Isolation and Characterization of Membrane‐Bound Arylamidases from Human Placenta and Kidney

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Abstract
Membrane‐bound arylamidases (EC 3.4.11.2) from human placenta and kidney were purified. The enzymes were solubilized from membrane fractions using trypsin. Purification procedures included DEAE‐cellulose column chromatography, concanavalin‐A–Sepharose 4B column chromatography, hydroxyapatite column chromatography and Sephadex G‐200 gel filtration. The final recoveries of placental and kidney enzymes were 22.0% and 20.8%, respectively. Polyacrylamide gel electrophoresis, analytical isoelectric focusing, immunoelectrophoresis and ultracentrifugation indicated the homogeneity of both purified enzymes. Equilibrium ultra‐centrifugation showed molecular weights of 193000 for placental enzyme and 211000 for kidney enzyme. Electrophoresis of the enzymes under denaturing conditions on dodecylsulfate/polyacrylamide gel indicated that each enzyme was a dimer consisting of two identical subunits. Isoelectric points of placental and kidney enzymes were 4.20 and 4.32 at 4°C, respectively. The amino acid compositions of the two membrane‐bound arylamidases were similar. The carbohydrate accounted for 18.2% of placental enzyme and 17.4% of kidney enzyme. The purified placental enzyme had a specific activity of 60.1 μmol min−1 (mg protein)−1 and the kidney enzyme had a specific activity of 96.4 μmol min−1 (mg protein)−1. Kcat values of placental and kidney enzymes were 187.8 s−1 and 328.6 s−1, respectively. The hydrolytic coefficient (Kcat/Km) of placental enzyme for l‐alanyl‐β‐naphthylamide was 2159 mM−1 s−1 and that of kidney enzyme was 3778 mM−1 s−1. Rabbit antibodies against placental and kidney membrane‐bound arylamidases inhibited the activities of the corresponding antigens, but the inhibitions were never complete. Three membrane‐bound arylamidases from placenta, kidney and renal cell carcinoma were immunochemically indistinguishable.