Studies on human placental alkaline phosphatase

When butanol extracts of microvilli and their fractions were analysed on starch gels, a considerable amount of PAP enzyme activity either failed to enter the gel system or caused a smear between the A and B zones of activity. This smearing was resolved into a discrete band of enzyme activity located between the A and B forms of the enzyme after the incorporation of Triton X-100 into the gel system. This band was called the M form of PAP and appears to be largely or wholly specific for microvilli. It was also found at very low concentration relative to the A and B forms in extracts of total placental tissue. Trypsin solubilization failed to release M-PAP from these membranes. It appeared to cleave a portion of a molecular weight of about 9K from the amino terminus, leaving an enzymatically active portion of PAP associated with the membrane. The available evidence demonstrates the existence of a new form of PAP in association with the microvillous plasma membranes of the placenta. This form of the enzyme seems to share at least part of the same enzymatic polypeptide with the non-membranous form (A-PAP) and appears to differ from it in having a hydrophobic anchoring domain at its carboxy-terminus. The precise relationship of the membrane binding domain to the bilayer is not yet understood. At this stage we can not speculate into the biogenetic relationship between membranous and non-membranous forms of PAP, however, the possibility should be considered that the pathway for the synthesis of secretory and membrane IgM may also be applicable to PAP forms, and therefore have a general biological significance. Direct biogenetic studies will be necessary to obtain complete information on this problem