BINDING OF DIHYDROARTEMISININ TO HEMOGLOBIN-H - ROLE IN DRUG ACCUMULATION AND HOST-INDUCED ANTIMALARIAL INEFFECTIVENESS OF ALPHA-THALASSEMIC ERYTHROCYTES

Dihydroartemisinin and other artemisinin derivatives are relatively in effective against Plasmodium falciparum infecting alpha-thalassemic er ythrocytes, namely hemoglobin (Hb) H or HbH/Hb Constant Spring erythro cytes, as compared with those infecting genetically normal erythrocyte s. The variant erythrocytes accumulate radiolabeled dihydroartemisinin to a much higher extent than the normal ones, and the accumulated dru g was retained after extensive washing, in contrast to the drug in nor mal erythrocytes which was mostly removed. At initial drug concentrati on of 1 mM, most (82-88%) of the drug was found in the cytosol fractio n of both variant and normal erythrocytes. Binding of the drug to hemo globins accounted for 40-70% of the total uptake. Hb H accounted for 1 0.9 +/- 2.7% and 12.4 +/- 6.2% of total protein in HbH and HbH/Hb Cons tant Spring erythrocytes. HbH bound with 28.7 +/- 6.7% of the drug, wh ereas HbH/Hb Constant Spring erythrocytes bound with 21.8 +/- 8.3% of the drug. Binding experiments showed that Hb H had 5-7 times the drug- binding capacity of Hb A. For Hb H, the maximum binding capacity (B-ma x) = 1.67 +/- 0.17 mol/mol Hb, and the dissociation constant (K-d) = 6 6 +/- 17 mu M, and for Hb A, B-max = 0.74 +/- 0.18 mol/mol Hb and K-d = 224 +/- 15 mu M. It is concluded that preferential binding of dihydr oartemisinin to Hb H over Hb A accounts partly for the higher accumula tion capacity of the alpha-thalassemic erythrocytes, which leads to it s antimalarial ineffectiveness.