Characterization of alkaline phosphatases from human first trimester placentas

Alkaline phosphatase from human first trimester placentas was purified, characterized, and compared with alkaline phosphatases from term placenta and liver. Three forms of first trimester placental alkaline phosphatase (I, IIa, and IIb) were isolated; their relative amounts were 35%, 39%, and 26%, respectively. Phosphatases I and IIa were found to be dimers, whereas phosphatase IIb appeared to be a tetramer consisting of two dimers of phosphatase I or IIa. Phosphatase I was indistinguishable from liver phosphatase by several criteria including apparent molecular weight (Mr = 165,000), size of the monomeric subunit (Mr = 77,000), heat liability, insensitivity to inactivation by antiserum against term placental alkaline phosphatase, and sensitivity to inactivation by antiserum against liver alkaline phosphatase. In addition, phosphatase I and liver phosphatase were equally sensitive to inhibition by amino acids, levamisole, l-p-bromotetramisole, and EDTA. Phosphatase IIa, in contrast, was indistinguishable from term placental alkaline phosphatase by the same criteria: apparent molecular weight (Mr = 115,000), size of the monomeric subunit (Mr = 63,000), heat stability, inactivation by antiserum against term placental alkaline phosphatase, and sensitivity to inhibition by various compounds. These findings clearly demonstrate the existence of two distinct placental alkaline phosphatases, one (phosphatase I) specific for the first trimester placenta and the other (phosphatase IIa) occurring in both first trimester and term placentas.