Biotin transport system

Receptors and Transport Systems Affinity labeling of receptors, 46, 572 nicotinic acetylcholine receptors, 46, 582 )8-adrenergic receptors, 46, 591 opiate receptors, 46, 601 amino acid transport proteins, 46, 607 the biotin transport system, 46, 613. 

Becker et al. specifically inactivated the biotin transport system of E. coli using biotin p-nitrophenyl ester. The lactose transport protein of E. coli was labeled by AT-bromoacetyl-/8-D-galactopyranosylamine. In the only reported attempt to isolate an affinity labeled protein, glucose 6-isothiocyanate, which is an affinity label for the glucose transport system in human erythrocytes, gave enough nonspecific labeling of other membrane proteins to render identification of the transport protein difficult. 

Recently, Prasad et al. cloned a mammalian Na+-dependent multivitamin transporter (SMVT) from rat placenta [305], This transporter is very highly expressed in intestine and transports pantothenate, biotin, and lipoate [305, 306]. Additionally, it has been suggested that there are other specific transport systems for more water-soluble vitamins. Takanaga et al. [307] demonstrated that nicotinic acid is absorbed by two independent active transport mechanisms from small intestine one is a proton cotransporter and the other an anion antiporter. These nicotinic acid related transporters are capable of taking up monocarboxylic acid-like drugs such as valproic acid, salicylic acid, and penicillins [5], Also, more water-soluble transporters were discovered as Huang and Swann [308] reported the possible occurrence of high-affinity riboflavin transporter(s) on the microvillous membrane. 

The uptake and accumulation of various amino acids in Lactobacillus arabinosus have been described. Deficiencies of vitamin B6, biotin, and pantothenic acid markedly alter the operation of these transport systems. Accumulation capacity is decreased most severely by a vitamin B6 deficiency. This effect appears to arise indirectly from the synthesis of abnormal cell wall which renders the transport systems unusually sensitive to osmotic factors. Kinetic and osmotic experiments also exclude biotin and pantothenate from direct catalytic involvement in the transport process. Like vitamin B6, they affect uptake indirectly, probably through the metabolism of a structural cell component. The evidence presented supports a concept of pool formation in which free amino acids accumulate in the cell through the intervention of membrane-localized transport catalysts. 

Other proteins that interact with biotin including egg yolk biotinbinding proteins and biotin transport components from several systems including yeast, mammalian intestinal cells, and bacteria [70-73]. Note that although there are other proteins (e.g., fibropellins) that have a motif [DENY]-jc(2)-[KRI]-[STA]-x(2)-V-G-jc-[DN]-jc[FW]-T-[KR] in common with strept(avidin), most if not all of these protein do not bind biotin (http //www.expasy.org/prosite/). 

Most dietary biotin is bound to protein, the amide linkage being broken prior to absorption. At least eight children have been described who have multiple carboxylase deficiency with low activities of several of the biotin-requiring carboxylases, i.e., multiple carboxylase deficiency (Table 38-1). Pharmacological doses of biotin restored the activities of the carboxylases in these patients, indicating that the defect was not in the apocarboxylases. Thus, the defect is presumably in the intestinal transport system, in holocarboxylase synthetase, or in some step in cellular uptake or intracellular transport of biotin. 

The above aflSnity labeling reagents were found to inhibit specifically the uptake of biotin, but not other nutrients, into yeast cells. From the reactivation studies, one can conclude that an essential cysteine or histidine residue was modified in the transport system. The biotinylated protein can be specifically adsorbed to avidin columns, and subsequent elution with thiols should yield biotin transport component (s) in an active form. 

Biotin enters tissues by a saturable transport system, and is then incorporated into biotin-dependent enzymes as the 8-aminolysine peptide, biocytin. Unlike other B vitamins, whose concentrative uptake into tissues can be achieved by facilitated diffusion followed by metabolic trapping, the incorporation of biotin into enzymes is relatively slow and cannot be considered part of the uptake process. On catabolism of the enzymes, biocytin is hydrolysed by biotinidase, permitting reutilization. 

