Nucleotide Pyrophosphatase of Rat Liver. A Comparative Study on the Enzymes Solubilized and Purified from Plasma Membrane and Endoplasmic Reticulum

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Abstract
Studies on the subcellular distribution of ratliver nucleotide pyrophosphatase activity revealed its presence in the plasma membrane and the endoplasmic reticulum only. The enzymes from either source were solubilized specifically with trypsin without an apparent change of their catalytic properties. A 2000-fold and 1600-fold purification, respectively, was achieved by a procedure including DEAE-cellulose and affinity-chromatography with AMP as ligand, gel filtration on Sephadex G-200 and gel electrophoresis. Both nucleotide pyrophosphatases were isolated as electrophoretically homogeneous soluble proteins. They were shown to contain carbohydrate moieties. The electrophoretic mobility of both enzymes in polyacrylamide gels was identical at three pH values. Dodecylsulfate gel electrophoresis indicated a molecular weight of 137000 for both glycoproteins. The enzymes hydrolyze a variety of purine and pyrimidine nucleotides yielding a 5′-nucleoside monophosphate. Adenosine 3′:5′-monophosphate, nucleic acids and phosphate monoesters are not cleaved, but p-nitrophenyl-thymidine 5′-monophosphate is readily hydrolyzed. In view of their substrate and inhibitor specificities the enzymes are considered nucleotide pyrophosphatases rather than phosphodiesterases.