Inositol phosphatase activity of the Escherichia coli agp-encoded acid glucose-1-phosphatase

When screening an Escherichia coli gene library for myo-inositol hexakisphosphate (InsP(6)) phosphatases (phytases), we discovered that the agp-encoded acid glucose-1-phosphatase also possesses this activity. Purified Agp hydrolyzes glucose-1-phosphate, p-nitrophenyl phosphate, and InsP(6) with pH optima, 6.5, 3.5, and 4.5, respectively, and was stable when incubated at pH values ranging from 3 to 10. Glucose-1-phosphate was hydrolyzed most efficiently at 55degreesC, while InsP(6) and p-nitrophenyl phosphate were hydrolyzed maximally at 60degreesC. The Agp exhibited K-m values of 0.39 mM, 13 mM, and 0.54 mM for the hydrolysis of glucose-1-phosphate, p-nitrophenyl phosphate, and InsP(6), respectively. High-pressure liquid chromatography (HPLC) analysis of inositol phosphate hydrolysis products of Agp demonstrated that the enzyme catalyzes the hydrolysis of phosphate from each of InsP(6), D-Inst(1,2,3,4,5)P-5. Ins(1,3,4,5,6)P-5, and Ins(1,2,3,4,6)P-5, producing D/L-Ins(1,2,4,5,6)P-5, D-Ins(1,2,4,5)P-4, D/L-Ins(1,4,5,6)P-4 and D/L-Ins(1,2,4,6)P-4, respectively. These data support the contention that Agp is a 3-phosphatase.