ABSTRACT
The hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) of human and the parasitic trematode, Schistosoma mansoni, were expressed at high levels in transformed Escherichia coli in their native forms. Guanosine 2',3'-dialdehyde 5'-phosphate (ox-GMP) was shown to bind irreversibly to both enzymes in a time-dependent manner. This binding was stabilized by sodium borohydride reduction, suggesting that a Schiff's base is formed between the dialdehyde groups of ox-GMP and the amino group of a lysine residue in the enzymes. This linkage formation applies also to inosine 2',3'-dialdehyde 5'-phosphate but not to adenosine 2',3'-dialdehyde 5'-phosphate. GMP was found to be protective against ox-GMP inactivation and [3H]ox-GMP labeling of both HGPRTases. 5-Phosphoribosyl-1-diphosphate (PRibPP) also protects human HGPRTase against the ox-GMP inactivation and [3H]ox-GMP labeling but provides virtually no protection against the ox-GMP inactivation and labeling of the schistosomal enzyme, even though PRibPP binds to the latter with a threefold higher affinity. These results imply that PRibPP and ox-GMP compete with each other for binding to the human HGPRTase but not for binding to the schistosomal enzyme. This discrepancy could be exploited for the purpose of designing selective inhibitors of the schistosomal HGPRTase. Guanosine 2',3'-dialdehyde (ox-guanosine) is nearly as active as ox-GMP in inhibiting schistosomal HGPRTase but much less potent in inhibiting human HGPRTase, suggesting that ox-guanosine and ox-GMP may bind equally well to the parasite enzyme. PRibPP can protect human but not schistosomal HGPRTase against the inactivation by ox-guanosine. Therefore, ox-GMP and ox-guanosine must be forming Schiff's bases with the same amino acid residues in each of the two HGPRTases.
