Inhibition of xanthine dehydrogenase from Triatoma infestans by some purine analogues.

Xanthine dehydrogenase (EC 1.2.1.37) activity was determined in the fat body of the Chagas' disease vector Triatoma infestans using a system in which phenazine methosulphate was associated with p-iodonitrotetrazolium violet as electron acceptors for the oxidation of the substrate xanthine. Under the standard conditions used, xanthine was the substrate of choice. The apparent Michaelis-Menten constant (Km) for xanthine was found to be 0.217 +/- 0.020 mM (mean +/- standard deviation of four independent determinations) in the absence of the inhibitors. In the presence of the purine derivatives used the average apparent Km varied from 0.216 mM to 0.219 mM, using the same enzyme preparation. In the presence of the inhibitors tested, competitive inhibition was observed with 8-azaxanthine (Ki = 0.160 mM), adenine hemisulphate (Ki = 0.184 mM) and 6-mercaptopurine (Ki = 0.008 mM). Noncompetitive inhibition was obtained with 8-azaguanine (Ki = 0.077 mM) most probably due to the formation of a dead-end complex between the enzyme, the substrate and the inhibitor.