HUMAN PLACENTAL L-TYROSINE TRANSPORT - A COMPARISON OF BRUSH-BORDER AND BASAL MEMBRANE-VESICLES

1. The mechanisms responsible for L‐tyrosine transport at both the maternal‐facing and fetal‐facing surfaces of the human full‐term placenta have been studied using isolated brush‐border and basal membrane vesicles under conditions where a direct comparison of the transport properties of the two membranes can be made. 2. Brush‐border vesicle uptake of L‐tyrosine was substantially into an osmotically active space. Transport was Na(+)‐independent, N‐ethylmaleimide‐sensitive (half‐maximal inhibition, Ki = 1.1 mM), and insensitive to pH over the range 5.5‐8.5. The initial rate of brush‐border L‐tyrosine uptake as a function of concentration showed saturation and obeyed Michaelis‐Menten kinetics with Michaelis constant (Km) and maximum velocity (Vmax) values of 54.2 microM and 1.28 pmol (mg protein)‐1 s‐1, respectively. Influx of L‐tyrosine was stereospecific and was virtually completely abolished by L‐phenylalanine, L‐tryptophan, L‐leucine or by 2‐aminobicycloheptane‐2‐carboxylic acid. These properties suggest that system L is responsible for brush‐border L‐tyrosine transport. 3. Basal membrane transport of L‐tyrosine was more complex and uptake was slower than that found in the brush border. Although, as in the brush‐border membranes, uptake was completely Na(+)‐independent, N‐ethylmaleimide was a less effective inhibitor, there was stimulation of transport at more alkaline pH and uptake did not show marked stereospecificity. An apparent Km of 168.9 microM and a Vmax of 0.31 pmol (mg protein)‐1 s‐1 were calculated for basal L‐tyrosine transport. There was clear inhibition by L‐ and D‐tyrosine, L‐phenylalanine and L‐tryptophan. 2‐Aminobicycloheptane‐2‐carboxylic acid was not as effective. 4. These findings suggest the existence of non‐identical carrier‐mediated transport systems for L‐tyrosine in brush‐border and basal membranes. Brush‐border transport resembles that by system L; L‐tyrosine transport at the basal membrane may be via system t. © 1990 The Physiological Society