic environments) [
116]. Third, it has been reported that the intracellular mechanism of molecular initiation of haemozoin formation occurs within neutral lipid predominant nanospheres, which envelop haemozoin inside
P. falciparum digestive vacuoles. It was suggested that haemozoin is formed at the interface between the aqueous medium of the food vacuole and the lipid nanospheres [
113]. Another study confirmed these findings, as molecular dynamic simulation showed that a precursor haemozoin dimer forms spontaneously in the absence of the competing hydrogen bonds of water, demonstrating that this substance probably self-assembles near a lipid/water interface
in vivo. Probably, haemozoin nucleation occurs at the digestive vacuole inner membrane, with crystallizations occurring in the aqueous rather than lipid phase, as indicated by cryogenic soft X-ray tomography [
117‐
120]. Thus, lipids mediate synthetic haemozoin formation very efficiently. Further weight is added to this lipid hypothesis by another recent study that demonstrated that haemozoin-associated neutral lipids alone are capable of mediating haemozoin formation at adequate rates under physiologically realistic conditions of ion concentrations to account for haemozoin formation [
121]. The combination of these recent insights makes a compelling case for the theory that lipids drive haemozoin assembly.