Activation of Artemisinin and Heme Degradation in Leishmania tarentolae Promastigotes: A Possible Link.

In this study, we focused on the activation of artemisinin (Art) in Leishmania tarentolae promastigotes (LtP) in comparison to reference compounds. Viability assays in different media in the presence of different iron sources (hemin/fetal calf serum) showed that IC50 values of Art in LtP were modulated by assay conditions, but overall were within the low micromolar range. Low temperature electron paramagnetic resonance (EPR) spectroscopy of LtP showed that Art shifted the redox state of the labile iron pool less than the EP ascaridole questioning its role as a major activator of Art in LtP. Based on the high reactivity of Art with hemin in previous biomimetic experiments, we focused on putative heme-metabolizing enzymes in Leishmania, which were so far not well described. Inhibitors of mammalian heme oxygenase (HO; tin and chromium mesoporphyrin) acted antagonistically to Art in LtP and boosted its IC50 value for several magnitudes. By inductively coupled plasma methods (ICP-OES, ICP-MS) we showed that these inhibitors do not block iron (heme) accumulation, but are taken up and act within LtP. These inhibitors blocked the conversion of hemin to bilirubin in LtP homogenates, suggesting that an HO-like enzyme activity in LtP exists. NADPH-dependent degradation of Art and hemin was highest in the small granule and microsomal fractions of LtP. Photometric measurements in the model Art/hemin demonstrated that hemin requires reduction to heme and that subsequently an Art/heme compl...
Source: Biochemical Pharmacology - Category: Drugs & Pharmacology Authors: Tags: Biochem Pharmacol Source Type: research