Schizonts of Plasmodium falciparum were cultured in medium containing a mixture of 10 micrograms/ml each of leupeptin, chymostatin, pepstatin, and antipain. The protease inhibitors did not inhibit macromolecular synthesis but were associated with decreased reinvasion of red cells and the accumulation of well preserved merozoites clustered around pigment granules (PCM, protease inhibitor clusters of merozoites). The parasite pellet from PCM cultures contained increased amounts of merozoite antigens, particularly at Mr 83, 73, 66, 45, and 17 kDa. The increases of the Mr 83, 73, and 45 kDa surface antigens observed in PCM had been observed also in similar merozoite clusters obtained by culturing schizonts in the presence of inhibitory antibodies. These three antigens are processed products of the abundant Mr 195 kDa schizont surface antigen. Liquid-phase double immunofluorescence of PCM demonstrated a residual red cell membrane through which monoclonal antibodies passed and reacted with the Mr 83, 73, and 45 kDa merozoite surface antigens or their precursors. The processes associated with normal reinvasion apparently involve protease(s), which plays a role(s) in the breakdown of the red cell membrane and the shedding of merozoite surface antigens. Interference with these processes by protease inhibitors is useful in increasing recoveries of merozoite antigens, as well as in elucidating mechanisms of reinvasion.

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