Ljjw. Smeijsters et al., THE EFFECT OF (S)-9-(3-HYDROXY-2-PHOSPHONYLMETHOXYPROPYL) ADENINE ON NUCLEAR AND ORGANELLAR DNA-SYNTHESIS IN ERYTHROCYTIC SCHIZOGONY IN MALARIA, Molecular and biochemical parasitology, 67(1), 1994, pp. 115-124
The very effective (ID50 = 47 nM) and selective antimalarial compound
(S)-9-(3-hydroxy-2-phosphonylmethoxypropyl) adenine (HPMPA) abruptly a
rrests Plasmodium falciparum-cultured schizonts at concentrations betw
een 1 and 10 x ID50 as soon as their DNA content reaches 8 times that
of the haploid ringform stage. Even very high HPMPA concentrations do
not inhibit the first 2-3 rounds of schizogonic DNA replication. Also,
in the presence of HPMPA, replication of the 6-kb mitochondrial and 3
5-kb chloroplast-like DNA proceeds normally and in close concert with
each other, both to a 16-fold amount within 5 h during the trophozoite
stage. Hence the in in vitro assays HPMPApp-sensitive plasmodial DNA
polymerase gamma-like enzyme (IC50 = 1 mu M) - assumed to be involved
in mitochondrial DNA replication - is not the target of HPMPA in vivo
(living parasites), nor seems to be the DNA polymerization activities
of the - in vitro also HPMPA-sensitive (IC50 = 38 mu M) - DNA polymera
se alpha or of any other nuclear DNA polymerase of Plasmodium. In vitr
o assays demonstrated that HPMPApp does not act as an alternative subs
trate for plasmodial polymerases, contradicting the suggestion that th
e observed delayed inhibition of plasmodial schizogony might be the re
sult of DNA strand breakage caused by HPMPApp incorporation. Neither d
o results support the idea that the HPMPA-induced arrest of DNA replic
ation might be due to chain termination as a result of such incorporat
ion. We investigated whether arrest of DNA replication by HPMPA in sch
izonts could be explained by inhibition of the DNA synthesis rate limi
ting ribonucleotide reductase enzyme. For the first time its in vitro
activity was demonstrated in Plasmodium, converting - in an allosteric
ally controlled way - rNDPs into dNDPs. However, this conversion is no
t inhibited by HPMPA or HPMPApp, excluding this enzyme as well as the
target of HPMPA. Putative alternative modes of action of HPMPA on Plas
modium are discussed.