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Mechanisms and Kinetics of Uptake and Efflux of L-Methionine in an Intestinal Epithelial Model (Caco-2)^1,2,3,

Department of Pharmaceutical Sciences, College of Pharmacy, Washington State University, Pullman, WA 99164-6510

Mechanism and kinetics of uptake and efflux of L-methionine in a human intestinal epithelial model system (Caco-2) were studied to understand the transcellular transport process and to determine its rate-limiting step. The kinetic studies indicated that uptakes from both the apical and basolateral sides were saturable [for apical uptake: K_m = 0.96 mmol/L, maximum flux = 673 pmol/(min·cm²); for basolateral uptake: K_m = 3.46 mmol/L, maximum flux = 3480 pmol/(min·cm²)], whereas the efflux from these two membranes was apparently linear for intracellular concentrations <6.5 mmol/L [for apical efflux, the apparent first-order rate constant = 1.01 x 10^-4 cm/min; for basolateral efflux, the rate constant = 1.78 x 10^-4 cm/min]. The results of inhibition studies indicate the apical uptake is partially active and Na⁺-dependent via a combination of amino acid transport systems B^0,+ and ASC, which is somewhat different from the less energy- and Na⁺-dependent basolateral uptake. The basolateral uptake is also more dependent on system L and exhibits high counter-exchange capability. Finally, the rate-limiting step in the apical to basolateral transport of methionine is determined to be the basolateral efflux.

KEY WORDS: • Caco-2 • methionine • uptake • efflux • kinetics

¹ Presented in abstract form at the 1993 Annual Meeting of the American Association of Pharmaceutical Scientists, Orlando, FL [Chen, J. & Hu, M. (1993) Uptake and efflux of L-methionine in the Caco-2 model system. Pharm. Res. 10 (suppl.): 5181 (abs.)].

² Supported by a grant from Zinpro Corporation.

³ 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.

⁴ To whom correspondence and reprint requests should be addressed.

Manuscript received 17 December 1993. Revision accepted 30 March 1994.

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