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 (Km approximating 0.1 mmol/L) converged to the data obtained for 35S-L-Met uptake under Na(+)-gradient and Na(+)-free conditions. The maximal velocity of 35S-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 35S-L-Met transport. A model of low affinity competitive inhibition (Ki approximately 17 mmol/L) described D-Met inhibition of 35S-L-Met transport. These data indicate that L- and D-Met are transported by a broad specificity system B type transporter. Maximal rates of 3H-L-HMB uptake were obtained under conditions of an internally directed H(+)-gradient (pHin = 7.5, pHout = 5.5). A model of intermediate affinity transport (Km approximately 1 mmol/L) described H(+)-dependent 3H-L-HMB uptake across the vesicles. The data from this and other studies indicate that a H(+)-dependent, nonstereo-specific system facilitates the uptake of the hydroxy analogues of linear amino acids, including HMB.
Broiler chicks were fed diets supplemented with DL-methionine or DL-2-hydroxy-4-methyl-thiobutanoic acid. At 4 wk of age the chicks were subdivided into thermoneutral (22 degrees C) and heat-exposed (32 degrees C) groups and maintained under these conditions for 48 h. Highly purified 3H-L-methionine (3H-L-Met) and 3H-L-2-hydroxy-4-methyl-thiobutanoic acid (3H-L-HMB) were used to evaluate treatment effects on the small intestinal passage of sources of supplemental methionine and on the transport of methionine sources across purified small intestinal brush border vesicles. 3H-L-Met was efficiently absorbed in the upper regions of the small intestine; however, 2.5-3.5% of dietary 3H from birds fed 3H-L-Met remained unabsorbed in the distal small intestine. Dietary 3H (15%) initially associated with 3H-L-HMB was not absorbed during passage down the length of the gut. The HPLC analysis indicated that only 10% of the radiolabeled material remaining in the terminal ileum eluted at the time expected for HMB. Partial breakdown of HMB to nonabsorbed, nonmethionine products during passage down the small intestine may contribute to the difference in biopotency of the two sources of supplemental dietary methionine. Heat exposure did not affect in vivo small intestinal passage or in vitro transport of 3H-L-Met and 3H-L-HMB across small intestinal brush border membrane vesicles.
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