The spermidine synthase of the malaria parasite Plasmodium falciparum: Molecular and biochemical characterisation of the polyamine synthesis enzyme

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 36.6 kDa 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 52 μM, 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 0.18 μM. 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 35 μM, indicating that P. falciparum spermidine synthase represents a putative drug target.