Rindi G, Laforenza U
Institute of Human Physiology, University of Pavia, Pavia, Italy.
Proc Soc Exp Biol Med. 2000 Sep;224(4):246-55. doi: 10.1046/j.1525-1373.2000.22428.x.
In the intestinal lumen thiamine is in free form and very low concentrations. Absorption takes place primarily in the proximal part of the small intestine by means of a dual mechanism, which is saturable at low (physiological) concentrations and diffusive at higher. Thiamine undergoes intracellular phosphorylation mainly to thiamine pyrophosphate, while at the serosal side only free thiamine is present. Thiamine uptake is enhanced by thiamine deficiency, and reduced by thyroid hormone and diabetes. The entry of thiamine into the enterocyte, as evaluated in brush border membrane vesicles of rat small intestine in the absence of H+ gradient, is Na+- and biotransformation-independent, completely inhibited by thiamine analogs and reduced by ethanol administration and aging. The transport involves a saturable mechanism at low concentrations of vitamin and simple diffusion at higher. Outwardly oriented H+ gradients enhance thiamine transport, whose saturable component is a Na+-independent electroneutral uphill process utilizing energy supplied by the H+ gradient, and involving a thiamine/ H+ 1:1 stoichiometric exchange. The exit of thiamine from the enterocyte, as evaluated in basolateral membrane vesicles, is Na+-dependent, directly coupled to ATP hydrolysis by Na+-K+-ATPase, and inhibited by thiamine analogs. Transport of thiamine by renal brush border membrane vesicles is similar to the intestinal as far as both H+ gradient influence and specificity are concerned. In the erythrocyte thiamine transport is a Na+-independent, electroneutral process yet with two components: saturable, prevailing at low thiamine concentrations, and diffusive at higher. The saturable (specific) component is missing in patients of the rare disease known as thiamine-responsive megaloblastic anaemia (TRMA), producing a general disturbance of thiamine transport up to thiamine deficiency. The TRMA gene is located in chromosome 1q23.3. Recently, the thiamine transporter has been cloned: it is a protein of 497 amino acid residues with high homology with the reduced-folate transporter.
在肠腔中，硫胺素呈游离形式且浓度极低。吸收主要发生在小肠近端，通过一种双重机制进行，该机制在低（生理）浓度下具有饱和性，在较高浓度下则为扩散性。硫胺素在细胞内主要磷酸化为硫胺素焦磷酸，而在浆膜侧仅存在游离硫胺素。硫胺素缺乏会增强硫胺素摄取，甲状腺激素和糖尿病则会使其降低。在不存在H⁺梯度的情况下，在大鼠小肠刷状缘膜囊泡中评估的硫胺素进入肠上皮细胞的过程，不依赖于Na⁺和生物转化，完全被硫胺素类似物抑制，乙醇给药和衰老会使其降低。该转运在低浓度维生素时涉及一种饱和机制，在较高浓度时为简单扩散。向外的H⁺梯度会增强硫胺素转运，其饱和成分是一个不依赖于Na⁺的电中性上坡过程，利用H⁺梯度提供的能量，涉及硫胺素/H⁺ 1:1化学计量交换。在基底外侧膜囊泡中评估的硫胺素从肠上皮细胞的排出，依赖于Na⁺，直接与Na⁺-K⁺-ATP酶水解ATP偶联，并被硫胺素类似物抑制。就H⁺梯度影响和特异性而言，肾刷状缘膜囊泡对硫胺素的转运与肠道相似。在红细胞中，硫胺素转运是一个不依赖于Na⁺的电中性过程，但有两个成分：饱和性成分在低硫胺素浓度时占主导，在较高浓度时为扩散性。在一种罕见疾病即硫胺素反应性巨幼细胞贫血（TRMA）患者中，饱和（特异性）成分缺失，导致硫胺素转运普遍紊乱直至硫胺素缺乏。TRMA基因位于1号染色体1q23.3。最近，硫胺素转运体已被克隆：它是一种由497个氨基酸残基组成的蛋白质，与还原型叶酸转运体具有高度同源性。
