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 Km 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, Ki 2.0±0.5×10−5m, but was not a substrate for the enzyme.

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