Recombinant MSP1-42 binds to heparin. (A) Recombinant merozoite antigens were tested for binding to heparin-BSA (5 μg/mL) or BSA alone (5 μg/mL). Binding to lactoferrin was used as a positive control and to normalize binding between assays. Data are mean binding ± SEM expressed as percentage binding of lactoferrin (3 assays in duplicate). Heparin-BSA had similar growth-inhibitory activity against P falciparum in vitro as heparin, whereas BSA was not inhibitory (data not shown). (B) Binding of heparin-BSA to MSP1-42 was concentration-dependent and saturable at low concentrations. Data are normalized to percentage binding of lactoferrin to heparin-BSA at 5 μg/mL. (C) Binding of heparin-BSA to MSP1-42 was inhibited with soluble heparin but not CSC. Data are expressed as the proportion of binding observed in the absence of inhibitors. Data are mean ± SEM (3 assays in duplicate).