Interference of MPA with de novo guanosine nucleotide synthesis. ATP, adenosine monophosphate; IMP, inosine monophosphate; XMP, xanthosine monophosphate; GMP, guanosine monophosphate; GTP, guanosine triphosphate; dGTP, deoxyguanosine triphosphate; IMPDH, inosine monophosphate dehydrogenase.