Biotin crystallizes from water as long, white needles. Its melting point is 232° to 233°C (450° to 451°F). 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 is inactivated by oxidative rancidity reactions (Scott et al., 1982). Biotin is gradually destroyed by direct exposure to ultraviolet (UV) radiation.
Structurally related analogs of biotin vary from zero biological activity, to partial biotin activity, to anti-biotin activity. Mild oxidation converts biotin to the sulfoxide, and strong oxidation converts it to the sulfone. Strong reagents result in sulfur replacement by oxygen, resulting in oxybiotin and desthiobiotin. Oxybiotin has partial biotin activity in chicks (one-third) and rats (one-tenth).
Biotin is present in feedstuffs in both bound and free forms, and much of the bound biotin is apparently unavailable to animal species. For poultry and swine (and presumably for ruminants), often less than half of the microbiologically determined biotin in a feedstuff is biologically available (Scott, 1981; Frigg, 1984, 1987; Sauer et al., 1988). 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, in a form called biocytin. Biotinidase is a mammalian enzyme that cleaves the biotin-lysine amide bond, freeing biotin for reuse in metabolism. Biotinidase is present in pancreatic secretions and intestinal cells of mammals, as well as systemically. Genetic deficiency of biotinidase causes a biotin deficiency syndrome in humans (Mock, 1990).
Studies with animals show that biotin is absorbed intact in the first third to half of the small intestine by active transport and diffusion (Bonjour, 1991; Zempleni and Mock, 1999). In addition, biotin is absorbed from the hind-gut of the pig. Fifty to sixty percent of infused biotin disappeared between the cecum and feces; this was accompanied by more than a fourfold increase of plasma biotin concentration and more than a sixfold increase of urinary biotin excretion (Barth et al., 1986).
McCormick and Olson (1984) reported that biotin is transported as a free water-soluble component of plasma. Most studies have found similar results, although biotin may also be partially (10% to 20%) bound to plasma biotinidase (Zempleni and Mock, 1999). The liver, brain, placenta and white blood cells actively accumulate biotin (Zempleni and Mock, 1999). The placenta has a sodium-dependent transporter that transports biotin, pantothenic acid and lipoic acid (Prasad et al., 1997). Prasad et al. (1999) have identified a similar multivitamin transporter for biotin, pantothenate and lipoate in the small intestine.
Virtually all living cells contain biotin, due to its role as enzyme cofactor (Cooper et al., 1997). The highest concentrations are found in the liver and kidney. Intracellular distribution of biotin corresponds to known localization of biotin-dependent enzymes (carboxylases). Biotin is also concentrated in the cell nucleus.
