Myo-Inositol-1-Phosphatase from the Pollen of Lilium longiflorum Thunb.

Abstract

A Mg(2+)-dependent, alkaline phosphatase has been isolated from mature pollen of Lilium longiflorum Thunb., cv. Ace and partially purified. It hydrolyzes 1l- and 1d-myo-inositol 1-phosphate, myo-inositol 2-phosphate, and beta-glycerophosphate at rates decreasing in the order named. The affinity of the enzyme for 1l- and 1d-myo-inositol 1-phosphate is approximately 10-fold greater than its affinity for myo-inositol 2-phosphate. Little or no activity is found with phytate, d-glucose 6-phosphate, d-glucose 1-phosphate, d-fructose 1-phosphate, d-fructose 6-phosphate, d-mannose 6-phosphate, or p-nitrophenyl phosphate. 3-Phosphosphoglycerate is a weak competitive inhibitor. myo-Inositol does not inhibit the reaction. Optimal activity is obtained at pH 8.5 and requires the presence of Mg(2+). At 4 millimolar, Co(2+), Fe(2+) or Mn(2+) are less effective. Substantial inhibition is obtained with 0.25 molar Li(+). With beta-glycerophosphate as substrate the K(m) is 0.06 millimolar and the reaction remains linear at least 2 hours. In 0.1 molar Tris, beta-glycerophosphate yields equivalent amounts of glycerol and inorganic phosphate, evidence that transphosphorylation does not occur.In higher plants this myo-inositol-1-phosphatase links myo-inositol biosynthesis to the myo-inositol oxidation pathway to produce an alternative path from d-glucose 6-phosphate to UDP-d-glucuronate that bypasses UDP-d-glucose dehydrogenase. myo-Inositol-1-phosphatase also furnishes free myo-inositol for reactions that lead to other cyclitols and cyclitol-containing compounds of biosynthetic and/or regulatory significance in plant growth and development.