Extracellular Ca²⁺-dependent inducible alkaline phosphatase from the extremely halophilic archaebacterium Haloarcula marismortui

When starved of inorganic phosphate, the extremely halophilic archaebacterium Haloarcula marismortui produces the enzyme alkaline phosphatase and secretes it to the medium. This inducible extracellular enzyme is a glycoprotein whose subunit molecular mass is 160 kDa, as estimated by sodium dodecyl sulfate-gel electrophoresis. The native form of the enzyme is heterogeneous and composed of multiple oligomeric forms. The enzymatic activity of the halophilic alkaline phosphatase is maximal at pH 8.5, and the enzyme is inhibited by phosphate. Unlike most alkaline phosphatases, the halobacterial enzyme require Ca²⁺ and not Zn²⁺ ions for its activity. Both calcium ions (in the millimolar range) and NaCl (in the molar range) are required for the stability of the enzyme.