Lower alkaline phosphatase activity and occurrence of an abnormal hybrid intestinal/tissue non-specific isoform in Down's syndrome amniotic fluids.

Quantitative and qualitative analysis of alkaline phosphatases (AP) was performed on amniotic fluid in 59 normal pregnancies and 14 Down's syndrome (DS) pregnancies at 16, 18 and 19 weeks of gestation. In DS cases, intestinal and placental isoenzyme levels were significantly reduced (P<0.001) and the AP electrophoretic pattern was seen to be modified on polyacrylamide gel electrophoresis. A unique component was detected. After extraction and purification of the abnormal isoenzyme, peptide fragments obtained after cyanogen bromide cleavage indicated a hybrid heterodimeric AP composed of intestinal and tissue non-specific subunits, as evaluated by SDS polyacrylamide gel electrophoresis.