Kinetic evidence for separate systems in transport of D- and L-methionine by rat small intestine

Abstract

The kinetics of D- and L-methionine uptake by rings of everted intestine in vitro are consistent with a saturable Michaelis-Menten component (Km = 11.7 and 1.7 mM; Vmax = 0.53 and 0.74 mumol X g-1 X min-1 for D- and L-methionine, respectively) plus a linear, diffusional one. All the data could be fit with a diffusion constant (Kd = 3.2 microliters X g-1 X min-1), which was essentially the same, independent of whether it was estimated by iteration or by using the extracellular marker, inulin. Similar results were obtained from in vivo perfusion experiments, except that the diffusional term was negligible. D-Methionine was found to inhibit L-methionine uptake by intestinal rings according to fully noncompetitive kinetics (Ki = 45 mM). Another set of experiments with jejunal brush-border membrane vesicles showed that D-methionine uptake is dependent on a Na+ gradient and is significantly inhibited by L-methionine and L-proline, but not by beta-alanine and alpha-methylaminoisobutyric acid. Our results indicate that, in rat jejunum, D-methionine is taken up through a Na+-dependent pathway distinct from the neutral amino acid (L-methionine) carrier and from the imino acid (L-proline, alpha-methylaminoisobutyric acid, beta-alanine) carrier.