Placental amino acid uptake. III. Transport systems for neutral amino acids

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Abstract
The human placenta concentrates nearly all amino acids intracellularly for transfer to the fetus. To clarify the mechanism and regulation of this process the specificity of the principal placental transport systems for neutral amino acids was determined. Using competitive inhibition techniques, 3 transport systems of overlapping specificity were elucidated. These correspond approximately to the A, L and ASC systems of Christensen et al. (1975, 1967). In the placenta the specificity of these systems is as follows: A system.sbd..alpha.-aminoisobutyric acid (AIB), glycine, proline, N-methylalanine, alanine, serine, threonine and glutamine; L system.sbd.isoleucine, valine, phenyalanine, BCH [DL-b-2-aminobicyclo-[2,2,1]-heptane-2-carboxylic acid], alanine, serine, threonine and glutamine; and ASC system.sbd.alanine, serine, threonine and glutamine. Placental AIB uptake previously was shown to increase with preincubation of tissue in vitro. This increase was found to be limited to the A system. Activity of the other 2 systems is essentially unaffected, demonstrating that the transport pathways are separately regulated.