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 b rush-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 t he absence of Na+, cis-inhibition experiments with neutral and cationi c 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 br oken down into two pathways, one sensitive to N-ethylmaleimide (NEM) a nd the other to L-glutamine and L-cystine. 3. The kinetics of L-methio nine influx in Na+-free conditions is described by a model involving t hree transport systems, here called a, b and c: systems a and b are ab le to interact with cationic amino acids but differ in their kinetic c haracteristics (system a: K-m = 2.2 +/- 0.3 mu M and V-max = 0.13 +/- 0.005 pmol (mg protein)(-1) (2 s)(-1); system b: K-m = 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 amin o acids or with alpha-(methylamino)isobutyrate (MeAIB). 5. The kinetic analysis of L-methionine influx under a Na+ gradient confirms the act ivity of the above described transport systems a and b. system a is no t affected by the presence of Na+ while system b shows a 3-fold decrea se 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 tw o 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 (m g protein)(-1) (2 s)(-1) 6. It is concluded that L-methionine is trans ported in the chicken jejunum by four transport systems, one with func tional 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 abs ence of Na+; a third (system c) which is Na+ independent and has simil ar properties to system L; and a fourth showing Na+ dependence and ten tatively identified with system B.