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The transepithelial transport and the intracellular accumulation of the large neutral amino acid L-phenylalanine (L-Phe) were studied in monolayers of Caco-2 cells, cultivated in a bicameral insert system, to characterize the mechanisms involved in the absorption of this essential amino acid by the human intestinal mucosa. In our model, L-Phe was transported selectively in the apical (AP)-to-basolateral (BL) direction. AP-to-BL transport of L-Phe was temperature dependent and Na(+) independent, increased in the absence of protein synthesis and showed competition with large neutral and cationic amino acids. By contrast, transport in the BL-to-AP direction mainly resulted from passive movement (probably paracellular passage and transcellular diffusion). L-Phe accumulation into Caco-2 cells was higher from the BL pole than from the AP pole and characterized by the incorporation of most of the accumulated molecules into newly synthesized proteins. In addition, L-Phe accumulation was Na(+) dependent from both poles, whereas only accumulation from the AP pole was sensitive to inhibition by both large neutral and cationic amino acids. These results suggest that the processes involved in AP-to-BL transport and AP accumulation of this amino acid are very different from those involved in BL-to-AP transport and BL accumulation.

The intestinal transport of L-proline (L-Pro) has been investigated in various animal species with the use of different tissue preparations. Because major qualitative differences have been observed among the species, it is difficult to extent the results obtained with animal models to humans. In addition, studies on human tissue are lacking because of difficulties in obtaining material for experiments. To characterize the mechanisms involved in the intestinal absorption of L-Pro in humans, the transport of this nonessential imino acid was studied in monolayers of human intestinal Caco-2 cells that were cultivated on microporous membranes. In this model, L-Pro was transported selectively in the apical (AP)-to-basolateral (BL) direction. This transport was significantly reduced by metabolic inhibitors and by an incubation at 4 degrees C; it was Na(+) dependent and showed competition with (methylamino)-alpha-isobutyric acid and L-hydroxyproline. By contrast, transport in the BL-to-AP direction resulted to a large extent from passive movement (paracellular passage and transcellular diffusion). L-Pro accumulation by Caco-2 cells was significantly greater from the AP pole than from the BL pole. About 30-50% of the accumulated molecules were incorporated into newly synthesized proteins in a process inhibited by cycloheximide, whereas the remainder were extensively metabolized into non-amino acid compounds. L-Pro accumulations from the AP and BL poles were both Na(+) dependent, but they exhibited different characteristics. AP accumulation was inhibited by competition with (methylamino)-alpha-isobutyric acid, L-hydroxyproline and, to a lesser extent, D-Pro, whereas BL accumulation was inhibited by competition with L-hydroxyproline, (methylamino)-alpha-isobutyric acid, alpha-aminoisobutyric acid, L-histidine and small neutral amino acids. The results indicate that AP-to-BL transport and AP accumulation of L-Pro exhibited very different characteristics than BL-to-AP transport and BL accumulation.