Hemozoin aggregation

As is often the case in multidisciplinary research, a number of different assays have been developed to monitor the inhibition of hemozoin aggregation (Table 1). Consequently, a great deal of confusion has arisen in the literature concerning the exact efficacy of a particular hemozoin inhibitor. For this reason, it is worthwhile to discuss the principal assays employed in the evaluation of antimalarial compounds and those assays specifically designed to evaluate heme aggregation inhibition. 

Scheme 1 Potential inhibition pathways to hemozoin aggregation...

Figure 4 a) Hemozoin aggregation mediated by bionucleating templates b) pH dependence of the heme aggregation c) SEM and d) XRD characterization of... 

Fig. (3). Hemozoin Production Mediated by Bionucleating Templates. Representative polymerization assay with 50 pM of hemin in 2 ml acetate buffer (500 mM, pH 4.8) at 37° C. BNTI and BNTII were used in 1 and 2 nmol amounts. Chloroquine (CQ, 100 pM) was included with BNT I and BNT n in inhibition reactions. Polyhistidine and bovine serum albumin in approximately 1 and 2 nmol amounts were used in protein control experiments. The blank control was the acetate buffer above. Base line amounts of insoluble aggregate are consistent with those previously reported under similar conditions.

This work is also notable in that Cole et al. performed detailed aggregation studies of the heme MPPIX using UV-vis and fluorescence spectroscopies to detect the formation of 71-71 hetero-metalloporphyrin assemblies under assay conditions. By employing UV-vis absorbance spectroscopy, the aggregation of porphyrin and metalloporphyrin systems may be examined. The in vitro assay system used for hemozoin... 

For over 300 years, the quinoline family of drugs, and chloroquine in particular, has been used as the primary treatment for malaria. Recent studies have demonstrated that this drug inhibits the aggregation of free heme into hemozoin, allowing levels of monomeric heme to rise until cell lysis occurs. Although the determined structure of hemozoin makes the polymer termination scheme proposed by Sullivan et al. unlikely, hemozoin drug heme interactions appear critical in the inhibition mechanism. Thus, researchers have identified the characteristics of... 

The French military physician Charles Louis Alphonse Lav-eran was credited with the 1880 discovery of the malaria protozoa Oscilliaria malarias, which was later termed Plasmodium. In malaria necropsies, Laveran noted the presence of dark pigmented bodies in the bloodstream and in the brain, spleen, and liver (7). This pigmented body was later characterized as a unique biomineral composed of a dimeric ferriprotopor-phyrin IX (Fe(III)PPIX) aggregate, which is known commonly as hemozoin (HZ). 

Figure 2 Modes of hemozoin inhibition. On a neutrai iipid dropiet tempiate (T), heme can aggregate to form the biomineral HZ. Antimalarials may inhibit this aggregation by binding heme substrate, interacting with the iipid tempiate or trapping heme bound to the template. All actions serve to prevent the formation of HZ.

Figure 4 Physical characterization of hemozoin. (a) The axial propionate linkages between heme units in FIZ are seen by FT-IR fingerprints of C = 0 and C-O stretching at 1664 and 1211 cm , respectively, (b) Characteristic 2 1 peaks are seen at T-. 21° and 24° 28 for (i) native and (ii) synthetic FIZ. Absent from these aggregates is the 23° 28 peak observed in the diffraction pattern of (iii) substrate hemin chloride, (c) Electron micrograph of FIZ in P. falciparum infected RBC where (i) is the host RBC, (ii) is the parasite, and (iii) is the DV. Image reproduced with kind permission of Springer Science and Business Media (23). (d) SEM image of uniform BFI crystals.

Biomimetic processes other than inorganic crystallisation have also been described. Wright and co-workers templated the production of the polymer hemozoin, a crystalline heme aggregate used by malarial parasites in a detoxification process. Understanding and controlling this could lead to new antimalarial drugs. Peptide dendrimers containing multiple repeats of the Ala-His-His sequence known to mediate hemozoin formation were prepared and indeed were shown to induce its production. ... 

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