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 is inactivated by oxidative rancidity reactions (Scott et al., 1982). It is gradually destroyed by ultraviolet (UV) radiation.
Structurally related analogs of biotin can vary from no activity, to partial replacement of biotin activity, to anti-biotin 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 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 presumably other species), often less than one-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 the form bound to protein in animal tissues, plant seeds and yeast. Naturally occurring biotin is often bound to the amino acid lysine.
Biotinidase, present in pancreatic juice and intestinal mucosa, releases biotin from biocytin (bound form of biotin) during the luminal phase of proteolysis. In most species that have been investigated, physiological concentrations of biotin are absorbed from the intestinal tract by a sodium-dependent active transport process, which is inhibited by dethiobiotin and biocytin (Said and Derweesh, 1991). Absorption of biotin by a Na+-dependent process was noted to be higher in the duodenum than the jejunum, which was in turn higher than that in the ileum, and it was concluded that the proximal part of the human small intestine was the site of maximum transport of biotin (Said et al., 1988). Biotin is absorbed intact in the first third to half of the small intestine (Bonjour, 1991). Also, biotin is absorbed from the hind gut of the pig; 50% to 61% of infused biotin disappears between the cecum and feces; this 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). Scholtissek et al. (1990) suggested that under basal conditions 1.7% to 17% of the swine requirement for biotin is provided by colonic bacteria.
Biotin appears to circulate in the bloodstream both free and bound to a serum glycoprotein, which also has biotinidase activity, catalyzing the hydrolysis of biocytin. In humans, 81% of biotin in plasma was free and the remainder bound (Mock and Malik, 1992). Information is very limited on biotin transport, tissue deposition, and storage in animals and humans. Mock (1990) reported that biotin is transported as a free water-soluble component of plasma, is taken up by cells via active transport and is attached to its apoenzymes. Said et al. (1992) reported that biotin is transported into human liver via a specialized, carrier-mediated transport system. This system is Na+ gradient-dependent and transports biotin via an electroneutral process.
All cells contain some biotin, with larger quantities in liver and kidney. Intracellular distribution of biotin corresponds to known location of biotin-dependent enzymes (carboxylases). Investigations of biotin metabolism in animals are difficult to interpret, as biotin-producing microorganisms are present in the intestinal tract distal to the cecum. Often the amount of biotin excreted in urine and feces together exceeds the total dietary intake, whereas urinary biotin excretion is usually less than intake. 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 meeting biotin requirements (Bender, 1992). 14C-labeled biotin showed the major portion of intraperitoneally injected radioactivity to be excreted in the urine and none in the feces or expired as CO2 (Lee et al., 1973). In rats and pigs, biliary excretion of biotin and metabolites is negligible (Zempleni et al., 1997).
