A colorimetric determination of inositol monophosphates as an assay for d-glucose 6-phosphate–1l-myoinositol 1-phosphate cyclase

Abstract

A rapid and convenient chemical assay for the enzyme d-glucose 6-phosphate–1l-myoinositol 1-phosphate cyclase is described. The 1l-myoinositol 1-phosphate formed enzymically was oxidized with periodic acid liberating inorganic phosphate, which was assayed. myoInositol 2-phosphate can be assayed in the same way. Glucose 6-phosphate and other primary phosphate esters gave only very small quantities of inorganic phosphate under the conditions described. The K_m of the enzyme for d-glucose 6-phosphate, 7.5±2.5×10⁻⁴m, was identical with that measured by the radiochemical method. 2-Deoxy-d-glucose 6-phosphate was a powerful competitive inhibitor, K_i 2.0±0.5×10⁻⁵m, but was not a substrate for the enzyme.