Maenz D D, Engele-Schaan C M
Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, Canada.
J Nutr. 1996 Feb;126(2):529-36. doi: 10.1093/jn/126.2.529.
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.
在鸡小肠刷状缘膜囊泡中对促进L-蛋氨酸（L-Met）、D-蛋氨酸（D-Met）和L-2-羟基-4-甲硫基丁酸（L-HMB）摄取的途径进行了表征。一种高亲和力转运模型（Km约为0.1 mmol/L）与在Na⁺梯度和无Na⁺条件下35S-L-Met摄取所获得的数据相符。在存在Na⁺梯度的情况下，35S-L-Met转运的最大速度几乎增加了两倍。D-Met、中性氨基酸的L-异构体和2-氨基双环[2.2.1]庚烷-2-羧酸的浓度（100 mmol/L）完全抑制了35S-L-Met的转运。一种低亲和力竞争性抑制模型（Ki约为17 mmol/L）描述了D-Met对~(35)S-L-Met转运的抑制作用。这些数据表明，L-和D-Met是通过一种广泛特异性的B型转运系统进行转运的。在内部导向的H⁺梯度（pHin = 7.5，pHout = 5.5）条件下获得了³H-L-HMB摄取的最大速率。一种中等亲和力转运模型（Km约为1 mmol/L）描述了跨囊泡的H⁺依赖性³H-L-HMB摄取。这项研究和其他研究的数据表明，一种H⁺依赖性、非立体特异性系统促进了包括HMB在内的线性氨基酸羟基类似物的摄取。
