The enzymes of pyrimidine biosynthesis in Babesia bovis and Babesia bigemina.

All six enzymes of de novo pyrimidine biosynthesis leading to the formation of UMP have been demonstrated in whole homogenates from two bovine Babesia species, B. bovis and B. bigemina. The specific activities of the respective enzymes were of the same order of magnitude as observed for the related parasite, Plasmodium berghei. The results indicate that both these parasites have the potential of obtaining their pyrimidine requirements by de novo synthesis. Subcellular fractionation established that dihydroorotate dehydrogenase, the fourth enzyme of the pathway, was of a particulate nature. Mammalian respiratory chain inhibitors and ubiquinone analogues caused inhibition of the Babesia dihydroorotate dehydrogenase. As observed for other eukaryotic systems, the dehydrogenase appears to be linked to a respiratory chain via ubiquinone.