Methionine and 2-hydroxy-4-methylthiobutanoic acid are transported by distinct Na+-dependent and H+-dependent systems in the brush border membrane of the chick intestinal epithelium

The pathways that facilitate the uptake of L-methionine (L-Met), D-methionine (D-Met) and L-2-hydroxy-4-methylthiobutanoic acid (L-HMB) were characterized in chick intestinal brush border membrane vesicles. A model of high affinity transport (K-m approximating 0.1 mmol/L) converged to the data obtained for S-35-L-Met uptake under Na+-gradient and Na+-free conditions. The maximal velocity of S-35-L-Met transport was almost threefold greater in the presence of a Na+-gradient. Concentrations (100 mmol/L) of D-Met, the L-isomers of neutral amino acids and 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid completely inhibited S-35-L-Met transport. A model of low affinity competitive inhibition (K-i similar to 17 mmol/L) described D-Met inhibition of S-35-L-Met transport. These data indicate that L- and D-Met are transported by a broad specificity system B type transporter. Maximal rates of H-3-L-HMB uptake were obtained under conditions of an internally directed H+-gradient (pH(in) = 7.5, pH(out) = 5.5). A model of intermediate affinity transport (K-m similar to 1 mmol/L) described H+-dependent H-3-L-HMB uptake across the vesicles. The data from this and other studies indicate that a H+-dependent, nonstereospecific system facilitates the uptake of the hydroxy analogues of linear amino acids, including HMB.