Does the bifunctional uridylate synthase channel orotidine 5'-phosphate? Kinetics of orotate phosphoribosyltransferase and orotidylate decarboxylase activities fit a noninteracting sites model

Orotate phosphoribosyltransferase and orotidine 5'-monophosphate decarboxylase exist as multienzyme complex in human malaria parasite Plasmodium falciparum

Plasmodium falciparum, the causative agent of the most lethal form of human malaria, totally depends on de novo pyrimidine biosynthetic pathway. Orotate phosphoribosyltransferase (OPRT) and orotidine 5'-monophosphate decarboxylase (OMPDC), the fifth and sixth enzymes in the pathway catalyzing formation of uridine 5'-monophosphate (UMP), remain largely uncharacterized in the protozoan parasite. In this study, we achieved purification of OPRT and OMPDC to near homogeneity from P. falciparum cultivated in vitro. The OPRT and OMPDC activities were co-eluted in all chromatographic columns during purification, suggesting the purified proteins exist as a multienzyme complex with a molecular mass of 140+/-8 kDa and contain two subunits each of OPRT and OMPDC. Monomeric forms of OPRT and OMPDC had molecular masses of 32+/-3 and 38+/-3 kDa, respectively, in agreement with those of proteins predicted from P. falciparum genome database. Interestingly, kinetic parameters and inhibitory constants of both OPRT and OMPDC activities were found to be different to those of the bifunctional human red cell UMP synthase. Our evidence provides the first example of OPRT and OMPDC existing as a multienzyme complex.

Uridine-5'-phosphate synthase: evidence for substrate cycling involving this bifunctional protein

Uridine 5'-phosphate (UMP) synthase contains two sequential catalytic activities for the synthesis of orotidine 5'-phosphate (OMP) from orotate (EC 2.4.2.10, orotate phosphoribosyltransferase) and the decarboxylation of OMP to form UMP (EC 4.1.1.23, OMP decarboxylase). Previous kinetic studies had indicated that partial channeling of OMP might occur [T.W. Traut and M.E. Jones (1977) J. Biol. Chem. 252, 8374-8381]; in the presence of a nucleotidase, there was no measurable formation of orotidine from OMP under conditions where OMP was maintained at a steady-state concentration [T.W. Traut (1980) Arch. Biochem. Biophys. 200, 590-594]. Recently claims were made that (i) the steady-state activities of UMP synthase could be modeled by Michaelis-Menten kinetics, and (ii) the nucleotidase activity in Ehrlich ascites cells was insufficient to degrade any significant amount of OMP [R.W. McClard and K.M. Shokat (1987) Biochemistry 26, 3378-3384]. The present studies show that UMP synthase has cooperative kinetics toward OMP, and that a substrate cycle involving orotate phosphoribosyltransferase, cytoplasmic nucleotidase, and uridine phosphorylase maintains the cyclic interconversion: orotate----OMP----orotidine----orotate, etc. It is therefore the complex steady-state kinetics of UMP synthase in the presence of OMP, and the existence of a substrate cycle that account for the results which were interpreted as channeling in the earlier studies.

Inhibition of orotidylate decarboxylase by 4(5H)-oxo-1-beta-D-ribofuranosylpyrazolo[3,4-d] pyrimidine-3-thiocarboxamide (APR-TC) in B lymphoblasts. Activation by adenosine kinase

The nucleoside allopurinol riboside-3-thiocarboxamide (APR-TC; 4-(5H)oxo-1-beta-D-ribofuranosylpyrazolo[3,4,d]pyrimidine-3-thioca rboxamide) demonstrates potent in vitro antiviral activity against various DNA and RNA viruses and cytostatic activity against a variety of cell lines in culture. The IC50 for APR-TC in the splenic derived B lymphoblast cell line, WI-L2, was 0.3 microM. Adenosine kinase-deficient WI-L2 cells were resistant to growth inhibition by APR-TC, indicating that adenosine kinase (EC 2.7.1.20) is responsible for phosphorylation of APR-TC to form the monophosphate derivative (APR-TC-5'P). A 4-hr incubation of cells with 50 microM APR-TC resulted in severe depletion of intracellular pyrimidine nucleotide pools and the accumulation of 3 microM APR-TC-5'P. The cytotoxicity of APR-TC was reversed by uridine, indicating that the active form of this compound inhibits the de novo pyrimidine biosynthetic pathway. Further, APR-TC-treated cells could not utilize the pyrimidine nucleotide precursor [6-14C]orotic acid, suggesting that the UMP synthase complex is the major cellular site of inhibition. In studies utilizing cell-free lysates of WI-L2, chemically prepared APR-TC-5'P provided potent inhibition of the orotidylate decarboxylase activity (ODCase, EC 4.1.1.23) of the UMP synthase complex. APR-TC-5'P was competitive with OMP, and a Ki value of 0.35 nM was determined.