Molecular heterogeneity of pregnancy specific β1-glycoprotein (SP1) in maternal blood and placenta
These studies on the physiology and biochemistry of the pregnancy-specific β1 globulin, SP1, were begun by extracting the protein from human placentae. It was found that, as in the maternal blood, two molecular species of the protein existed in the placenta. The smaller molecule, with β1 electrophoretio mobility, is readily extractable with phosphate buffered saline. The second larger molecule, with α2 electrophoretic mobility, is more tightly bound in the placenta and requires a detergent to be solubilized. These two proteins are designated SP1β and SP1α respectively. It was also found that the placenta always contained large amounts of the larger molecule, SP1α, which was difficult to extract. It was, therefore, speculated that SP1α was the placental precursor to SP1β and was released more irregularly and in a smaller amount from the placenta into the maternal circulation than SP1β. Later, it was, however, found that SP1β could be converted to SP1α. It was suggested that SP1α represents the combination of the placental protein SP1β with some other protein present in the placenta and in the maternal serum. The SP1 antigens in late pregnancy serum and in placental extracts were purified on positive affinity chromatography with immobilised SP1 antiserum and on a negative affinity column with immobilised antiserum to non-pregnant human serum. Both SP1α and SP1β were bound by the positive column but only SP1α was bound by the negative column. This was adduced as further support for the hypothesis that SP1α is formed by the combination of SP1β with another protein. This was confirmed when the two SP1 proteins were subjected to crossed immunoelectrophoresis with an antiserum to non-pregnant human serum in the first dimension gel, SP1α was absorbed but SP1β was unaffected. Similarly incubation with such an antiserum caused precipitation of SP1α but not SP1β. The α and β forms of the SP1 have been studied in pregnancy serum and in the placenta. Only two forms of the protein, SP1α and SP1β, could be found; both reacting with antisera against SP1. On gel chromatography these two forms could be separated, with intermediate effluent containing a varying mixture of both proteins. The front running protein, pure SP1α, had an α2 electrophoretic mobility and a molecular weight of 430,000 daltons, while the slower moving component, pure SP1β, had the electrophoretic mobility of a β1-globulin and a molecular weight of 90,000. Immunoelectrophoresis of the effluent fractions from gel chromatography showed rocket shaped immunoprecipitates whose morphology depended on the mixture of SP1α and SP1β. These findings have also been confirmed by crossed immunoelectrophoresis. When SP1 antiserum was absorbed with either SP1α or SP1β it resulted in the removal of all antibodies to either SP1α or SP1β .It was found that SP1β did not bind to hydroxylapatite, while SP1α was bound. This property can be used to achieve a simple, clear separation of the two SP1 proteins. Small hydroxylapatite columns can be used for the separate measurements of SP1α and SP1β on a routine basis.