Spectrophotometric assay for d-ribose-5-phosphate ketol-isomerase and for d-ribulose-5-phosphate 3-epimerase

Spectrophotometric assays for d-ribose-5-phosphate ketol-isomerase and d-ribulose-5-phosphate 3-epimerase are reported. These assays are based upon the increase in absorption at 290 mμ due to the keto group of the products formed from ribose 5-phosphate. The millimolar extinction coefficient for the formation or disappearance of ketopentose phosphate was determined by a number of methods and a value of 0.072 was adopted. The applications of the assays and possible sources of interference are discussed.