Isolation and characterization of a flavin-containing placental protein (PP3).

Placental protein 3 (PP3) has been isolated to purity from saline extracts of human term placentas. PP3 turned out to be a flavin-containing protein and thus appears to be an enzyme. The prosthetic group was identified as flavin-adenine dinucleotide (FAD) which was found to be noncovalently bound to the protein. The physical characterization of PP3 showed that the molecules are composed of two identical subunits having molecular weights of 55000 daltons which are noncovalently linked. Each subunit appears to contain one FAD group. In addition PP3 was found to have an electrophoretic mobility in between the alpha 1 and alpha 2 globulins, an isoelectric point in the range of 5.3-5.7, a sedimentation coefficient of 6.3 S and an extinction coefficient of 15.7 (E1%(1cm) 280 nm). Immunochemical methods were used to detect and quantitate PP3 in extracts of placentas and other human tissues. From one human term placenta an average of around 4 mg PP3 could be extracted; PP3 was also found to occur in extracts of adult human stomach. In concentrated extracts of other human tissues and in human body fluids this protein could not be detected, at least not in concentrations higher than 2 mg/dl.