The properties of a
purine phosphoribosyltransferase from late trophozoites of the human
malaria parasite, Plasmodium falciparum, are described.
Enzyme activity with
hypoxanthine,
guanine and
xanthine as substrates eluted in parallel during
hydroxylapatite, size exclusion and
DEAE-Sephadex chromatography as well as during chromatofocusing experiments. Furthermore,
enzyme activity with all three
purine substrates changed in parallel during heat inactivation of
enzyme preparations and upon cold storage (4 degrees C) of the
enzyme. When considered together, these results support the view that the phosphoribosyltransferase is capable of utilizing all three
purine bases as substrates. Additional characterization revealed that the apparent molecular weight and isoelectric point of this
enzyme are 55,500 and 6.2, respectively, and that the apparent Km for 5-phosphoribosyl-1-pyrophosphate ranges from 13.3 to 21.4 microM, depending on the
purine base serving as substrate. The apparent Km values for
hypoxanthine,
guanine and
xanthine were found to be 0.46, 0.30 and 29 microM, respectively. Other experiments showed that several
divalent cations and
sulfhydryl reagents produce a marked reduction of
enzyme activity whereas
dithiothreitol activates the
enzyme. It should be noted that the ability to utilize
xanthine as a substrate serves to distinguish the P. falciparum
enzyme from its counterpart in the parasite's host cell, the human erythrocyte. The human
enzyme shows only barely detectable activity with
xanthine while the parasite
enzyme displays similarly high levels of activity with all three
purine substrates. Thus, the parasite
enzyme might prove to be selectively susceptible to inhibition by
xanthine analogs and related compounds.