[Partial purification of bilirubin binding protein from the human term placenta (author's transl)]

Fetal bilirubin may pass through the placenta into the maternal circulation with yet unknown mechanisms and be metabolized in the maternal liver because of deficiency of bilirubin conjugating and excreting activity in the fetal liver. To elucidate bilirubin transport mechanisms in placenta we partially purified bilirubin binding protein from 105,000 X g supernatant of the villous homogenates from the human term placenta. The supernatant was brought to 50% saturation with ammonium sulfate. The precipitates dissolved in 0.25 M sucrose buffer (pH 7.4) were chromatographed on a Sepharose 6B column (2.6 X 91.5 cm) with the same buffer. Each fraction was analyzed for protein and bilirubin binding activity. The highest bilirubin binding activity was eluated at void volume. After rechromatography the void volume fraction was centrifuged in a linear sucrose density gradient. The 4.8S fraction with the highest bilirubin binding activity was then applied onto a Sepharose 6B column (0.9 X 58 cm) and rechromatographed with 0.4 M sucrose buffer. After rechromatography the most active fraction was analyzed using sodium dodecylsulfate-polyacrylamide gel disc electrophoresis. The molecular weight of the placental bilirubin binding protein was found to be about 76,000 both in Sepharose 6B gel filtration and gel electrophoresis. The results may indicate that the bilirubin binding protein was of single polypeptide chain distinct from ligandin, Z-protein, and alpha-fetoprotein.