Biotin unavailable

Alimentary biotin deficiency is rare. It may, however, occur in patients on long-term parenteral nutrition lacking biotin or in persons who frequently consume raw egg white. Raw egg white contains a biotin-binding glycoprotein, called avidin, which renders biotin biologically unavailable. Pharmacological doses of the vitamin (1-10 mg/d) are then used to treat deficiency symptoms. There are no reports of toxicity for daily oral doses up to 200 mg and daily intravenous doses of up to 20 mg [2]. 

Biotin deficiency is characterized by anorexia, nausea, vomiting, glossitis, depression, and dry, scaly dermatitis. Biotin deficiency occurs when avidin, a biotinbinding glycoprotein, is present. Avidin, which is found in raw egg whites, binds the biotin, making it nutritionally unavailable. 

Biotin deficiency is rare but under laboratory conditions it can be induced by feeding subjects with large amounts of raw egg white which contains the protein, avidin, which has a binding site for the imidazole moiety of biotin, thus making it unavailable. Avidin is denatured by heat and, therefore, biotin binding occurs only in raw egg albumen. Symptoms of biotin deficiency include scaly dermatitis, hair loss, loss of appetite, nausea, hallucinations and depression. 

The relative bioavailability of free and bound biotin is not entirely clear (179,180) for example, the biotin present in many cereals and oilseeds is largely unavailable to animals. In addition, the bioavailability of biotin can be significantly impaired by avidin, a biotin-binding protein found in egg white. The physiology and metabolism of biotin has been recently reviewed (179,180). 

Dietary biotin bound to avidin (Section 11.6) is unavailable, but intravenously administered avidin-biotin is biologically active. Cells in culture are not inhibited by the addibon of avidin to the culmre medium, and can take up the avidin-biotin complex by pinocytosis followed by lysosomal hydrolysis, releasing free biotin. Unlike other B vitamins, for which concentrative uptake into tissues is achieved by facilitated diffusion, followed by metabolic trapping, the incorporation of biotin into enzymes is slow and cannot be considered part of the uptake process. 

A second well-known sulfur-containing coenzyme is biotin Biotin, a member of the B vitamin complex, plays an essential nutritional role in carboxylation reactions. It is the coenzyme of carboxylases and catalyzes fundamental metabolic processes. Its relative unavailability from natural sources spurred great interest in synthetic approaches, the most recent of which appeared in xhe synthesis of... 

A few years later egg-white injury in chicks was shown to be associated with a deficiency of biotin in the tissues, despite its abundance in the diet [145]. In the same year, avidin was isolated from egg-white and its ability to inactivate biotin in vitro was demonstrated [146]. Gyorgy and Rose [235] fed rats with avidin but only found very small amounts of biotin in the faeces until the faeces were steamed. Biotin was then released from the avidin-biotin complex. As the result of further experiments, it was concluded that the fundamental cause of egg-white injury is the unavailability of biotin due to its fixation to avidin, so that biotin is not absorbed from the intestinal tract and is excreted in the faeces [237]. A similar conclusion was reached by Sullivan and Nicholls [603] who showed that when egg-white is cooked, avidin is denatured and rendered incapable of binding biotin. Egg-white injury has been produced experimentally in man and can be cured by the administration of biotin [607]. In a recent study, Peters [497] reported that raw egg-white has a direct toxic effect which is not associated with its action in causing biotin deficiency. It would appear, therefore, that further studies on egg-white injury must be more closely associated with a critical analysis of the different components of egg-white. 

Biotin is widely distributed in foods, and deficiency is unknown, except among people maintained for many months on total parenteral nutrition and a very small number of people who eat abnormally large amounts of uncooked egg. There is a protein in egg white, avidin, that binds biotin extremely tightly and renders it unavailable for absorption. Avidin is denatured by cooking and then loses its ability to bind biotin. The amount of avidin in uncooked egg white is relatively small, and problems of biotin deficiency have occurred only in people eating abnormally large amounts — a dozen or more raw eggs a day - for some years. 

Native ovoflavoprotein (49 kDa, pf=5.1) has, as does ovomucoid, certain antinutritional effects, as it inhibits serine proteases (trypsin, chymotrypsin and also microbial proteases) and has antiviral activity. Ovomacroglobulin (ovostatin) is an inhibitor of serine, cysteine, thiol and metalloproteases and shows antimicrobial activity. Some antinutritional effects are also seen in the basic glycoprotein avidin in raw egg white (relative molecular weight of the monomer is 15.6 kDa). It contains four identical subunits (pf = 9.5), each of which binds one molecule of biotin to give an unavailable complex. However, the denatured avidin, for example in hard-boiled eggs, does not interact with biotin. The interaction of riboflavin with flavoprotein (32 kDa, pf = 4.0) has, on the contrary, a positive influence on vitamin stability. Cystatin acts as cysteine protease inhibitor, and shows antimicrobial, antitumor and immunomodulating activities. 

Biotin is stable when heated, in the Hght, in neutral and strongly acidic solutions. It is unstable in alkaline media. It is easily oxidised by hydrogen peroxide and other oxidants to a mixture of isomeric (-1-)- and (-)-sulfoxides (5-88 and 5-89), eventually to biotin sulfone (5-90). Sulfoxides also form as metaboHc products of microorganisms. They are ftiUy available, whereas sulfone is unavailable as a vitamin. Nitrogen of a ureido ring maybe nitrosated to nitrosobiotin (5-91) in the presence of nitrites or nitrogen oxides. 

A proteinaceous suErstance in raw egg white which ties up the vitamin biotin so that it is unavailable. Cooking egg white inactivates avidin. 

NOTE WELL Raw egg white contains an antivitamin, a protein called avidin, which binds to biotin and renders it unavailable. However, avidin is denatured when eggs are cooked, thus making biotin available. 

Eggs, raw Uncooked eggs. Eaten rav(, usually with salt and pepper added. Faddists claim that raw eggs Increase potency. Raw eggs contain the anlivitamin. avidin, which binds biotin and makes it unavailable. 

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