Biotin crystallizes from water as long white needles. Its melting point is 232° to 233°C. Free biotin is soluble in dilute alkaline solutions and hot water and practically insoluble in fats and organic solvents. Biotin is quite stable under ordinary conditions. It is destroyed by nitric acid, other strong acids, strong bases and formaldehyde and also inactivated by oxidative rancidity reactions (Scott et al., 1982). Also, it is gradually destroyed by ultraviolet (UV) radiation.
Structurally related analogs of biotin can vary from no activity to partial replacement value to antibiotin activity. Mild oxidation converts biotin to the sulfoxide, and strong oxidation converts it to sulfone. Strong agents result in sulfur replacement by oxygen, resulting in oxybiotin and desthiobiotin. Oxybiotin has some biotin activity for chicks (one-third) but less for rats (one-tenth).
Biotin exists in natural materials in both bound and free forms, with much of the bound biotin apparently not available to animal species. For poultry, and presumably for pigs, often less than half of the microbiologically determined biotin in a feedstuff is biologically available (Scott, 1981; Frigg 1984, 1987). Naturally occurring biotin is found partly in the free state (fruit, milk, vegetables) and partly in a form bound to protein in animal tissues, plant seeds and yeast. Naturally occurring biotin is often bound to the amino acid lysine.
The few animal studies of biotin metabolism revealed that biotin is absorbed as the intact molecule in the first third to half of the small intestine (Bonjour, 1984). There is also absorption of biotin from the hind gut of the pig, with disappearance of 50% and 61% of infused biotin between the cecum and feces that was accompanied by a more than a fourfold increase of plasma biotin concentration and a more than sixfold increase of urinary biotin excretion (Barth et al., 1986). Kopinski and Leibholz (1985) reported that post-ileal absorption was 10% to 15% of that from the small intestine after oral ingestion. Of a labeled biotin dose infused into the cecum of mini-pigs, 80% appeared in portal blood (Drochner and Volker, 1984) with the largest proportion appearing in feces. Kopinski et al. (1989a, b) reported similar findings in that absorption of free biotin in the post-ileal digestive tract was about 8% as efficient as that from a similar labeled dose of orally administered biotin. Kopinski et al. (1989c) observed that even with extensive microbial synthesis of biotin in the post-ileal tract, low concentrations of biotin in plasma and tissue and the presence of deficiency signs indicate that post-ileal synthesized biotin is of little benefit to the pig. Information on biotin transport, tissue deposition and storage in animals and humans is very limited. McCormick and Olson (1984) reported that biotin is transported as a free water-soluble component of plasma, taken up by cells via active transport, and attached to its apoenzymes. Whitehead and Bannister (1980) reported that plasma biotin concentration gives a good indication of biotin intake in young pigs.
All cells contain some biotin, with largest quantities in liver and kidney. Intracellular distribution of biotin corresponds to known localization of biotin-dependent enzymes (carboxylases). Cooper et al. (1997) investigated the distribution of biotin in tissues of pigs and chickens using an indirect peroxidase-antiperoxidase immunohistochemical technique. In both species, immunoreactive biotin was detected in many tissues, including liver, kidney, pancreas, testis, brain, choroid plexus, adipose tissue adrenal gland, cardiac and skeletal muscle, skin and lymphoid tissues and epithelium of the respiratory and digestive systems. Investigations into biotin metabolism in animals are difficult to interpret, as biotin-producing microorganisms exist in the intestinal tract distal to the cecum. Often the amount of biotin excreted in urine and feces together exceeds total dietary intake, whereas urinary biotin excretion is usually less than intake.
