Provitamin B5 (pantothenol) inhibits growth of the intraerythrocytic malaria parasite

Abstract

Pantothenic acid, a precursor of the crucial enzyme cofactor coenzyme A, is one of a relatively few nutrients for which the intraerythrocytic parasite has an absolute and acute requirement from the external medium. In some organisms the provitamin pantothenol can serve as a source of pantothenic acid; however, this was not the case for the human malaria parasite Plasmodium falciparum. Instead, pantothenol inhibited the in vitro growth of P. falciparum via a mechanism that involves competition with pantothenate and which can be attributed to inhibition of the parasite's pantothenate kinase. Oral administration of pantothenol to mice infected with the murine parasite Plasmodium vinckei vinckei resulted in a significant inhibition of parasite proliferation. This study highlights the potential of the coenzyme A biosynthesis pathway in general, and pantothenate kinase in particular, as an antimalarial drug target.

FIG. 1.

(A) Chemical structures of pantothenic acid and its alcohol, pantothenol. (B) Pantothenate dependence of the growth of P. falciparum in in vitro culture. Removal of the water-soluble vitamins from the culture medium resulted in a cessation of parasite growth. Restoration of just one of these, pantothenate, restored parasite growth to control levels. Pantothenol was unable to substitute for pantothenate in supporting parasite growth. The data are averaged from three or more independent experiments. Error bars represent the SEM.

FIG. 2.

(A) Inhibition of the in vitro growth of P. falciparum and human Jurkat cells by pantothenol. The dose-response curves show the effect of increasing concentrations of pantothenol on [³H]hypoxanthine incorporation by parasites cultured (for 96 h) in medium containing 1 (filled circles), 2 (open squares), 4 (filled triangles), or 8 μM pantothenate (open triangles) or by Jurkat cells cultured (for 96 h) in medium containing 1 μM pantothenate (open circles). (B) Linear relationship between the IC₅₀ for inhibition of [³H]hypoxanthine incorporation by pantothenol and the concentration of pantothenate in the culture medium. (C) Dose-response curves for the inhibition of [³H]hypoxanthine incorporation as a function of the ratio of the concentration of pantothenol to the concentration of pantothenate in the medium (symbols represent the same pantothenate concentrations as in panel A). The dose-response curves are superimposable, demonstrating the competitive and specific nature of the effect of pantothenol on parasite proliferation. The data are averaged from three independent experiments. Error bars in panels A and B represent SEM and, where not shown, fall within the symbols. Error bars are omitted from panel C for clarity.

FIG. 3.

(A) Accumulation of [¹⁴C]pantothenate into isolated P. falciparum trophozoites in the presence (open circles) or absence (closed circles) of 1 mM pantothenol. [[¹⁴C]Pantothenate]_i/[[¹⁴C]Pantothenate]_o is the ratio of the concentration of [¹⁴C]pantothenate inside the parasite to that in the extracellular medium. (B) H⁺-coupled transport of [¹⁴C]pantothenate across the parasite plasma membrane in the presence or absence of 1 mM pantothenol or 0.2 mM phloretin. (C) Effect of pantothenol on the rate of phosphorylation of [¹⁴C]pantothenate in a parasite lysate. The dashed line in panel C represents the dose-response curve for the effect of pantothenol on [³H]hypoxanthine incorporation by the parasite (averaged from Fig. 2C). Complete inhibition of pantothenate phosphorylation in the lysate coincided with full inhibition of [³H]hypoxanthine incorporation. The data in panels A and B are averaged from two independent experiments (error bars represent the range divided by 2), and those in panel C are averaged from three experiments (error bars represent SEM). Where not shown, error bars fall within the symbols.

FIG. 4.

Effect of oral administration of pantothenol on the proliferation of P. vinckei vinckei in an in vivo 4-day suppression test. (A) Parasitemia in mice measured the day after 4 days of treatment with pantothenol or solvent (water) control. Error bars represent SEM. (B) Survival of mice in the days following the end of the 4-day treatment regimen with pantothenol (open circles) or solvent (water; filled circles).