Assay of inorganic phosphate in the mild pH range, suitable for measurement of glycogen phosphorylase activity

A spectrophotometric assay of orthophosphate at mild pH is described. Phosphomolybdate complex is reduced by ascorbic acid at pH 5.0 in the presence of Zn2+. The chromophore produced is measured at 850 nm. The method is simple and sensitive, and the reaction proceeds normally in the presence of "interfering substances" such as metal-chelating agents and thiol compounds. The method could be widely employed for the assay of phosphate-releasing reaction from labile organic phosphates, such as for the assay of glycogen phosphorylase.