The human placenta is known to concentrate nearly all amino acids intracellularly for transfer to the fetus. To clarify the mechanism and regulation of this process we have determined the specificity of the principal placental transport systems for neutral amino acids. With the use of competitive inhibition techniques, three transport systems of overlapping specificity have been elucidated. These correspond approximately to the "A", "L", and "ASC" systems of Christensen and associates. In the placenta the specificity of these systems is as follows: A system - alpha aminoisobutyric acid (AIB), glycine, proline, N-methylalanine, alanine, serine, threonine, and glutamine; L system - isoleucine, valine, phenylalanine, BCH, alanine, serine, threonine, and glutamine; and ASC system - alanine, serine, threonine, and glutamine. Placental AIB uptake previously has been shown to increase with preincubation of tissue in vitro. This increase has now been found to be limited to the A system. Activity of the other two systems is essentially unaffected, demonstrating that the transport pathways are separately regulated.

The surface coat of syncytial trophoblast from term human placentas was studied using cytochemical methods (colloidal iron, alcian blue-lanthanum nitrate, dialyzed iron) in coordination with tissue enzyme digestions (trypsin, neuraminidase) and sialic acid analyses. The presence of at least two highly acidic anionic components that contribute significantly to the surface negativity of trophoblast has been demonstrated. The first of these, sialic acid, was removed with neuraminidase. Tissue digestion with this glycosidase was accompanied by a decrease in trophoblast surface staining with colloidal iron, a decrease in tissue sialic acid, and an increase in the concentration of sialic acid in the incubating medium. Results from methylation experiments were consistent with the presence of sialic acid. The second anionic component(s) was identified by removal with trypsin of a glycocalyx constituent that stained with both colloidal iron and lanthanum. After trypsinization, tissue sialic acid levels were not significantly different from control values, and no detectable sialic acid was present in the incubating medium. The identity of this anionic component has not been established. Both sialic acid and nonsialic acid acidic components are distributed in higher density on membrane of microvilli than on intermicrovillous surface membrane. In addition, the sialic acid moieties appear to be clustered in the glycocalyx.

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