Human alkaline phosphatases. I. Purification and some structural properties of the enzyme from human liver

Alkaline phosphatase (EC 3.1.3.1) from human liver was purified to homogeneity. It is a glycoprotein of about 136000 molecular weight. Analysis of the subunit structure by sedimentation equilibrium in 6M urea and by dodecylsulfate polyacrylamide gel electrophoresis of the denatured enzyme indicated molecular weights of about 35000 and 32000, respectively. Thus, the human liver alkaline phosphatase seems to be a tetramer composed of identical or very similar subunits. The purified enzyme has a specific activity of 1360 mumol/(min x mg prot.), corresponding to a molecular activity of 3170s-1. The enzyme retains full activity in 1% sodium dodecylsulfate for several hours. The isoelectric point of the native enzyme is 4.2. After treatment with neuraminidase the isoelectric point increases to 6.5.