Properties of High-Molecular-Mass Placental Alkaline Phosphatases in Normal Pregnancy Sera

Abstract

We examined the appearance of high-molecular-mass placental alkaline phosphatases (HPLAPs) in the serum of normal pregnant women by means of polyacrylamide gel electrophoresis (PAGE) in the presence of Triton X-100. The HPLAPs were undetectable or only slightly detectable by PAGE in the absence of Triton X-100. The HPLAPs were detected in all sera sampled during the last trimester of pregnancy. The catalytic activities of total placental alkaline phosphatase (TPLAP) and HPLAPs were correlated (r = 0.96) and the ratio of HPLAPs/TPLAP catalytic activity was 0.20 (0.06) (mean and SD) in 40 serum samples from pregnant women. The HPLAPs appear to be formed from a common dimeric placental alkaline phosphatase (PLAP) (common-PLAP), as judged by the fact that they were formed again after removal of HPLAPs from serum by gel filtration. The formation of HPLAPs was more prominently observed with the faster fractions of gel filtration. The apparent molecular mass of the HPLAPs in pregnancy serum was 720 K Da by gel filtration. HPLAPs were not converted to common-PLAP by phosphatidylinositol-specific phospholipase (PIPL) C and PIPL-D treatments. The HPLAPs were selectively incorporated into liposomes consisting of phosphatidylcholine/cholesterol, and most of the PIPL-D-treated PLAP could form HPLAPs, while a small amount of PLAP could not form HPLAPs. On the other hand, HPLAPs in pregnant women's sera and HPLAPs prepared from partially purified PLAP in vitro could be converted to common-PLAP by brief treatment with subtilisin. However, the highly purified PLAP could not form HPLAPs in the presence of Triton X-100. These results suggest that PIPL-D-resistant and PLAP-associated serum protein may regulate the conversion of PLAP to HPLAP in the presence of Triton X-100.