Figure 1. Enzymatic conversion of 2-amino-6-mercapto-7-methylpurine riboside (methylthioguanosine, a guanosine analog: MESG) into ribose 1-phosphate and 2-amino-6-mercapto-7-methylpurine by purine nucleoside phosphorylase (PNP) in the presence of inorganic phosphate released from ATP (substrate) by ATPase (enzyme to be tested).