Biotin uptake has been extensively studied in micro-organisms such as Lactobacillus plantarum, Saccharomyces cerevisiae, and Escherichia coli (30 and references cited). The micro-organisms are able to recover biotin from the medium and to concentrate it intracellularly by an active process, that is, against a concentration gradient, mediated by a specialized protein and dependent on an energy source. The biosynthesis of the transport system is regulated by the level of external biotin (30). Since biotin is not biosynthesized by mammalian cells, it must be obtained from exogenous sources by absorption. This uptake of free biotin has been studied, in relation to biotin deficiency, in the small intestine, liver, kidney, and placenta (reviewed in 31). Different models have been utilized (whole animal studies, everted sacs, brush border membrane vesicles. . . ), but the preferred model is cultured cell lines (31). It seems that biotin uptake in... 

Biotin is transported across the blood-brain barrier. The transporter is saturable and structurally specific for the free carboxylate group on the valeric acid side chain. Transport into the neuron also appears to involve a specific transport system with subsequent trapping of biotin by covalent binding to brain proteins, presumably carboxylases. 

Biotin concentrations are 3- to 17-fold greater in plasma from human fetuses compared to those in their mothers in the second trimester, consistent with active placental transport. The microvillus membrane of the placenta contains a saturable transport system for biotin that is Na dependent and actively accumulates biotin within the placenta, consistent with SMVT. 

Animal studies and studies using brush border membrane vesicles from human kidney cortex indicate that biotin is reclaimed from the glomerular filtrate against a concentration gradient by a saturable, Na -depen-dent, structurally specific system, but biocytin does not inhibit tubular reabsorption of biotin. Subsequent egress of biotin from the tubular cells occurs via a basolateral membrane transport system that is not dependent on Na. Studies of patients with biotinidase deficiency surest that there may be a role for biotinidase in the renal handling of biotin. 

The water-soluble vitamins generally function as cofactors for metabolism enzymes such as those involved in the production of energy from carbohydrates and fats. Their members consist of vitamin C and vitamin B complex which include thiamine, riboflavin (vitamin B2), nicotinic acid, pyridoxine, pantothenic acid, folic acid, cobalamin (vitamin B12), inositol, and biotin. A number of recent publications have demonstrated that vitamin carriers can transport various types of water-soluble vitamins, but the carrier-mediated systems seem negligible for the membrane transport of fat-soluble vitamins such as vitamin A, D, E, and K. 

The biotin-dependent decarboxylases of anerobic microorganisms are transmembrane proteins. In addition to their roles in the metabolism of ox-aloacetate, methylmalonyl CoA, and glutaconyl CoA, they serve as energy transducers. They transport 2 mol of sodium out of the cell for each mole of substrate decarboxylated. The resultant sodium gradient is then used for active transport of substrates by sodium cotranspoit systems, or maybe used to drive ATP synthesis in a similar manner to the proton gradient in mammalian mitochondria (Buckel, 2001). 

Biotin is essential for cell proliferation. Peripheral blood mononuclear cells appear to take up biotin by a system that is distinct from the sodium-dependent multivitamin transporter that is responsible for intestinal and renal uptake of biotin (Section 11.1). In response to mitogenic stimuli the uptake of biotin increases several-fold, with no change in the activity of the sodium-dependent transporter. At the same time, there is an increase in the rate of expression of methylcrotonyl CoA, propionyl CoA carboxylases, and holocarboxylase... 

Biotin forms part of several enzyme systems and is necessary for normal growth and body function. Biotin functions as a cofactor for enzymes involved in carbon dioxide fixation and transfer. These reactions are important in the metaboHsm of carbohydrates, fats, and proteins, as well as promotion of the synthesis and formation of nicotinic acid, fatty acids, glycogen, and amino acids (5—7). Biotin is absorbed unchanged in the upper part of the small intestine and distributed to all tissues. Highest concentrations are found in the Hver and kidneys. Little information is available on the transport and storage of biotin in humans or animals. A biotin level in urine of approximately 160 nmol/24 h or 70 nmol/L, and a circulating level in blood, plasma, or serum of approximately 1500 pmol/L seems to indicate an adequate supply of biotin for humans. However, reported levels for biotin in the blood and urine vary widely and are not a reHable indicator of nutritional status. 

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