Purine metabolism in the protozoan parasite Eimeria tenella.

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Abstract
Crude extracts of the oocysts of E. tenella, a protozoan parasite of the coccidium family that develops inside the cecal 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 3 activities are associated with 1 enzyme hypoxanthine-xanthine-quinine phosphoribosyltransferase (HXGPRTase), which was 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 [sodium dodecyl sulfate] gel electrophoresis. Kinetic studies suggest differences in purine substrate specificity between E. tenella HXGPRTase and chicken liver HGPRTase [hypoxanthine-guanine phosphoribosyltransferase]. Allopurinol preferentially inhibits the parasite enzyme by competing with hypoxanthine; a Ki .apprx. 22 .mu.M.