Neutral lipids associated with haemozoin mediate efficient and rapid beta-haematin formation at physiological pH, temperature and ionic composition

作者：Ambele Melvin A; Egan Timothy J^*

来源：Malaria Journal, 2012, 11(1): 337.

DOI：10.1186/1475-2875-11-337

• Haemozoin
• beta-haematin
• Neutral lipids
• Kinetics
• Rates
• Autofluorescence

Background: The malaria parasite disposes of host-derived ferrihaem (iron(III) protoporphyrin IX, Fe(III) PPIX) by conversion to crystalline haemozoin in close association with neutral lipids. Lipids mediate synthetic haemozoin (beta-haematin) formation very efficiently. However, the effect on reaction rates of concentrations of lipid, Fe(III) PPIX and physiologically relevant ions and biomolecules are unknown. %26lt;br%26gt;Methods: Lipid emulsions containing Fe(III) PPIX were prepared in aqueous medium (pH 4.8, 37 degrees C) to mediate beta-haematin formation. The reaction was quenched at various times and free Fe(III) PPIX measured colorimetrically as a pyridine complex and the kinetics and yields analysed. Products were also characterized by FTIR, TEM and electron diffraction. Autofluorescence was also used to monitor beta-haematin formation by confocal microscopy. %26lt;br%26gt;Results: At fixed Fe(III)PPIX concentration, beta-haematin yields remained constant with decreasing lipid concentration until a cut-off ratio was reached whereupon efficiency decreased dramatically. For the haemozoin-associated neutral lipid blend (NLB) and monopalmitoylglycerol (MPG), this occurred below a lipid/Fe(III)PPIX (L/H) ratio of 0.54. Rate constants were found to increase with L/H ratio above the cut-off. At 16 mu M MPG, Fe(III)PPIX concentration could be raised until the L/H ratio reached the same ratio before a sudden decline in yield was observed. MPG-mediated beta-haematin formation was relatively insensitive to biologically relevant cations (Na+, K+, Mg2+, Ca2+), or anions (H2PO4-, HCO3-, ATP, 2,3-diphosphoglycerate, glutathione). Confocal microscopy demonstrated beta-haematin formation occurs in association with the lipid particles. %26lt;br%26gt;Conclusions: Kinetics of beta-haematin formation have shown that haemozoin-associated neutral lipids alone are capable of mediating beta-haematin formation at adequate rates under physiologically realistic conditions of ion concentrations to account for haemozoin formation.

• 出版日期2012-10-8