7-methyl-6-Thioguanosine is a chromophoric substrate that is used to quantify inorganic phosphate (Pi). In the presence of purine nucleoside phosphorylase, it reacts with Pi to produce 7-methyl-6-thioguanine and ribose 1-phosphate, with the increase in 7-methyl-6-thioguanine followed by continuous spectrophotometry at an absorbance of 355-360 nm.1 7-methyl-6-Thioguanosine is also used to measure changes in Pi levels in coupled enzyme systems, including assays of the activities of protein phosphatases, phosphorylase kinases, GTPases, and ATPases.[1],[2],[3],[4]

Reference:
[1]. Webb, M.R. A continuous spectrophotometric assay for inorganic phosphate and for measuring phosphate release kinetics in biological systems. Proceedings of the National Academy of Sciences of the United States of America 89(11), 4884-4887 (1992).
[2]. Cheng, Q., Wang, Z.X., and Killilea, S.D. A continuous spectrophotometric assay for protein phosphatases. Analytical Biochemistry 310(2), 156-162 (2002).
[3]. Wang, Z.X., Cheng, Q., and Killilea, S.D. A continuous spectrophotometric assay for phosphorylase kinase. Analytical Biochemistry 230(1), 55-61 (1995).
[4]. Tan, Y.C., Wu, W.N., Zheng, Y., et al. Characterization of the interactions between the small GTPase RhoA and its guanine nucleotide exchange factors. Analytical Biochemistry 310(2), 156-162 (2014).