MECHANISMS AND KINETICS OF UPTAKE AND EFFLUX OF L-METHIONINE IN AN INTESTINAL EPITHELIAL MODEL (CACO-2)

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.cm(2)); for basolateral uptake: K-m 3.46 mmol/L, maximum flux = 3480 pmol/(min cm(2))], 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-O,B-+ 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.