Mechanisms of transepithelial transport of amino acids in human placental syncytiotrophoblast

Abstract

Brush border and basal plasma membrane vesicles prepared from human full-term placental syncytiotrophoblast have been used to study the mechanisms of transepithelial transfer of amino acids from mother to fetus. Such studies indicated marked differences in the distribution of amino acid transport systems between the brush border and the basal surface of the human syncytiotrophoblast. Transport of α-(methylamino)isobutyrate by bursh border membrane vesicles was stimulated in the presence of an Na + gradient from the outside to the inside of the vesicles, while by basal membrane vesicles α-(methylamino)isobutyrate was transported only Na+-independently. L-Tyrosine transport was not enhanced by an inwardly directed gradient of Na + in either brush border or basal membrane vesicles. The initial rate of L-tyrosine transport as a function of concentration showed saturation and obeyed Michaelis-Menten kinetics in both membrane vesicles. The calculated values of Km and Vmax for the L-tyrosine transport in brush border membranes were 54.2 μM and 1.28 pmol (mg protein) −1 s −1 , and in basal membranes were 168.9 μM and 0.31 pmol (mg protein) −1 , s −1 , respectively. N-Ethylmaleimide inactivated L-tyrosine transport in a concentration-dependent manner in both membranes, however the transport systems at the brush border were more sensitive to this reagent than those at the basal side. L-Tyrosine transport by basal membrane vesicles was stimulated at more alkaline pH, while at brush border side transport was insensitive to changes in external pH over the range of 5.5 to 8.5. Transstimulation of L-tyrosine transport was observed in brush border membrane vesicles but not in basal membrane vesicles.