Biomedical subjects
Publications and source records attributed to D D Randall.
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3-Phosphoglycerate phosphatase and phosphoglycolate phosphatase were found in leaves of all 52 plants examined. Activities of both phosphatases varied widely between 1 to 20 micromoles per minute per milligram chlorophyll. Plants were grouped into two categories based upon the relative ratio of activity of 3-phosphoglycerate phosphatase to phosphoglycolate phosphatase. This ratio varied between 2:1 to 4:1 in the C(4)-plants except corn leaves which had a low level of 3-phosphoglycerate phosphatase. This ratio was reversed and varied between 1:2 to 1:6 in all C(3)-plants except one bean variety which had large amounts of both phosphatases. By differential grinding procedures for C(4) plants a major part of the 3-phosphoglycerate phosphatase was found in the mesophyll cells and P-glycolate phosphatase in the bundle sheath cells. Phosphoglycolate phosphatase, but not 3-phosphoglycerate phosphatase, was located in chloroplasts of C(3)- and C(4)- plants. Formation of 3-phosphoglycerate phosphatase increased 4- to 12-fold during greening of etiolated sugarcane leaves. This cytosol phosphatase displayed a diurnal variation in sugarcane leaves by increasing 50% during late daylight hours and early evening. It is proposed that the soluble form of 3-phosphoglycerate phosphatase is necessary for carbon transport between the bundle sheath and mesophyll cells during photosynthesis by C(4)-plants. In C(3)- and C(4)-plants this phosphatase initiates the conversion of 3-phosphoglycerate to serine which is an alternate metabolic pathway to glycolate metabolism and photorespiration.
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.