Some Aspects of Thiamin Transport in Mammals

Rindi, G.

doi:10.3177/jnsv.38.special_379

Cited by 9 publications

(6 citation statements)

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“…In humans, single oral doses of T + that are higher than 2.5–5 mg are largely unabsorbed (9, 10), with intestinal uptake following saturation kinetics (11). The process of T + intestinal absorption, as studied in vitro in human tissue, involves two mechanisms (5, 12, 13). At concentrations lower than 1 μmol/l, T + is transported mainly by an active, carrier‐mediated system that involves the intracellular phosphorylation of the vitamin (14).…”

Section: Thiamine Intestinal Absorptionmentioning

confidence: 99%

Thiamine Intestinal Transport and Related Issues: Recent Aspects

Rindi

Laforenza

2000

Proc Soc Exp Biol Med

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Abstract. 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 aminoacid residues with high homology with the reduced‐folate transporter.

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Section: Thiamine Intestinal Absorptionmentioning

confidence: 99%

Thiamine Intestinal Transport and Related Issues: Recent Aspects

Rindi

Laforenza

2000

Proc Soc Exp Biol Med

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“…The high affinity transport of thiamine by normal lymphoblasts is abolished in the presence of a 100 fold excess of unlabeled thiamine. Under such conditions, some uptake continues from a low affinity (K m in the mM range) transport mechanism [ 3 ] and/or from diffusion [ 20 ] that characterizes thiamine uptake in all mammalian cells examined to date. Using an expanded range of thiamine concentrations from that shown in fig.…”

Section: Resultsmentioning

confidence: 99%

“…After thorough drying (overnight at 60°C), the amount of labeled thiamine taken up by the cells was determined by scintillation counting [ 25 ]. For each concentration, the uptake in the presence of a 100-fold excess of unlabelled thiamine was performed to assess the contribution to uptake from a low affinity (K m in the mM range) component [ 3 ] and/or from diffusion [ 20 ].…”

Section: Methodsmentioning

confidence: 99%

Mitochondria from cultured cells derived from normal and thiamine-responsive megaloblastic anemia individuals efficiently import thiamine diphosphate

Song

Singleton

2002

BMC Biochem

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Background Thiamine diphosphate (ThDP) is the active form of thiamine, and it serves as a cofactor for several enzymes, both cytosolic and mitochondrial. Isolated mitochondria have been shown to take up thiamine yet thiamine diphosphokinase is cytosolic and not present in mitochondria. Previous reports indicate that ThDP can also be taken up by rat mitochondria, but the kinetic constants associated with such uptake seemed not to be physiologically relevant. Results Here we examine ThDP uptake by mitochondria from several human cell types, including cells from patients with thiamine-responsive megaloblastic anemia (TRMA) that lack a functional thiamine transporter of the plasma membrane. Although mitochondria from normal lymphoblasts took up thiamine in the low micromolar range, surprisingly mitochondria from TRMA lymphoblasts lacked this uptake component. ThDP was taken up efficiently by mitochondria isolated from either normal or TRMA lymphoblasts. Uptake was saturable and biphasic with a high affinity component characterized by a K m of 0.4 to 0.6 μM. Mitochondria from other cell types possessed a similar high affinity uptake component with variation seen in uptake capacity as revealed by differences in V max values. Conclusions The results suggest a shared thiamine transporter for mitochondria and the plasma membrane. Additionally, a high affinity component of ThDP uptake by mitochondria was identified with the apparent affinity constant less than the estimates of the cytosolic concentration of free ThDP. This finding indicates that the high affinity uptake is physiologically significant and may represent the main mechanism for supplying phosphorylated thiamine for mitochondrial enzymes.

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“…Beside it the role of some phosphorylated form of thiamin in the nerve conduction process is well established [2]. As in general the amounts of TDP and total Vitamin B1 will hardly differ in fresh blood samples, the concentration of total Vitamin B1 will reflect the Vitamin B1 status of a patient very well.…”

Section: Introductionmentioning

confidence: 99%