Transport mechanisms of the imino acid l-proline in the human intestinal epithelial Caco-2 cell line

The intestinal transport of L-proline (L-Pro) has been investigated in vari ous animal species with the use of different tissue preparations. Because m ajor qualitative differences have been observed among the species, it is di fficult to extent the results obtained with animal models to humans. in add ition, studies on human tissue are lacking because of difficulties in obtai ning material for experiments. To characterize the mechanisms involved in t he intestinal absorption of L-Pro in humans, the transport of this nonessen tial imino acid was studied in monolayers of human intestinal Caco-2 cells that were cultivated on microporous membranes. In this model, L-PrO was tra nsported selectively in the apical (AP)-to-basolateral (BL) direction. This transport was significantly reduced by metabolic inhibitors and by an incu bation at 4 degreesC; it was Na+ dependent and showed competition with (met hyl-amino)-alpha -isobutyric acid and L-hydroxyproline, By contrast, transp ort in the BL-to-AP direction resulted to a large extent from passive movem ent (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 new ly synthesized proteins in a process inhibited by cycloheximide, whereas th e remainder were extensively metabolized into non-amino acid compounds. L-P ro accumulations from the AP and BL poles were both Na+ dependent, but they exhibited different characteristics. AP accumulation was inhibited by comp etition with (methylamino)-alpha -isobutyric acid, L-hydroxyproline and, to a lesser extent, D-Pro, whereas BL accumulation was inhibited by competiti on with L-hydroxyproline, (methylamino)-alpha -isobutyric acid, cr-aminoiso butyric acid, L-histidine and small neutral amino acids. The results indica te that AP-to-BL transport and AP accumulation of L-Pro exhibited very diff erent characteristics than BL-to-AP transport and BL accumulation.