BACKGROUND: Biocytin is an important end product of intraluminal digestion of dietary protein-bound biotin. Limited studies are available regarding the ability of the small intestine to transport biocytin and about the mechanism involved. The aim of the present study was to delineate these issues. METHODS: Transport of [3H]-biocytin was examined using everted sacs from rat intestine. RESULTS: Mucosal-to-serosal transport of low (0.022 mumol/L) and high (5 mumol/L) concentrations of biocytin were linear for up to 20 minutes of incubation. Transport of biocytin as a function of concentration (0.022-5 mumol/L) was linear (r = 0.99) and occurred at a rate of 22,062 fmol.g tissue (wet wt)-1.15 min-1. Addition of high concentrations of unlabeled biocytin, biotin, biotin methyl ester, and lysine did not cause a significant inhibition of the transport of [3H]-biocytin. Furthermore, transport of biocytin was independent of Na+ concentration, pH, energy, and temperature. Compared with transport of equimolar concentrations of free biotin, transport of biocytin (0.022 mumol/L) was significantly lower in both the jejunum and the ileum. CONCLUSIONS: (1) Biocytin transport in rat intestine is lower than that of free biotin and occurs via simple physical diffusion. (2) In the rat, efficient absorption and optimal bioavailability of dietary protein-bound biotin necessitates its conversion to free biotin.
BACKGROUND: Biocytin is an important end product of intraluminal digestion of dietary protein-bound biotin. Limited studies are available regarding the ability of the small intestine to transport biocytin and about the mechanism involved. The aim of the present study was to delineate these issues. METHODS: Transport of [3H]-biocytin was examined using everted sacs from rat intestine. RESULTS: Mucosal-to-serosal transport of low (0.022 mumol/L) and high (5 mumol/L) concentrations of biocytin were linear for up to 20 minutes of incubation. Transport of biocytin as a function of concentration (0.022-5 mumol/L) was linear (r = 0.99) and occurred at a rate of 22,062 fmol.g tissue (wet wt)-1.15 min-1. Addition of high concentrations of unlabeled biocytin, biotin, biotin methyl ester, and lysine did not cause a significant inhibition of the transport of [3H]-biocytin. Furthermore, transport of biocytin was independent of Na+ concentration, pH, energy, and temperature. Compared with transport of equimolar concentrations of free biotin, transport of biocytin (0.022 mumol/L) was significantly lower in both the jejunum and the ileum. CONCLUSIONS: (1) Biocytin transport in rat intestine is lower than that of free biotin and occurs via simple physical diffusion. (2) In the rat, efficient absorption and optimal bioavailability of dietary protein-bound biotin necessitates its conversion to free biotin.