Biotin protein-bound, digestion

Biotin in the diet is largely protein bound, and digestion of these proteins by gastrointestinal enzymes produces biotinyl peptides, which may be further hydrolyzed by intestinal bio-tinidase to release biotin. Avidin, a protein found in raw egg whites, binds biotin tightly and prevents its absorption. The peptide biocytin (e-N-biotinyl lysine) is resistant to hydrolysis by proteolytic enzymes in the intestinal tract but together with biotin is readily absorbed. A biotin carrier, the sodium-dependent multivitamin transporter (SMVT)... 

The RDA for biotin has not been established. The requirement for biotin has been established as 30 to 100 pg/day. Biotin is produced by the gut microflora, which, it has been estimated, supplies half of our requirement. The rate of transport of free biotin by the gut cell is higher in the jejunum than in the ileum and is very low in the colon. Despite the low level of transport in the colon, it is apparently sufficient to allow supply by vitamin produced in the gut. The digestion of dietary proteins results in the release of the constituent amino acids and of biotin in the form of lysyl-biotin. Lysyl-biotin is further cleaved to lysine and biotin by biotinidase, an enzyme of the gut mucosa. The enzyme is present in milk and in the bloodstream as well, Biotin in human blood plasma occurs at a concentration of about 4 nM (Velazquez ef /, 1995). Nearly all of the biotin in human milk is free and not bound to proteins. Breast milk contains biotin at concentrations of about 25 nM (MiKik et al., 1997a),... 

---