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

Institute of Biological Chemistry, Washington State University, Pullman, Washington 99164

A Mg²⁺-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 -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 Mg²⁺. At 4 millimolar, Co²⁺, Fe²⁺ or Mn²⁺ are less effective. Substantial inhibition is obtained with 0.25 molar Li⁺. With -glycerophosphate as substrate the K_m is 0.06 millimolar and the reaction remains linear at least 2 hours. In 0.1 molar Tris, -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.

¹ Supported by Grant GM-22427 from the National Institute of General Medical Sciences, National Institutes of Health, United States Public Health Service. Scientific Paper No. 6173, Project 0266, College of Agriculture Research Center, Washington State University, Pullman, WA 99164.

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