Transport mechanisms of the large neutral amino acid L-phenylalanine in the human intestinal epithelial Caco-2 cell line

The transepithelial transport and the intracellular accumulation of the lar ge 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 a mino acids. By contrast, transport in the BL-to-AP direction mainly resulte d from passive movement (probably paracellular passage and transcellular di ffusion). 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 f rom the AP pole was sensitive to inhibition by both large neutral and catio nic amino acids. These results suggest that the processes involved in AP-to -BL transport and AP accumulation of this amino acid are very different fro m those involved in BL-to-AP transport and BL accumulation.