Nucleoside hydrolases from Trypanosoma cruzi

Four distinct nucleoside cleaving enzymes were detected in extracts of Trypanosoma cruzi epimastigotes. Two of these were nucleoside phosphorylases: one was specific for pyrimidines and the other for purines. The other two enzymes were nucleoside hydrolases. These hydrolytic enzymes were partially purified and their substrate and inhibitor specificities were studied. One of the hydrolases was designated inosine/guanosine hydrolase. It required purine substrates containing a 9-beta-D-ribofuranosyl substituent. Neither 2'-deoxyinosine, 2'-deoxyguanosine, nor hypoxanthine arabinoside served as substrates. The kinetic patterns obtained from combined product analysis and the mutual competitive inhibition of this enzyme by inosine and guanosine suggested that these substrates were cleaved at a common catalytic site. Inhibitor studies with several deoxyinosines have demonstrated the importance of the 2'-, 3'-, and 5'-hydroxyl groups for efficient binding to the enzyme. Adenosine, which did not serve as a substrate, was a potent competitive inhibitor (Ki = 8 microM) with respect to both inosine and guanosine. This enzyme had a particle weight of 106,000 +/- 10,000 as determined by Sephadex chromatography. The other hydrolase was specific for 2'-deoxyinosine. It only accepted purine substrates with a 6-oxo- and a 9-beta-D-2'-deoxyribofuranosyl substituent. Inosine, hypoxanthine arabinoside and a variety of deoxyinosines were not substrates nor did they strongly inhibit this enzyme. This enzyme had a particle weight of 19,000 +/- 2,000 as determined by Sephadex chromatography.