Isolation and characterization of cell membranes from human placenta.

Two enriched plasma membrane subfractions were obtained from syncytiotrophoblast isolated from human placenta. They were isolated from a "crude" plasma membrane fraction at the buffer-24% and 24-30% (w/w) sucrose interfaces of a sucrose gradient; another enriched plasma membrane fraction was isolated from the microsomal fraction at buffer-24% (w/w) sucrose interface and was similar to that isolated from the "crude" plasma membrane fraction at the same sucrose density. Although all three subfractions contain a high specific activity in 5'-nucleotidase and alkaline phosphatase, the specific activity was twofold higher in the lighter than in the heavier subfractions. The activities of succinate dehydrogenase, monoamine oxidase, acid phosphatase and glucose-6-phosphatase indicated very low contamination with other organelles. Polyacrylamide-gel electrophoresis resolved the polypeptides of the plasma membrane subfractions into about 14 major protein bands; no differences were observed in the patterns of the two enriched plasma membrane subfractions derived from the "crude" plasma membrane fraction.