title: 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.
abstract: 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.
authors.affiliation: Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, Canada.
authors.forename: D D
authors.initials: DD
authors.lastname: Maenz
authors.forename: C M
authors.initials: CM
authors.lastname: Engele-Schaan
mesh_terms: D000818:Animals; D001693:Biological Transport, Active; D002645:Chickens / Q000378:metabolism* / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D004848:Epithelium / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure; D006859:Hydrogen / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D007421:Intestine, Small / Q000378:metabolism* / Q000502:physiology / Q000648:ultrastructure; D007536:Isomerism; D008715:Methionine / Q000031:analogs & derivatives* / Q000032:analysis / Q000493:pharmacokinetics*; D008871:Microvilli / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure; D012964:Sodium / Q000502:physiology*; D013379:Substrate Specificity; D013462:Sulfur Radioisotopes
chemical_list: D013462:Sulfur Radioisotopes; C008391:alpha-hydroxy-gamma-methylmercaptobutyric acid; D006859:Hydrogen; D012964:Sodium; D008715:Methionine