L‐proline transport by brush border membrane vesicles prepared from human placenta.

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Abstract
Brush border microvillous plasma membranes were prepared from syncytiotrophoblast of human term placenta by differential centrifugation. Such plasma membranes form closed, osmotically active, right-side-out vesicles into which L-proline (188 .mu.M) is transported in a time-dependent manner. There is no detectable metabolism of L-proline within the vesicles during 30 min incubation. Transient accumulation of L-proline to levels of up to 3 times its equilibrium value occurs in the presence of an inward gradient of NaCl. Proline and Na reach electrochemical equilibrium by 30 min, at which stage there is about 100 pmol L-proline mg protein-1. This transient accumulation is abolished by the prior equilibration of the NaCl gradient, or by the replacement of Na by inwardly directed gradients of other cations. Entry of the amino acid into the vesicles is influenced by the permeability of the anion in the medium and by an imposed K diffusion potential. L-Proline transport across the brush border membrane of human placental syncytiotrophoblast is a Na-dependent, electrogenic process. Studies of the transport processes indicate saturation at higher concentrations of L-proline with a Km of about 1 mM; Vmax averaged about 2 nmol mg protein-1 min-1 but varied considerably between preparations. L-Proline (188 .mu.M) transport is inhibited competitively by the presence of many amino acids and by the dipeptide glycyl-L-proline. The Ki for inhibition by methyl AIB [.alpha.-isobutyrate] is 300 .mu.M. These findings are discussed in relation to the mechanism of transplacental amino acid transfer.

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