3-Phosphoglycerate Phosphatase in Plants: III. Activity Associated with Starch Particles

A particulate form of 3-phosphoglycerate phosphatase represents about 20% of this activity in spinach (Spinacia oleracea var. Longstanding Bloomsdale) leaves. By differential and isopycnic sucrose density gradient centrifugation, all the particulate activity was found in starch grains that pelleted through 2.5 m sucrose. This particulate phosphatase was extremely stable, had a pH optimum of 5.8, and an apparent Michaelis constant (3-phosphoglycerate) of 9 x 10(-4)m. No cation requirement for activity could be demonstrated, and the enzyme was inhibited by 0.5 mm Zn(2+) or Cu(2+). The enzyme was most active in catalyzing the hydrolysis of 3-phospho-d-glycerate, but it was not substrate specific. The phosphatase from the starch grains could not be removed by washing, dialysis, homogenization, or treatment with a French pressure cell, but it was solubilized by prolonged sonication or by addition of 0.25 m MgCl(2), when the particles were suspended in 0.8 m sucrose. The solubilized enzyme was partially purified. The properties of the enzyme on the particles or after solubilization were similar to those previously described for the cytosol form of this phosphatase. It is conjectured that the phosphatase of the starch grain regulates glucan synthesis by controlling a 3-phosphoglycerate pool which is an effector for ADP-glucose pyrophosphorylase.