Steady-State Kinetics of the Schistosomal Hypoxanthine-Guanine Phosphoribosyltransferase

Schistosomiasis is a trematode infection of some 200 million people. The hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) of the major etiologic agent, Schistosoma mansoni, has been proposed as a potential target for antischistosomal chemotherapy [Dovey, H.F., McKerrow, J.H., & Wang, C.C. (1984) Mol. Biochem. Parasitol. 11, 157-167]. The steady-state kinetic mechanism for the schistosomal HGPRTase has been determined by including both hypoxanthine and guanine in the forward and reverse reactions under identical conditions. Double-reciprocal plots of initial velocity versus the concentration of one substrate, at a series of fixed concentrations of the other, give groups of intersecting straight lines indicating a sequential mechanism for the schistosomal HGPRTase-catalyzed reactions. In product inhibition studies, the results show that magnesium pyrophosphate (MgPPi) is a noncompetitive inhibitor with respect to dimagnesium phosphoribose pyrophosphate (Mg₂PRPP), hypoxanthine, and guanine. Also, magnesium inosine monophosphate (MgIMP) and magnesium guanosine monophosphate (MgGMP) are noncompetitive inhibitors with respect to hypoxanthine or guanine, respectively, but are competitive inhibitors to Mg₂PRPP. Furthermore, Mg₂PRPP is a competitive inhibitor with respect to MgIMP and MgGMP but is a noncompetitive inhibitor to MgPPi. The minimum kinetic model which fits the experimental data is an ordered bi-bi mechanism, where the substrates bind to the enzyme in a defined order (first Mg₂PRPP followed by the purine bases), while products are released in sequence (first MgPPi followed by MgIMP or MgGMP). The ordered release of products of the schistosomal enzyme apparently is different from that reported for the human HGPRTase in which the release of products is in rapid equilibrium random fashion [Giacomello, A., & Salerno, C. (1978) J. Biol. Chem. 253, 6038-6044]. These results suggest that the design of a highly specific inhibitor of the schistosomal HGPRTase, which binds exclusively to the enzyme-purine nucleotide binary complex, may be possible.