Amino Acid Phosphatase Activity of Alkaline Phosphatase

Abstract

Alkaline phosphatase (ALP) hydrolyzed phosvitin and amino acid phosphates demonstrating nonisotropy at different pH. Orthovanadate, a protein phosphatase inhibitor, more specifically inhibited the serine and tyrosine phosphatase activities of ALP than that of threonine phosphatase at concentrations > 0.1 mM or 0.01 mM, respectively. Calyculin A and okadaic acid at increased concentrations increased ALP amino acid phosphatase activity. Bisphosphonates, such as disodium-1-hydroxy-1-aminopropylidine-1,1-diphosphonate (APD) and ethane-1-hydroxy-1,1-diphosphonate (HEBP), at increased concentrations, inhibited ALP amino acid phosphatase activity. These results suggest that ALP may function as a protein phosphatase. In terms of protein kinase inhibitors, N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide, N-(6-aminoheyxl)-5-chloro-1-naphthalenesulfomide hydrochloride and 4',5,7-trihydroxyisoflavone had little effect on ALP amino acid phosphatase activity. Staurosporine slightly enhanced ALP serine and threonine phosphatase activities at a concentration of 0.1 mM. These results suggest that protein phosphatase activity does not depend on the protein kinase activity of ALP, since duality between the former and the latter is not supported. ALP may function less as a protein kinase than as a protein phosphatase. The coupling mechanism of phosphate dynamics may be regulated indirectly.