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Multiple pathways for L-methionine transport in brush-border membrane vesicles from chicken jejunum

Juan F. Soriano-García, Mònica Torras-Llort, Ruth Ferrer and Miquel Moretó

1. The intestinal transport of L-methionine has been investigated in brush-border membrane vesicles isolated from the jejunum of 6-week-old chickens. L-Methionine influx is mediated by passive diffusion and by Na⁺-dependent and Na⁺-independent carrier-mediated mechanisms.

2. In the absence of Na⁺, cis-inhibition experiments with neutral and cationic amino acids indicate that two transport components are involved in L-methionine influx: one sensitive to L-lysine and the other sensitive to 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH). The L-lysine-sensitive flux is strongly inhibited by L-phenylalanine and can be broken down into two pathways, one sensitive to N-ethylmaleimide (NEM) and the other to L-glutamine and L-cystine.

3. The kinetics of L-methionine influx in Na⁺-free conditions is described by a model involving three transport systems, here called a, b and c: systems a and b are able to interact with cationic amino acids but differ in their kinetic characteristics (system a: Km = 2·2 ± 0·3 µM and V_max = 0·13 ± 0·005 pmol (mg protein)^-1 (2 s)^-1; system b: Km = 3·0 ± 0·3 mM and V_max = 465 ± 4·3 pmol (mg protein)^-1 (2 s)^-1); system c is specific for neutral amino acids, has a K_m of 1·29 ± 0·08 mM and a V_max of 229 ± 5·0 pmol (mg protein)^-1 (2 s)^-1 and is sensitive to BCH inhibition.

4. The Na⁺-dependent component can be inhibited by BCH and L-phenylalanine but cannot interact either with cationic amino acids or with -(methylamino)isobutyrate (MeAIB).

5. The kinetic analysis of L-methionine influx under a Na⁺ gradient confirms the activity of the above described transport systems a and b. System a is not affected by the presence of Na⁺ while system b shows a 3-fold decrease in the Michaelis constant and a 1·4-fold increase in V_max. In the presence of Na⁺, the BCH-sensitive component can be subdivided into two pathways: one corresponds to system c and the other is Na⁺ dependent and has a K_m of 0·64 ± 0·013 mM and a V_max of 391 ± 2·3 pmol (mg protein)^-1 (2 s)^-1.

6. It is concluded that L-methionine is transported in the chicken jejunum by four transport systems, one with functional characteristics similar to those of system b^{o, +} (system a); a second (system b) similar to system y⁺, which we suggest naming y⁺m to account for its high V_max for L-methionine transport in the absence of Na⁺; a third (system c) which is Na⁺ independent and has similar properties to system L; and a fourth showing Na⁺ dependence and tentatively identified with system B.

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