Absorption and metabolism of biotin
Most biotin in foods is present as biocytin (ε-amino-biotinyllysine), which is released on proteolysis, then hydrolyzed by biotinidase in the pancreatic juice and intestinal mucosal secretions, to yield free biotin. The extent to which bound biotin in foods is biologically available is not known.
Free biotin is absorbed from the small intestine by active transport. Biotin circulates in the bloodstream both free and bound to a serum glycoprotein that has biotinidase activity, catalyzing the hydrolysis of biocytin.
Biotin enters tissues by a saturable transport system and is then incorporated into biotin-dependent enzymes as the ε-amino-lysine peptide, biocytin. Unlike other B vitamins, where concentrative uptake into tissues can be achieved by facilitated diffusion followed by metabolic trapping, the incorporation of biotin into enzymes is relatively slow, and cannot be considered part of the uptake process. On catabolism of the enzymes, biocytin is hydrolyzed by biotinidase, permitting reutilization.