Background：

ALPP (Alkaline phosphatase placental type; also SEAP， PALP-1 and ALP) is a 66-68 kDa glycoprotein member of the alkaline phosphatase family of molecules. It is principally expressed by 2nd and 3rd trimester synctiotrophoblasts and tumor cells， and represents one of four alkaline phosphatase isozymes. Experimentally， it cleaves phosphate monoesters into alcohol and phosphate. Human ALPP is synthesized as a 535 amino acid (aa) preproprecursor. It contains a 22 aa signal sequence， a 484 mature region (aa 23-506)， and a 29 aa C-terminal propeptide that is cleaved to generate a GPI linkage at Asp506. The ALPP gene is highly allelic and may generate three potential splice variants that involve the signal sequence. The ALPP enzyme utilizes one Mg and two Zn ions， and functions as either a homodimer， or a heterodimer with the related Intestinal (ALPI) and Germ cell (GCAP) family members on the cell surface. ALPP is reported to oligomerize under certain conditions. Over aa 23-506， human ALPP shares 96%