Human fetal intestinal alkaline phosphatase: molecular heterogeneity and immunological detection in amniotic fluids

Human fetal intestinal alkaline phosphatase (fIALP) is present in amniotic fluids as free dimers (Mr 140,000) or membrane-bound through phosphatidylinositol residues. Extraction of corresponding particulate material with Triton X-100, resulted in release of tetrameric high Mr fIALP forms (Mr 380,000). In individual amniotic fluids, as well as in meconeum, both dimeric and tetrameric fIALP are sialylated to various extents. When measured by a double sandwich-ELISA, up to 10-fold higher fIALP antigen levels were found in amniotic fluids than when determined by an enzyme antigen immunoassay, based upon fIALP enzyme activity measurements. Frequency analysis of fIALP antigen levels, showed a more symmetrical distribution than analysis of fIALP enzyme activities; likewise, the lower 95% confidence limit, calculated for the fIALP antigen distribution curve, overlapped less with the bulk of values. In cystic fibrosis amniotic fluids, measurements of fIALP antigen levels resulted in a lower false-negativity outcome than fIALP enzyme activity measurements, whereas in amniotic fluids of trisomy pregnancies fIALP enzyme activities and fIALP antigen levels were equally unpredictive.