Glutamine amidotransferase/
aminodeoxychorismate synthase (
GAT-ADCS) is a bifunctional
enzyme involved in the synthesis of
p-aminobenzoate, a central component part of
folate cofactors.
GAT-ADCS is found in eukaryotic organisms autonomous for
folate biosynthesis, such as plants or parasites of the phylum Apicomplexa. Based on an automated screening to search for new inhibitors of
folate biosynthesis, we found that
rubreserine was able to inhibit the
glutamine amidotransferase activity of the plant
GAT-ADCS with an apparent IC(50) of about 8 μM. The growth rates of Arabidopsis thaliana, Toxoplasma gondii, and Plasmodium falciparum were inhibited by
rubreserine with respective IC(50) values of 65, 20, and 1 μM. The correlation between
folate biosynthesis and growth inhibition was studied with Arabidopsis and Toxoplasma. In both organisms, the
folate content was decreased by 40-50% in the presence of
rubreserine. In both organisms, the addition of
p-aminobenzoate or
5-formyltetrahydrofolate in the external medium restored the growth for inhibitor concentrations up to the IC(50) value, indicating that, within this range of concentrations,
rubreserine was specific for
folate biosynthesis.
Rubreserine appeared to be more efficient than
sulfonamides,
antifolate drugs known to inhibit the invasion and proliferation of T. gondii in human fibroblasts. Altogether, these results validate the use of the bifunctional
GAT-ADCS as an efficient
drug target in eukaryotic cells and indicate that the chemical structure of
rubreserine presents interesting anti-parasitic (
toxoplasmosis,
malaria) potential.