Prooxidant activity of beta-hematin (synthetic malaria pigment) in arachidonic acid micelles and phospholipid large unilamellar vesicles

Intraerythrocytic malaria parasite has evolved a unique pathway to detoxify hemoglobin-derived heme by forming a crystal of Ferri-protoporphyrin IX di mers, known as hemozoin or "malaria pigment," The prooxidant activity of be ta -hematin (BH), the synthetic malaria pigment obtained from hematin at ac idic pH, was studied in arachidonic acid micelles and phospholipid Large Un ilamellar Vesicles (LUVs) and compared to that of alpha -hematin (AH, Ferri -protoporphyrin IX-hydroxicie) and hemin (HE, Ferri-protoporphyrin-chloride ). Lipid peroxidation was measured as production of thiobarbituric acid rea ctive substances (TBARS). The extent of peroxidation induced by either AH o r BH was strongly dependent upon the content of pre-existing hydroperoxides and efficiently inhibited by triphenylphosphine, a deoxygenating agent abl e to reduce hydroperoxides to hydroxides and by lipophilic scavengers. BH p rooxidant activity was linearly related to the material, whereas that of AH seemed dependent on the aggregation state of the porphyrin. Maximal activi ty was observed when AH was present in concentration lower than 2 muM. In t his case a shift of spectra in the Soret region, leading to the increase of the O.D. 400/385 nm ratio, suggested a transition toward a less aggregated state. BH prooxidant activity was significantly lower than that of monomer ic AH, yet. higher than that of AH aggregates. Differently from AH aggregat es, BH-induced peroxidation was unaffected by GSH and inhibited rather than enhanced by acidic pH (5.7) and chloroquine. UV/Vis spectroscopy of AH agg regates at acidic pH, low GSH concentrations and chloroquine suggests a shi ft of An aggregates toward the less aggregated state, more active as peroxi dation catalyst. (C) 2001 Elsevier Science Inc. All rights reserved.