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.