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The Transport of Lysine across Monolayers of Human Cultured Intestinal Cells (Caco-2) Depends on Na⁺-Dependent and Na⁺-Independent Mechanisms on Different Plasma Membrane Domains^1,2,

Istituto Nazionale della Nutrizione, via Azdeatina 546, 00178 Rome ^* Dipartimento Scienze Biochimiche, Università La Sapienza, 00185 Rome Italy

To characterize the mechanisms involved in the intestinal absorption of the essential amino acid L-lysine from the diet, the transepithelial transport of L-lysine was studied in monolayers of cultured human intestinal cells (Caco-2) grown and differentiated on microporous membrane supports. L-lysine was transported mainly in the apical (AP) to basolateral (BL) direction and the BL to AP transport was one order of magnitude lower at all concentrations tested. Nonlinear regression analysis of the transport in the AP to BL and the BL to AP direction identified, in both cases, single saturable components with similar Km but different V_max and a nonsaturable diffusional component. The AP to BL L-lysine transport was highly energy- and sodium-dependent and was unaffected by an unfavorable concentration gradient. Selective replacement of sodium ions in the AP or the BL compartment and determination of both AP to BL transport and the intracellular soluble lysine pool showed that uptake occurs via a sodium-independent mechanism, not significantly influenced by membrane potential, whereas efflux is a sodium-dependent process. Competition experiments showed that L-lysine uptake is highly stereospecific and is shared by cationic and large neutral amino acids. This study demonstrates the presence of a sodium-dependent mechanism of lysine efflux across the BL membrane of intestinal cells, which may be essential for lysine transport into the blood circulation. Overall, these results support the use of the Caco-2 cell model for studies of intestinal nutrient transport.

KEY WORDS: • transepithelial transport • amino acid transport • bioavailability • Caco-2 human cells

¹ Supported by National Research Council of Italy, Special Project RAISA, Subproject 4, Paper 1889 and Target Project Biotechnology and Bioinstrumentation and by the Italian Ministry of Education, MPI 60%.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence and reprint requests should be addressed.

Manuscript received 28 November 1994. Revision accepted 9 May 1995.