Cysteine biosynthesis inhibitors of Entamoeba histolytica from secondary metabolites of microorganisms

Human amebiasis, an intestinal infection, is caused by the protozoan parasite Entamoeba histolytica. An antibacterial and antiprotozoal drug, metronidazole, has been used to treat amebiasis for several decades. However, metronidazole is not effective against cysts of the protozoa, and resistant protozoa have already appeared. Therefore, we screened novel drug candidates of human amebiasis from secondary metabolites of microorganisms.

We focused on a sulfur assimilatory cysteine biosynthetic pathway to screen selective anti-amebic compounds. Cysteine plays essential roles in synthesis of various proteins, proliferation, adherence, and defense against oxidative stress in E. histolytica. This pathway consists of two steps catalyzed by serine acetyltransferase (SAT) and cysteine synthase (CS). In contrast, cysteine is biosynthesized from methionine and serine in mammals. The two enzymes, SAT and CS, are specific to E. histolytica, and each of three isoenzymes of SAT1-3 and CS1-3 are known. We selected these enzymes as targets and screened their inhibitors from 300 compounds in the Kitasato Natural Products Library and more than 3,000 samples of microbial cultured broths.

Several compounds were found to inhibit SAT or CS from the Natural Products Library and the broth screening. For example, ascofuranone (IC₅₀ 120 µM) and naphthacemycin A₉ (IC₅₀ 19 µM) in the Natural Products Library inhibited SAT1 and CS1, respectively. Exophillic acid isolated from the culture broth of a fungus Exophiala sp. FKI-7082 inhibited CS1 at the IC₅₀ value at 19 µM. We will report the structures of inhibitors and their inhibitory activities against enzymes of E. histolytica in this presentation.