The Enzymatic Exchange of l-Serine with O-Phospho-l-serine Catalyzed by a Specific Phosphatase

During the course of experiments on the enzymatic synthesis of phosphatidylserine it was found that extracts of rat liver catalyzed the following reactions.P-r-serinel ---f L-serine + Pi(1) P-n-serine + n-serine + Pi(2)An enzyme from chicken liver catalyzing the same reactions was discovered independently by Neuhaus and Byrne (I).This paper will describe the partial purification and some of the properties of the enzyme from rat liver.A preliminary report on some aspects of this work has been published (2).EXPERIMENTAL Materials and Methods-nn-Serine-3-C14 was purchased from the Volk Radiochemical Company.n-Serine-3-V and n-serine-3-V were generous gifts of Dr. Manfred Karnovsky.Orthophosphate labeled with Ps2 was obtained from the Atomic Energy Commission.ATP labeled in the two terminal phosphates with Ps2 was prepared by a method similar to that of Lowenstein (3).Pi was determined by a procedure based on that of Gomori (4).Protein was determined by the method of Warburg and Christian (5).P-Serine and serine were separated by chromatography on Whatman No. 43 filter paper with the ascending technique.The solvent system was 70 per cent ethanol containing 0.02 M citrate buffer of pH 4.8.The RF value for serine in this system was 0.55 and for P-serine, 0.27.Synthesis of P%Yerine-Neuhaus and Korkes (6) have described a convenient procedure for the preparation of P-serine in unlabeled form.However, it is not easy to adapt this procedure for the preparation of P32-labeled serine.Attempts to use the procedure of Plimmer (7) for this synthesis were unsuccessful, but the following modification of the method was found satisfactory.Carrier-free P32, 1 mc., was added to a test tube containing 0.2 ml. of 85 per cent orthophosphoric acid.The solution was kept in an oven at 110" overnight.After the tube was cooled, 0.4 gm. of phosphoric anhydride