Although acidocalcisomes have been well characterized morphologically
in other apicomplexan parasites, no such characterization has been done
in Plasmodium spp. Here, we report that Plasmodium falciparum
merozoites possess electron-dense organelles rich in phosphorus and
calcium, as detected by X-ray microanalysis of intact cells, which are
similar to the acidocalcisomes of other apicomplexans, but of more
irregular form. In agreement with these results malaria parasites
possess large amounts of short- and long-chain polyphosphate (polyP),
which are associated with acidocalcisomes in other organisms. PolyP
levels were highest in the trophozoite stage of the parasite. Treatment
of isolated trophozoites with chloroquine resulted in a significant
hydrolysis of polyP. Taken together, these results provide evidence
that acidocalcisomes from Plasmodium falciparum do not differ
significantly from acidocalcisomes of other apicomplexan parasites.