Purine Metabolism in the Protozoan Parasite Eimeria Tenella

Crude extracts of the oocysts of Eimeria tenella, a protozoan parasite of the coccidium family that develops inside the caecal epithelial cells of infected chickens, do not incorporate glycine or formate into purine nucleotides; this suggests lack of capability for de novo purine synthesis by the parasite. The extracts, however, contain high levels of activity of the purine salvage enzymes: hypoxanthine, guanine, xanthine, and adenine phosphoribosyltransferases and adenosine kinase. The absence of AMP deaminase from the parasite indicates that E. tenella cannot convert AMP to GMP; the latter thus has to be supplied by the hypoxanthine, xanthine, or guanine phosphoribosyltransferase of the parasite. These three activities are associated with one enzyme (HXGPRTase), which has been purified to near homogeneity in high yield (71-80%) in a single step by GMP-agarose affinity column chromatography. The size of the enzyme subunit is estimated to be 23,000 daltons by NaDodSO4 gel electrophoresis. Kinetic studies suggest differences in purine substrate specificity between E. tenella HXGPRTase and chicken liver HGPRTase. Allopurinol preferentially inhibits the parasite enzyme by competing with hypoxanthine; a Ki approximately 22 microM.