Thiamine uptake

Thiamin active transport is sodium-independent and requires an outwardly directed proton gradient (i.e., it is dependent on a proton-pumping ATPase). Antimetabolites, such as pyrithiamin, compete with thiamin for active transport. There is a similar proton-dependent thiamin uptake system in renal tubules (Hindi and Laforenza, 2000 Dudeja et al., 2001). 

Dudeja PK, Tyagi S, Kavilaveettil RJ, Gill R, and Said HM (2001) Mechanism of thiamine uptake by human jejunal brush-border membrane vesicles. American Journal of Physiology Cell Physiology 281, C786-92. 

Laforenza, U., Patrini, C., Alvisi, C., Faelli, A., Licandro, A., and Rindi, G. (1997). Thiamine uptake in human intestinal biopsy specimens, including observations from a patient with acute thiamine deficiency. Am. J. Clin. Nutr. 66, 320-326. 

Thiamine absorption occurs primarily in the proximal small intestine by both a saturable (thiamine transporter) process at low concentration (Ipmol/L, or lower) and by simple passive diffusion beyond that, though percentage absorption diminishes with increased dose. The absorbed thiamine undergoes intracellular phosphorylation, mainly to the pyrophosphate, but at the serosal side 90% of the transferred thiamine is in the firee form. Thiamine uptake is enhanced by thiamine deficiency and reduced by thyroid hormone, diabetes, and ethanol ingestion. The gene for the specific thiamine transporter has been identified, and the transporter cloned. Thiamine is carried by the portal blood to the liver. The firee vitamin occurs in the plasma, but the coenzyme, TPP, is the primary cellular component. Approximately 30 mg is stored in the body with 80% as the pyrophosphate, 10% as triphosphate, and the rest as thiamine and its monophosphate. About half of the body stores are found in skeletal muscles, with much of the remainder in heart, liver, kidneys, and nervous tissues (including the brain, which contains most of the triphosphate). 

ThiT/EcfAA T - thiamine uptake Firmicutes 21 Cloned from Lactobacillus casei, binding 41... 

HMP, forming first HMP-P (8, salvage activity) and then HMP-PP (9, biosynthetic activity)/ Structural studies have provided an explanation for this novel dual kinase activity." The ABC transporter involved in thiamin uptake has also been identified and the periplasmic thiamin-binding protein has been structurally characterized. ... 

Strangely, cellular ThDP levels are 2-3 times lower in humans than in rodents, and this is also true for the brain (Gangolf et al. 2010a). This is probably linked to very low circulating thiamin levels in humans, possibly because intestinal thiamin absorption is less efficient. This would explain the higher sensitivity of humans to thiamin deficiency, in particular in association with alcohol consumption partial inhibition of intestinal thiamin uptake by alcohol would be sufficient to critically affect thiamin absorption. 

In animals, insufficient thiamin uptake affects the nervous system and leads to severe syndromes. 

Bettendorlf, L., and Wins, P., 1994. Mechanism of thiamine transport in neuroblastoma cells. Inhibition of a high affinity carrier by sodium channel activators and dependence of thiamine uptake on membrane potential and intracellular ATP. Journal of Biological Chemistry. 269 14379-14385. 

It is known that chronic treatment with alcohol induces compensatory response in the form of stimulation of thiamine uptake and its conversion to TDP. That is why the appearance of clinical symptoms of thiamine deficiency may be markedly delayed despite of systemic shortages of thiamine (Parkohomenko et al. 2011). Increased levels of TDP in the blood of alcoholic people with adequate dietary supply of thiamine seem to support such a hypothesis. 

Thiamine. Thiamine plays crucial roles in cellular metabolism, and impaired thiamine uptake results in a variety of disorders. 

Most coenzymes have aromatic heterocycles as major constituents. While enzymes possess purely protein structures, coenzymes incorporate non-amino acid moieties, most of them aromatic nitrogen het-erocycles. Coenzymes are essential for the redox biochemical transformations, e.g., nicotinamide adenine dinucleotide (NAD, 13) and flavin adenine dinucleotide (FAD, 14) (Scheme 5). Both are hydrogen transporters through their tautomeric forms that allow hydrogen uptake at the termini of the quinon-oid chain. Thiamine pyrophosphate (15) is a coenzyme that assists the decarboxylation of pyruvic acid, a very important biologic reaction (Scheme 6). 

A compound that includes an aminopyrimidine ring as well as the quaternary salt present in thiamine shows preferential inhibition of absorption of that co-factor by coccidia parasites over uptake by vertebrates. The compound is thus used in poultry where coccidiosis is an economically important disease. Condensation of ethoxymethylenemalononitrile (42-1) with the amidine (42-2) leads to the aminopyrimidine (42-4), probably via the intermediate addition-elimination intermediate (42-3). The nitrile group is then reduced to the methylamino derivative (42-5) by means of hthium aluminum hydride. Exhaustive methylation, for example by reaction with formaldehyde and formic acid, followed by methyl iodide leads to the quaternary methiodide (42-6). The quaternary salt is then displaced by bromine, and the resulting benzyhc-like cylic halide (42-7) is displaced by 2-picoline (42-8). There is thus obtained amprolium (42-9) [43]. 

The higher potency of nicarbazin (57) compared with earlier agents then set a standard for new compounds. Moreover, parasite resistance to the existing agents was becoming problematic. In 1960 amprolium (58) was introduced. This compound had an entirely different mode of action. That the drug reduces the uptake of thiamine (59) by (or... 

Some thiamin is phosphorylated to thiamin monophosphate in the intestinal mucosa, although this is not essential for uptake, and isolated membrane vesicles wUl accumulate free thiamin against a concentration gradient. Thiamin does not accumulate in the mucosal cells there is sodium-dependent active transport across the basolateral membrane, so that the mucosal concentration of thiamin is lower than that in the serosal fluid (Hindi et al., 1984 Hindi and Laforenza, 2000 Dudeja et al., 2001). 

The absorption of thiamin is impaired in alcoholics, leading to thiamin deficiency (Section 6.4.4). In vitro, tissue preparations show normal uptake of the vitamin into the mucosal cells in the presence of ethanol, but impaired transport to the serosal compartment. The sodium-potassium-dependent ATPase of the basolateral membrane responsible for the active efQux of thiamin into the serosal fluid is inhibited by ethanol (Hoyumpa et al., 1977). 

Both thiamin monophosphate and free thiamin are found in cerebrospinal fluid. Uptake of thiamin monophosphate into cells in the central nervous system involves extracellular hydrolysis to free thiamin, probably catalyzed... 

Thiamin that is not bound to plasma proteins is rapidly filtered at the glomerulus. Diuresis increases the excretion of the vitamin, and patients who are treated with diuretics are potentially at risk of thiamin deficiency. Some of the diuretics used in the treatment of hypertension may also inhibit cardiac (and other tissue) uptake of thiamin, thus further impairing thiamin status, which may be a factor in the etiology of heart failure (Suter and Vetter, 2000). 

Plaitakis A, Nicklas WJ, Van Woert MH, Hwang EC, and Bed S (1981) Uptake and metabolism of serotonin and amino acids in thiamine deficiency. Advances in Experimental Medicine and Biology 133, 391 16. 

---