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Oxidative inactivation of brain alkaline phosphatase responsible for hydrolysis of phosphocholine

Journal of Neurochemistry, ISSN: 0022-3042, Vol: 72, Issue: 1, Page: 355-362
1999
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Alkaline phosphatase, one of the enzymes responsible for the conversion of phosphocholine into choline, was purified from bovine brain membrane, where the phosphatase is bound as glycosylphosphatidylinositol-linked protein, and subjected to oxidative inactivation. The phosphatase activity, based on the hydrolysis of p-nitrophenyl phosphate and phosphocholine, decreased slightly after the exposure to HO. Inclusion of Cu in the incubation with 1 mM HO led to a rapid decrease of activity in a time- and concentration-dependent manner. In comparison, the HO/Cu system was much more effective than the HO/Fe system in inactivating brain phosphatase. In a further study, it was observed that the hydroxy radical scavengers mannitol, ethanol, or benzoate failed to prevent against HO/ Cu-induced inactivation of the phosphatase, excluding the involvement of extraneous hydroxy radicals in metal-catalyzed oxidation. In addition, it was found that both substrates, p-nitrophenyl phosphate and phosphocholine, and an inhibitor, phosphate ion, at their saturating concentrations exhibited a remarkable, although incomplete, protection against the inactivating action of HO/ Cu. A similar protection was also expressed by divalent metal ions such as Mg or Mn. Separately, it was found that HO/Fe- induced inactivation was prevented by p-nitrophenyl phosphate or Mg but not phosphate ions. Thus, it is implied that phosphocholine-hydrolyzing alkaline phosphatase in brain membrane might be one of enzymes susceptible to metal-catalyzed oxidation.

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