title: Enzymes of de novo pyrimidine biosynthesis in Babesia rodhaini.
abstract: The pathway of de novo pyrimidine biosynthesis in the rodent parasitic protozoa Babesia rodhaini has been investigated. Specific activities of five of the six enzymes of the pathway were determined: aspartate transcarbamylase (ATCase: E.C. 2.1.3.2); dihydroorotase (DHOase: E.C. 3.5.2.3); dihydroorotate dehydrogenase (DHO-DHase: E.C. 1.3.3.1); orotate phosphoribosyltransferase (OPRTase: E.C. 2.4.2.10); and orotidine-5'-phosphate decarboxylase (ODCase: E.C. 4.1.1.23). Michaelis constants for ATCase, DHO-DHase, OPRTase, and ODCase were determined in whole homogenates. Several substrate analogs were also investigated as inhibitors and inhibitor constants determined. N-(phosphonacetyl)-L-aspartate was shown to be an inhibitor of the ATCase with an apparent Ki of 7 microM. Dihydro-5-azaorotate inhibited the DHO-DHase (Ki, 16 microM) and 5-azaorotate (Ki, 21 microM) was an inhibitor of the OPRTase. The UMP analog, 6-aza-UMP (Ki, 0.3 microM) was a potent inhibitor of ODCase, while lower levels of inhibition were found with the product, UMP (Ki, 120 microM) and the purine nucleotide, XMP (Ki, 95 microM). Additionally, menoctone, a ubiquinone analog, was shown to inhibit DHO-DHase.
mesh_terms: D000818:Animals; D001221:Aspartate Carbamoyltransferase / Q000378:metabolism; D001403:Babesia / Q000201:enzymology*; D004080:Dihydroorotase / Q000378:metabolism; D004081:Dihydroorotate Oxidase / Q000378:metabolism; D007700:Kinetics; D009962:Orotate Phosphoribosyltransferase / Q000378:metabolism; D009964:Orotidine-5'-Phosphate Decarboxylase / Q000378:metabolism; D011743:Pyrimidines / Q000096:biosynthesis*
chemical_list: D011743:Pyrimidines; D004081:Dihydroorotate Oxidase; D001221:Aspartate Carbamoyltransferase; D009962:Orotate Phosphoribosyltransferase; D004080:Dihydroorotase; D009964:Orotidine-5'-Phosphate Decarboxylase