The demonstration of two pregnancy-associated proteins with SP1 determinants in placental extracts.

The SP1 content of the placenta has been examined. It was found that, as in pregnancy serum, two variants of the protein existed in the placenta. The smaller molecule, with a beta 1 electrophoretic mobility, is readily extractable with phosphate buffered saline. The second larger molecule is more tightly bound in the placenta and requires Triton X-100 to be solubilized. The hypothesis is put forward that the larger alpha variant is the precursor form which is related more irregularly and in a smaller amount from the placenta into the maternal circulation.