Critical importance of the de novo pyrimidine biosynthesis pathway for Trypanosoma cruzi growth in the mammalian host cell cytoplasm.

The intracellular parasitic protist Trypanosoma cruzi is the causative agent of Chagas disease in Latin America. In general, pyrimidine nucleotides are supplied by both de novo biosynthesis and salvage pathways. While epimastigotes-an insect form-possess both activities, amastigotes-an intracellular replicating form of T. cruzi-are unable to mediate the uptake of pyrimidine. However, the requirement of de novo pyrimidine biosynthesis for parasite growth and survival has not yet been elucidated. Carbamoyl-phosphate synthetase II (CPSII) is the first and rate-limiting enzyme of the de novo biosynthetic pathway, and increased CPSII activity is associated with the rapid proliferation of tumor cells. In the present study, we showed that disruption of the T. cruzi cpsII gene significantly reduced parasite growth. In particular, the growth of amastigotes lacking the cpsII gene was severely suppressed. Thus, the de novo pyrimidine pathway is important for proliferation of T. cruzi in the host cell cytoplasm and represents a promising target for chemotherapy against Chagas disease.

Molecular interaction of the first 3 enzymes of the de novo pyrimidine biosynthetic pathway of Trypanosoma cruzi.

The first 3 reaction steps of the de novo pyrimidine biosynthetic pathway are catalyzed by carbamoyl-phosphate synthetase II (CPSII), aspartate transcarbamoylase (ATC), and dihydroorotase (DHO), respectively. In eukaryotes, these enzymes are structurally classified into 2 types: (1) a CPSII-DHO-ATC fusion enzyme (CAD) found in animals, fungi, and amoebozoa, and (2) stand-alone enzymes found in plants and the protist groups. In the present study, we demonstrate direct intermolecular interactions between CPSII, ATC, and DHO of the parasitic protist Trypanosoma cruzi, which is the causative agent of Chagas disease. The 3 enzymes were expressed in a bacterial expression system and their interactions were examined. Immunoprecipitation using an antibody specific for each enzyme coupled with Western blotting-based detection using antibodies for the counterpart enzymes showed co-precipitation of all 3 enzymes. From an evolutionary viewpoint, the formation of a functional tri-enzyme complex may have preceded-and led to-gene fusion to produce the CAD protein. This is the first report to demonstrate the structural basis of these 3 enzymes as a model of CAD. Moreover, in conjunction with the essentiality of de novo pyrimidine biosynthesis in the parasite, our findings provide a rationale for new strategies for developing drugs for Chagas disease, which target the intermolecular interactions of these 3 enzymes.

Trypanosome alternative oxidase as a target of chemotherapy.

Parasites have developed a variety of physiological functions necessary for their survival within the specialized environment of the host. Using metabolic systems that are very different from those of the host, they can adapt to low oxygen tension present within the host animals. Most parasites do not use the oxygen available within the host to generate ATP, but rather employ systems anaerobic metabolic pathways. The enzymes in these parasite-specific pathways are potential targets for chemotherapy.Cyanide-insensitive trypanosome alternative oxidase (TAO) is the terminal oxidase of the respiratory chain of long slender bloodstream forms of the African trypanosome, which causes sleeping sickness in human and nagana in cattle. TAO has been targeted for the development of anti-trypanosomal drugs because it does not exist in the host. Recently, we found the most potent inhibitor of TAO to date, ascofuranone, a compound isolated from the phytopathogenic fungus, Ascochyta visiae.

Purification of active recombinant trypanosome alternative oxidase.

Trypanosome alternative oxidase (TAO) is the terminal oxidase of the respiratory chain in long slender bloodstream forms of African trypanosomes. TAO is a cytochrome-independent, cyanide-insensitive quinol oxidase. These characteristics are distinct from those of the bacterial quinol oxidases, proteins that belong to the heme-copper terminal oxidase superfamily. The inability to purify stable TAO has severely hampered biochemical studies of the alternative oxidase family. In the present study, we were able to purify recombinant TAO to homogeneity from Escherichia coli membranes using the detergent digitonin. Kinetic analysis of the purified TAO revealed that the specific inhibitor ascofuranone is a competitive inhibitor of ubiquinol oxidase activity.

Strain-specific difference in amino acid sequences of trypanosome alternative oxidase.

Cyanide-insensitive trypanosome alternative oxidase (TAO) is the terminal oxidase of the respiratory chain of long slender bloodstream forms of the African trypanosome, which causes sleeping sickness in human and nagana in cattle. TAO has been targeted for the development of anti-trypanosomal drugs because it does not exist in the host. The cDNA for TAO has been cloned from Trypanosoma brucei brucei EATRO110 strain and has been used for further characterization. In this study, we found amino acid sequence of the C-terminal part of TAO from the strain that we are using, T. b. brucei TC221, is considerably different from that of the EATRO110 strain.

Overproduction of highly active trypanosome alternative oxidase in Escherichia coli heme-deficient mutant.

Cyanide-insensitive trypanosome alternative oxidase (TAO) is the terminal oxidase of the respiratory chain of long slender bloodstream forms of the African trypanosome, which causes sleeping sickness in humans and nagana in cattle. TAO has been targeted for the development of anti-trypanosomal drugs, because it does not exist in the host. In this study, we established a system for overproduction of highly active TAO in Eschericia coli heme-deficient mutant. Kinetic analysis of recombinant enzyme and TAO in Trypanosoma brucei brucei mitochondria revealed that recombinant TAO retains the properties of native enzyme, indicating that recombinant TAO is quite valuable for further biochemical study of TAO.