2 MAJOR PATHWAYS OF ZINC(II) ACQUISITION BY HUMAN PLACENTAL SYNCYTIOTROPHOBLAST

Uptake of zinc into placental villous syncytiotrophoblast is the first step in its transfer from mother to fetus. To help characterise physi ologically significant pathways of zinc accumulation by these cells, w e incubated cultured layers of syncytiotrophoblast cells derived from human near-term placental tissue with serum ultrafiltrate (containing the zinc complexed with low molecular mass serum constituents), dialys ed serum (containing the zinc bound to the serum proteins) and whole s erum, each of whose endogenous zinc was tracer-labelled with Zn-65(II) . Zinc label from both fractions of serum readily entered a rapidly la belled EDTA-sensitive cellular compartment, probably representing zinc bound to the outside cell surface and in accumulative fashion, an EDT A-resistant compartment, probably consisting largely of internalised c ellular zinc. Movement of zinc into the EDTA-resistant pool was strong ly temperature-dependent and did not occur via the EDTA-sensitive pool from either serum source. Transfer of zinc from the low molecular mas s serum fraction into the EDTA-resistant pool was saturable, the conce ntration giving half-maximal rate being 1.2 mu mol/l nonprotein-bound zinc. No nonsaturable component was detected. Zinc from the serum prot ein-bound fraction entered by a saturable component, already saturated at physiological total protein-bound zinc concentration, and by an ap parently nonsaturable component not appreciably accounted for by nonsp ecific fluid-phase endocytosis. The results show that zinc is acquired by placental syncytiotrophoblast from the low molecular mass serum zi nc pool probably by a carrier-mediated process, and at least as import antly, from the zinc bound to serum protein, possibly by an endocytic mechanism. (C) 1995 Wiley-Liss, Inc.