title: Optimization of a multi-well colorimetric assay to determine haem species in Plasmodium falciparum in the presence of anti-malarials.
abstract: BACKGROUND
  The activity of several well-known anti-malarials, including chloroquine (CQ), is attributed to their ability to inhibit the formation of haemozoin (Hz) in the malaria parasite. The formation of inert Hz, or malaria pigment, from toxic haem acquired from the host red blood cell of the parasite during haemoglobin digestion represents a pathway essential for parasite survival. Inhibition of this critical pathway therefore remains a desirable target for novel anti-malarials. A recent publication described the results of a haem fractionation assay used to directly determine haemoglobin, free haem and Hz in Plasmodium falciparum inoculated with CQ. CQ was shown to cause a dose-dependent increase in cellular-free haem that was correlated with decreased parasite survival. The method provided valuable information but was limited due to its low throughput and high demand on parasite starting material. Here, this haem fractionation assay has been successfully adapted to a higher throughput method in 24-well plates, significantly reducing lead times and starting material volumes.
  
  
  METHODS
  All major haem species in P. falciparum trophozoites, isolated through a series of cellular fractionation steps were determined spectrophotometrically in aqueous pyridine (5 % v/v, pH 7.5) as a low spin complex with haematin. Cell counts were determined using a haemocytometer and a rapid novel fluorescent flow cytometry method.
  
  
  RESULTS
  A higher throughput haem fractionation assay in 24-well plates, containing at most ten million trophozoites was validated against the original published method using CQ and its robustness was confirmed. It provided a minimum six-fold improvement in productivity and 24-fold reduction in starting material volume. The assay was successfully applied to amodiaquine (AQ), which was shown to inhibit Hz formation, while the antifolate pyrimethamine (PYR) and the mitochondrial electron transporter inhibitor atovaquone (Atov) demonstrated no increase in toxic cellular free haem.
  
  
  CONCLUSIONS
  This higher throughput cellular haem fractionation assay can easily be applied to novel anti-malarials with a significantly decreased lead time, providing a valuable tool with which to probe the mechanisms of action of both new and established anti-malarials.
authors.affiliation: Division of Pharmacology, Department of Medicine, University of Cape Town, Observatory 7925, Cape Town, South Africa. cmbjil001@myuct.ac.za.
authors.forename: Jill M
authors.initials: JM
authors.lastname: Combrinck
authors.affiliation: Department of Chemistry, Vanderbilt University, Station B 351822, Nashville, TN, 37235, USA. kim.y.fong@vanderbilt.edu.
authors.forename: Kim Y
authors.initials: KY
authors.lastname: Fong
authors.affiliation: Division of Pharmacology, Department of Medicine, University of Cape Town, Observatory 7925, Cape Town, South Africa. Gibhard@uct.ac.za.
authors.forename: Liezl
authors.initials: L
authors.lastname: Gibhard
authors.affiliation: Division of Pharmacology, Department of Medicine, University of Cape Town, Observatory 7925, Cape Town, South Africa. Peter.Smith@uct.ac.za.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Smith
authors.affiliation: Department of Chemistry, Vanderbilt University, Station B 351822, Nashville, TN, 37235, USA. David.Wright@vanderbilt.edu.
authors.forename: David W
authors.initials: DW
authors.lastname: Wright
authors.affiliation: Department of Chemistry, University of Cape Town, Private Bag, Rondebosch, 7701, South Africa. Timothy.Egan@uct.ac.za.
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Egan
mesh_terms: D000655:Amodiaquine / Q000494:pharmacology; D000962:Antimalarials / Q000494:pharmacology*; D053626:Atovaquone / Q000494:pharmacology; D002738:Chloroquine / Q000494:pharmacology; D003124:Colorimetry / Q000379:methods*; D006418:Heme / Q000032:analysis*; D010963:Plasmodium falciparum / Q000187:drug effects*; D011739:Pyrimethamine / Q000494:pharmacology; D053058:Trophozoites / Q000187:drug effects*
chemical_list: D000962:Antimalarials; D000655:Amodiaquine; D006418:Heme; D002738:Chloroquine; D053626:Atovaquone; D011739:Pyrimethamine