The gene encoding
spermidine synthase was cloned from the human
malaria parasite Plasmodium falciparum. Northern and Western blot analyses revealed a stage specific expression during the erythrocytic schizogony with the maximal amount of transcript and
protein in mature trophozoites. Immunofluorescence assays (IFAs) suggest a cytoplasmatic localisation of the
spermidine synthase in P. falciparum. The
spermidine synthase polypeptide of 321
amino acids has a molecular mass of 36.6kDa and contains an N-terminal extension of unknown function that, similarly, is also found in certain plants but not in animal or bacterial orthologues. Omitting the first 29
amino acids, a truncated form of P. falciparum
spermidine synthase has been recombinantly expressed in Escherichia coli. The
enzyme catalyses the transfer of an aminopropyl group from
decarboxylated S-adenosylmethionine (dcAdoMet) onto
putrescine with Km values of 35 and 52microM, respectively. In contrast to mammalian
spermidine synthases,
spermidine can replace to some extent
putrescine as the aminopropyl acceptor. Hence, P. falciparum
spermidine synthase has the capacity to catalyse the formation of
spermine that is found in small amounts in the erythrocytic stages of the parasite. Among the
spermidine synthase inhibitors tested against P. falciparum
spermidine synthase,
trans-4-methylcyclohexylamine (4MCHA) was found to be most potent with a Ki value of 0.18microM. In contrast to the situation in mammals, where inhibition of
spermidine synthase has no or only little effect on cell proliferation, 4MCHA was an efficient inhibitor of P. falciparum cell growth in vitro with an IC50 of 35microM, indicating that P. falciparum
spermidine synthase represents a putative drug target.