Multiple pathways for L-methionine transport in brush-border membrane vesicles from chicken jejunum.
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 microM and Vmax = 0.13 +/- 0.005 pmol (mg protein)-1 (2 s)-1; system b: Km = 3.0 +/- 0.3 mM and Vmax = 465 +/- 4.3 pmol (mg protein)-1 (2 s)-1); system c is specific for neutral amino acids, has a Km of 1.29 +/- 0.08 mM and a Vmax 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 alpha-(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 Vmax. 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 Km of 0.64 +/- 0. 013 mM and a Vmax 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 bo, + (system a); a second (system b) similar to system y+, which we suggest naming y+m to account for its high Vmax 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.
Soriano-García
J F
JF
Departament de Fisiologia-Divisio IV, Facultat de Farmàcia, Universitat de Barcelona, E-08028 Barcelona, Spain.
Torras-Llort
M
M
Ferrer
R
R
Moreto
M
M
D000596:Amino Acids / Q000494:pharmacology*; D000598:Amino Acids, Cyclic; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002645:Chickens; D003553:Cystine / Q000494:pharmacology; D004058:Diffusion; D005033:Ethylmaleimide / Q000494:pharmacology; D005973:Glutamine / Q000494:pharmacology; D007413:Intestinal Mucosa / Q000378:metabolism*; D007583:Jejunum; D007700:Kinetics; D008239:Lysine / Q000494:pharmacology; D008297:Male; D008715:Methionine / Q000378:metabolism*; D008871:Microvilli / Q000187:drug effects / Q000378:metabolism*; D009994:Osmolar Concentration; D010649:Phenylalanine / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology
D000596:Amino Acids; D000598:Amino Acids, Cyclic; D005973:Glutamine; C006504:2-aminobicyclo(2,2,1)heptane-2-carboxylic acid; D010649:Phenylalanine; D003553:Cystine; D012964:Sodium; D008715:Methionine; D008239:Lysine; D005033:Ethylmaleimide