Biotin intestinal synthesis

Another of the water-soluble B vitamins known to have a function for some bacteria and animals is biotin. A role in carbohydrate metabolism has been suggested. A protein substance called avidin, when fed in the form of large amounts of raw egg yolk, combines with biotin, and may thus cause a biotin deficiency in some species. Somewhat similar symptoms have been reported in human subjects (Sydenstricker et al., 1942). Intestinal synthesis and the antivitamin may explain the failure to confirm this observation. No definite requirement for the diet can be stated. 

In general, the combined urinary and fecal excretion of biotin exceeds the dietary intake. It seems likely that the fecal excretion of biotin is an indication of intestinal synthesis, whereas urinary excretion is a reflection of the dietary intake. Published reports of normal values of biotin in blood vary too widely for diagnostic use without carefully controlled observations. 

The use of the insoluble sulfonamides for the study of intestinal synthesis has been most productive. The demonstration that folic acid and biotin... 

An important factor in biotin s availability is that some of the vitamin is derived from synthesis by intestinal microorganisms this is demonstrated by the fact that three to six... 

Biotin -member of the B-complex family Water soluble Metabolism of fats synthesis of ascorbic acid healthy skin hair balding and greying. Soya beans, brown rice, nuts, fruit, brewer s yeast and milk. It can be synthesised by intestinal bacteria. 100 pg No... 

As a result of this resorption and the protein binding of plasma biotin, which reduces filtration at the glomerulus, renal clearance of biotin is only 40% of that of creatinine. This efficient conservation of biotin, together with the recycling of biocytin released from the catabolism of biotin-containing enzymes, may be as important as intestinal bacterial synthesis of the vitamin in explaining the rarity of deficiency. 

On the basis of studies in patients who developed deficiency during total parenteral nutrition, and who are therefore presumably wholly reliant on an exogenous source of the vitamin - with no significant contribution from intestinal bacterial synthesis - the provision of 60 fxg of biotin per day for adults receiving total parenteral nutrition is generally recommended (Bitsch et al., 1985). 

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. 

Pantothenic acid is taken in as dietary CoA compounds and dCphosphopantetheine and hydrolyzed by pyrophosphatase and phosphatase in the intestinal lumen to dephospho-CoA, phosphopantetheine, and pantetheine. This is further hydrolyzed to pantethenic acid. The vitamin is primarily absorbed as pantothenic acid by a saturable process at low concentrations and by simple diffusion at higher ones. The saturable process is facilitated by a sodium-dependent multivitamin transporter, for which biotin and lipoate compete. After absorption, pantothenic acid enters the circulation and is taken up by cells in a manner similar to its intestinal adsorption. The synthesis of CoA from pantothenate is regulated by pantothenate kinase, which itself is subject to negative feedback from the products CoA and acyi-CoA. The steps involved were outlined above. Pantothenic acid is excreted in the urine after hydrolysis of CoA compounds by enzymes that cleave phosphate and the cys-teamine moieties. Only a small fraction of pantothenate is secreted into milk and even less into colostrum. 

Biotin is widely distributed in foods. Beef liver, yeast, peanuts, kidney, chocolate, and egg yolk are especially rich sources. The intestinal flora synthesizes biotin. Fecal excretion reflects this enteric synthesis. Total daily urinary and fecal excretion exceeds the dietary intake. 

For animal feed, biotin is also of high importance. [110] Whereas ruminants have normally a sufficient supply of biotin provided by their fodder and by the amounts of biotin synthesised in the gastrointestinal tract, deficiencies occur more often in pigs, especially in piglets. Poultry tends to underutilise the biotin in their feed, and their enteral biotin synthesis is poor. Turkeys have an especially high demand for biotin. The use of sulfonamides and other antibiotics in animal husbandry affects the intestinal flora and may necessitate biotin-fortified feed (Tab. 7.4). 

Biotin is widely distributed in many foods. It is synthesized by intestinal flora, and in balance studies the total output of biotin in urine plus faeces is three- to sixfold greater than the intake, reflecting bacterial synthesis. It is not known to what extent this is available to the host (see also Problem 10.4). 

A considerable amount of biotin is synthesized by human intestinal bacteria, as evidenced by the fact that 3 to 6 times more biotin is excreted in the urine and feces than is ingested. But synthesis in the gut may occur too late in the intestinal passage to be absorbed well and play much of a direct role as a biotin source. Also, several variables affect the microbial synthesis in the intestines, including the carbohydrate source of the diet (starch, glucose, sucrose, etc.), the presence of other B vitamins, and the presence or absence of antimicrobial drugs and antibiotics. 

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