6 research outputs found
Synthetic Hemozoin (β-Hematin) Crystals Nucleate at the Surface of Neutral Lipid Droplets that Control Their Sizes
Emulsions
of monopalmitoylglycerol (MPG) and of a neutral lipid
blend (NLB), consisting of MPG, monostearoylglycerol, dipalmitoylglycerol,
dioleoylglycerol, and dilineoylglycerol (4:2:1:1:1), the composition
associated with hemozoin from the malaria parasite Plasmodium falciparum, have been used to mediate
the formation of β-hematin microcrystals. Transmission electron
microscopy (TEM), electron diffraction, and electron spectroscopic
imaging/electron energy loss spectroscopy (ESI/EELS) have been used
to characterize both the lipid emulsion and β-hematin crystals.
The latter have been compared with β-hematin formed at a pentanol/aqueous
interface and with hemozoin both within P. falciparum parasites and extracted from the parasites. When lipid and ferriprotoporphyrin
IX solutions in 1:9 v/v acetone/methanol were thoroughly premixed
either using an extruder or an ultrasound bath, β-hematin crystals
were found formed in intimate association with the lipid droplets.
These crystals resembled hemozoin crystals, with prominent {100} faces.
Lattice fringes in TEM indicated that these faces made contact with
the lipid surface. The average length of these crystals was 0.62 times
the average diameter of NLB droplets, and their size distributions
were statistically equivalent after 10 min incubation, suggesting
that the lipid droplets also controlled the sizes of the crystals.
This most closely resembles hemozoin formation in the helminth worm Schistosoma mansoni, while in P. falciparum, crystal formation appears to be associated with the much more gently
curved digestive vacuole membrane, which apparently leads to formation
of much larger hemozoin crystals, similar to those formed at the flat
pentanol–water interface
Insights into the Role of Heme in the Mechanism of Action of Antimalarials
By using cell fractionation and measurement of Fe(III)heme-pyridine,
the antimalarial chloroquine (CQ) has been shown to cause a dose-dependent
decrease in hemozoin and concomitant increase in toxic free heme in
cultured <i>Plasmodium falciparum</i> that is directly correlated
with parasite survival. Transmission electron microscopy techniques
have further shown that heme is redistributed from the parasite digestive
vacuole to the cytoplasm and that CQ disrupts hemozoin crystal growth,
resulting in mosaic boundaries in the crystals formed in the parasite.
Extension of the cell fractionation study to other drugs has shown
that artesunate, amodiaquine, lumefantrine, mefloquine, and quinine,
all clinically important antimalarials, also inhibit hemozoin formation
in the parasite cell, while the antifolate pyrimethamine and its combination
with sulfadoxine do not. This study finally provides direct evidence
in support of the hemozoin inhibition hypothesis for the mechanism
of action of CQ and shows that other quinoline and related antimalarials
inhibit cellular hemozoin formation