| Abstract: | 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-DHasc. 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 K, of 7μM. Dihydro-5-azaorotate inhibited the DHO-DHase (K, 16 μM) and 5-azaorotate (Ki, 21 μM) was an inhibitor of the OPRTase. The UMP analog, 6-aza-UMP (Ki, 0.3 μM) was a potent inhibitor of ODCase, while lower levels of inhibition were found with the product. UMP (Ki, 120 μM) and the purine nucleotide, XMP (K1, 95 μM). Additionally, menoctone, a ubiquinone analog, was shown to inhibit DHO-DHase. |
