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Alkaline Phosphatase (EC 3.1.3.1) in Serum Is Inhibited by Physiological Concentrations of Inorganic Phosphate¹

Biochemistry Department, Fort Wayne State Developmental Center (S.P.C., J.D.M., M.J.), Fort Wayne, Indiana 46835; the Department of Mathematical Sciences (Y.Z.), Indiana University and Purdue University, Fort Wayne, Indiana 46805; and the Department of Medicine, Southern Illinois University (J.W.), Springfield, Illinois 62702

Address all correspondence and requests for reprints to: Dr. Stephen P. Coburn, Fort Wayne State Developmental Center, 4900 St. Joe Road, Fort Wayne, Indiana 46835. E-mail: coburn{at}ipfw.edu

Natural and artificial manipulation of tissue nonspecific alkaline phosphatase activity indicates that pyrophosphate, phosphoethanolamine, and pyridoxal 5'-phosphate are among the natural substrates for this enzyme. Although inorganic phosphate has been recognized as a competitive inhibitor of this enzyme for many years, the influence of phosphate on alkaline phosphatase activity in serum under physiological conditions has not been previously reported. We examined the kinetics of tissue nonspecific alkaline phosphatase from bovine kidney and sera from 49 patients with a wide range of endogenous phosphate concentrations using pyridoxine 5'-phosphate as a substrate at pH 7.4. For the bovine kidney enzyme, the K_m was 0.42 ± 0.04 µmol/L, and the K_i for phosphate was 2.4 ± 0.2 µmol/L. Analysis of the kinetics using pyridoxine 5'-phosphate in undiluted serum from 10 subjects with phosphorus ranging from 0.5–2.1 mmol/L and alkaline phosphatase activity ranging from 41–165 nmol/min·mL gave estimates for the K_m of 56 ± 11 µmol/L and for the K_i of 540 ± 82 µmol/L for phosphate. This indicates that under physiological conditions alkaline phosphatase activity toward pyridoxine 5'-phosphate is reduced approximately 50% by the normal phosphate concentration and that it will increase or decrease significantly in response to changes in phosphate concentration within the ranges observed clinically.

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