ONTOGENY OF AMINO-ACID-TRANSPORT SYSTEM-A IN RAT PLACENTA

Amino acid transport System A has previously been demonstrated in apic al membranes derived from rat placenta, as well as in apical and basal membranes derived from human placenta. We have studied Na+-dependent alpha-(methylamino)isobutyric acid (MeAIB) transport in apical and bas al predominant membrane fractions prepared from 14 and 20 day gestatio n rat placenta. Marker enzyme recoveries did not differ significantly between age groups. Markers for intracellular organelles were also fou nd to be comparable. Na+-dependent MeAIB transport was not sensitive t o freezing and could be found in all membrane components tested. Kinet ic parameters were studied-K-m=852 +/- 215 mu M, V-max=718 +/- 126 pmo l/5 sec/mg protein-20 day apical; K-m=748 +/- 269 mu M, V-max=610 +/- 176 pmol/5 sec/mg protein-20 day basal-predominant; K(m)614 +/- 261 mu M, V-max=123 +/- 45 pmol/5 sec/mg protein-14 day apical. Kinetic para meters could not be determined in the 14 day gestation basal-predomina nt fraction because of the small amount of uptake present. We conclude that System A like activity is found in both apical and basal predomi nant membrane fractions derived from rat placenta, and that this activ ity increases over the last one third of gestation. (C) 1996 W. B. Sau nders Company Ltd