[40] Purification and characterization of 5′-nucleotidase/FAD pyrophosphatase from human placenta

5'-Nucleotidase (NT) is widely distributed in vertebrate tissues and human trophoblastic microvilli are rich in NT activity. The primary structure of human placental NT has been deduced from cDNA analysis. The mature protein has a calculated molecular mass of 61 kDa and contains a stretch of hydrophobic amino acids at the C terminus that is believed to be exchanged for a glycosylphosphatidylinositol membrane anchor. Four potential N-glycosylation sites have been identified on placental NT. The native enzyme exists as a homodimer linked by interchain disulfide bridges that are essential for its activity. 5'-Nucleotidase catalyzes the dephosphorylation of a wide range of 5'-nucleotides. The physiological roles of NT are not well understood but may include the provision of nucleosides for cellular uptake and the enzyme may play a modulating role in ATP and ADP regulatory actions at the cell surface. High-affinity binding of riboflavin to a preparation of microvillous membrane vesicles is discussed, and it is suggested that this might represent the first step in riboflavin transport across the placenta. The results of in vitro perfusion and incubation studies indicate that placental riboflavin transport is a saturable process that differs from mechanisms in other epithelia.