Screening and Characterization of Antimalarial Heme Polymerase Inhibitors from Garlic Cloves

Abstract

Aim: Garlic (Allium sativum L) aqueous extract was investigated to identify antimalarial compounds inhibiting heme polymerization. Methods: Solvent fraction of aqueous garlic extract was tested in heme polymerization assay and antimalarial assay to identify active factor. Mass spectroscopy, TLC and optical spectroscopy was used to characterize the active factor and mechanism of inhibiton. Results: Solvent fractionation and silica chromatography of aqueous garlic extract yields partially purified active constituent. The crude garlic extract has a high level of heme polymerization inhibition activity. Mass spectroscopy analysis of the high activity fraction indicates quercetin as a promising hit with an acceptable limit of error. Pure quercetin was found to inhibit heme polymerization and inhibit parasite growth in a dose dependent manner with an activity comparable to the activity present in the purified garlic aqueous fractions. Quercetin forms two distinct complexes with hemin as evident by TLC Chromatogram of hemin and quercetin mixture. ESI-MS analysis of quercetin-hemin reaction mixture gives two prominent peaks; 1st peak with m/z 929 (Hemin+Q+Li+3H) and 2nd peak with m/z 1244.7 (H+2Q+Na) with a clear indication of the formation of quercetin: hemin (1:1) and 2:1 complexes. The dissociation constant (Kd) of quercetin-hemin is 9.35 µM. Conclusions: In summary, aqueous garlic extract has heme polymerization inhibitor with high antimalarial activity. Quercetin is the main active constitution responsible for the activity and it inhibits heme polymerization by chelating free available hemin for polymerization.