7-Methyl-6-thioguanosine

7-Methyl-6-thioguanosine (MESG) is a chromogenic substrate that can be converted to 7-methyl-6-thioguanine in the presence of purine nucleoside phosphorylase and inorganic phosphate. 7-Methyl-6-thioguanosine can be used to quantify inorganic phosphate. 7-Methyl-6-thioguanosine can also be used to determine the activities of purine nucleoside phosphorylase and protein phosphatases.