STRUCTURE-ACTIVITY-RELATIONSHIPS FOR THE BINDING OF LIGANDS TO XANTHINE OR GUANINE PHOSPHORIBOSYL TRANSFERASE FROM TOXOPLASMA-GONDII

Preliminary characterization of Toxoplasma gondii phosphoribosyltransf erase activity towards purine nucleobases indicates that there are at least two enzymes present in these parasites. One enzyme uses hypoxant hine, guanine, and xanthine as substrates, while a second enzyme uses only adenine. Furthermore, competition experiments using the four poss ible substrates suggest that there may be a third enzyme that uses xan thine. Therefore, sixty-eight purine analogues and thirteen related de rivatives were evaluated as ligands of T. gondii phosphoribosyltransfe rase, using xanthine or guanine as substrates, by examining their abil ity to inhibit these reactions in vitro. Inhibition was quantified by determining apparent K-i values for compounds that inhibited these act ivities by greater than 10% at a concentration of 0.9 mM. On the basis of these data, a structure-activity relationship for the binding of l igands to these enzymes was formulated using hypoxanthine (6-oxopurine ) as a reference compound. It was concluded that the following structu ral features of purine analogues are required or strongly preferred fo r binding to both enzymes: (1) a pyrrole-type nitrogen (lactam form) a t the 1-position; (2) a methine (=CH-), a pyridine type nitrogen (=N-) , or an exocyclic amino or oxo group at the 2-position; (3) no exocycl ic substituents at the 3-position; (4) an exocyclic oxo or thio group in the one or thione tautomeric form at the 6-position; (5) a pyridine -type nitrogen (=N-) or a methine group at the 7-position; (6) a methi ne group at the 8-position; (7) a pyrrole-type nitrogen or a carbon at the 9-position; and (8) no exocyclic substituents at the 9-position. These findings provide the basis for the rational design of additional ligands of hypoxanthine, guanine, and xanthine phosphoribosyltransfer ase activities in T, gondii.