PROPERTIES OF HIGH-MOLECULAR-MASS PLACENTAL ALKALINE-PHOSPHATASES IN NORMAL-PREGNANCY SERA

We examined the appearance of high-molecular-mass placental alkaline p hosphatases (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 activiti es of total placental alkaline phosphatase (TPLAP) and HPLAPs were cor related (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 p hosphatase (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 fra ctions of gel filtration. The apparent molecular mass of the HPLAPs in pregnancy serum was 720 KDa by gel filtration. HPLAPs were not conver ted to common-FLAP by phosphatidylinositol-specific phospholipase (PIP L) C and PIPL-D treatments. The HPLAPs were selectively incorporated i nto liposomes consisting of phosphatidylcholine/cholesterol, and most of the PIPL-D-treated FLAP could form HPLAPs, while a small amount of FLAP could not form HPLAPs. On the other hand, HPLAPs in pregnant wome n's sera and HPLAPs prepared from partially purified FLAP in vitro cou ld be converted to common-PLAP by brief treatment with subtilisin. How ever, the highly purified FLAP could not form HPLAPs in the presence o f Triton X-100. These results suggest that PIPL-D-resistant and PLAP-a ssociated serum protein may regulate the conversion of FLAP to HPLAP i n the presence of Triton X-100.