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J. Biochem, 1987, Vol. 101, No. 4 939-947
© 1987 Japanese Biochemical Society

other

Phosphotyrosine Phosphatase: A Novel Phosphatase Specific for Phosphotyrosine, 2'-AMP and p-Nitrophenylphosphate in Rat Brain

Noboru MOTOYAMA, Kouichi TAKIMOTO, Masato OKADA and Hachiro NAKAGAWA

Division of Protein Metabolism, Institute for Protein Research, Osaka University Suita, Osaka 565

A unique phosphatase that selectively hydrolyzed phosphotyrosine and 2'-AMP at alkaline pH and p-nitrophenylphosphate at neutral pH was isolated from a cytosolic fraction of rat brain. The purified enzyme appeared homogeneous on SDS-polyacrylamide gel electrophoresis and its molecular weight was estimated to be 42,000. The molecular weight of the native enzyme was 45, 000 as determined by molecular sieve chromatography. These findings indicate that the native enzyme is a monomer protein. At pH 8.6, the enzyme hydrolyzed L-phosphotyrosine, D-phosphotyrosine, 2'-AMP, p-nitrophenylphosphate, 3'-AMP, 2'-GMP, and 3'-GMP; the ratio of its activities with these substrates was 100: 96: 115: 68: 39: 25: 16. Its Km values for L-phosphotyrosine, 2'-AMP, and p-nitrophenylphosphate were 0.8 × 10–4 M, 1.4 × 10–4 M, and 1.7 × 10–4 M, respectively. At pH 7.4, the enzyme hydrolyzed p-nitrophenylphosphate, L-phosphotyrosine, and D-phosphotyrosine; the ratio of its activities with these compounds was 100: 17: 17, and its Km values for L-phosphotyrosine and p-nitrophenylphosphate were 1.8 × 10–4 M and 2.0 × 10–4 M, respectively. The enzyme activity was dependent on Mn2+ or Mg2+, and was strongly inhibited by 5'-nucleotides, pyrophosphate, and Zn2+. The 5'-nucleotides caused competitive inhibition of the binding of substrates. The enzyme was not sensitive to inhibitors of some well-characterized phosphatases such as NaF, molybdate, L(+)tartrate, tetramisole, vanadate, and lithium salt. The physiological role of the enzyme is discussed with respect to its activities toward phosphotyrosine, 2'-AMP, and p-nitrophenylphosphate.


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