Biotin

Biotin (hexahydro-2-oxo-11 l-thicno[3.4-djiniidazole-4-pcntanoic acid, obsolete names vitamin 11, coenzyme R. molecular weight 244 \ is a water-soluble vitamin.

Abbreviations

CoA coenzyme A

SMVT sodium-dependent multivitamin transporter

Hgur* W.37 Biotin

Nutritional summary

Function: Biotin is essential for lipid metabolism, amino acid breakdown, replenishing of tricarboxylic acid cycle intermediates, and some nuclear functions. Requirements: Adequate daily intakes for infants are 0.7 puiz kg. Children need 8-20jig/day, depending on their age. Adults should get 30p.g/day. Lactation increases tins amount to 35 jxg/day. I lemodialysis increases requirements. Sources: Intestinal bacteria probably provide enough to satisfy requirements under most circumstances. Soybeans. Ii\er. cauliflower, mushrooms, bean sprouts, and eggs each provide at least 20% of adult needs per serving.

Deficiency: I he consequences of diminished biotin status include impaired glucose tolerance, mental dysfunction (Bregola et <//.. 19%), myalgia, hyperesthesia and paresthesia, anorexia, nausea, dry eyes, maeulo-squamous. seborrheic dermatitis (similar to the changes seen with essential fatty acid deficiency), angular cheilitis, hair loss (which may be complete in patients with short-bowel syndrome), impaired immune response, possibly teratogenicity. Biotin deficiency may be aggravated by pantothenic acid deficiency.

Excessive intake: No toxicity has been observed following consumption of 20 mg day. Dietary and other sources

Foods contain both free and bound biotin (epsilon-N-biotinyl-L-lysinec metabolites such as bisnorbiotin and biotin sulfoxide have no biotin activity. Good sources include soybeans (O.fipgg), li\er (I.Opgg), cauliflower and mushrooms (0.17pgg). egg yolk (K.5 p.g/yo!k), legumes (especially sprouts), grains, and nuts. Milk contains only 3 5 pg 1; most vegetables, fruits, and meats are poor sources. Intakes may be between 28 and 100p.g per day (Dakshinamurti, 1994).

A wide variety of intestinal flora probably produces much more biotin than is normally consumed with food, but only a small fraction of this appears to contribute to the body's supplies.

Digestion and absorption

Biotin-containing food proteins are broken down by the usual digestive proteases; biotin is then released by biotinidase (EC3.5.I.I2) from the resulting lysylbiotin

(biocytin) or ly sy 1 btoi i n-contain i ng peptides. A high-affinity, low-capacity sodium multivitamin cotransporter (SMVT. SI C5A6) in the small intestine mediates apical uptake ufbiotin (and also of pantothenate and lipoate) into enterocytes (Prasad et al,, I99X; Chatleijee et a!,, 1999). The fact that SMVT is also expressed in the colon (Said, 1999) may explain why deficiency of this critically important vitamin is so rare.

Biotin transport across the basolateral membrane is mediated by a sodium-independent, etectrogenic mechanism (Said et al., 19X8).

Note: Heat-labile avidin in egg white tightly binds biotin and prevents its uptake from the intestinal lumen.

Transport and cellular uptake

Blood circulation: Biotin in blood is transported at least in part, with biotinidase (EC3.5.1.12) which acts as a biotinyl-transferase in this case (Hymes et al.. 1999).

One or more anion acid carriers, sharing characteristics or identical with the intestinal transporter, mediate uptake by liv er cells. Inside the hepatocytc (and probably all other cells) some biotin is transported with biotinidase into the nucleus and transferred to histones.

Blood brain barrier; The SMVT (SLC5A6) mediates biotin transfer across the blood-brain barrier (Prasad et al.. 1998).

Maternofetal transfer: The SMVT participates in hiotin transport across the brush border membrane ofthc syntrophoblast of the placenta, but knowledge on the involvement of additional transporters is -.till evoK ing (Prasad et al.. 1998).

Metabolism

Biotin can be linked covalently to four carboxylases (acetyl-CoA carboxylase. EC&.4,1.2: propionyl-CoA-carboxylase. EC&.4.1.3; pyruvate carboxylase, EC6.4.1.1: 3-methylcrotonyl-CoA carboxylase. EC6.4,1.4 > through the action of biotin-J propionyl-Co A-carboxylase (ATP-hydrolysing)] ligase (EC6.3.4.10). The reaction is driven by the hydrolysis of ATP to AMP.

Protein-bound biotin is recycled very efficiently. As the biotin-containing enzymes are broken down in due course by intracellular proteases, lysylbiotin or peptides containing lysylbiotin are generated. Biotinidase (EC3.5.1.I2) then releases the free biotin. which thus becomes available again at the site where it is needed for incorporation into proteins.

Beta-oxidation successively shortens the side chain to bisnorbiotin and tetranorbi-otin. A small amount of bisnorbiotin methylketone arises from the non-enzymic decarboxylation of beta-keto-biotin. an intermediate product of biotin beta-oxidation. In the ring moiety (thiophane) of biotin, the thiocther sulfur can be oxidized to btotin-l-sulfoxide. biotin-d-sulfoxide, or biotin sulfone: the mechanism of this oxidation is not known. I lie combination of side-chain catabolism and sulfur oxidation generates a wide spectrum of catabohtcs, none of which has significant biotin activ itv: some of them (such as tetranorbiotm-l-sulfoxide) have greatly decreased affinity to avidin. which can make them difficult to detect with binding assays (Zemplcni et al.. 1997).

Biotin-

|propiony(-CoA

carboxylase]

ligase

Biotin-

|propiony(-CoA

carboxylase]

ligase

Pigur* 10.38 Biotin becomes linked to carboxylases

Pigur* 10.38 Biotin becomes linked to carboxylases

Biotin sulfoxide

FigurĀ« 10.39 Inactive biotin metabolites

Storage

Mitochondrial biolinyl-acetyl-CoA carboxylase (F.C6.4.L2) appears to comprise the majority of biotin reserves in a wide range of tissues (1.2mg in liver atone). Stores may be depleted within 5-7 weeks of intake cessation.

Excretion

The high-affinity SMVT (Prasad et at.. 1998) recovers filtered biotin from the tubular lumen. Little is known about biotin transport across the ceil and into the pericapillary

Pyruvate carboxylase

ATP ADP

^Pvruvale carboxylase C-OH

Pyruvate Oxaloacetate

Figure 10.40 Riatirt-depenrlem pyruvate ciirhctxylaie space. Excess vitamin is excreted with urine as biotin. bisnorbiotin, biotin sulfoxide, bisnorbiotin methyl ketone, and biotin sulfonc (3:2:1:0.4:0.1): additional intermediates of biotin catabolism are excreted in much smaller amounts (Zempleni et ul., 1997).

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