Uptake of riboflavin by rat intestinal mucosa in vitro.

The uptake of riboflavin was studied in everted sacs of rat intestine using [14C]riboflavin and [3H]polyethylene glycol to define the mechanism of mucosal membrane transport. Three-minute incubations were used throughout. Initial studies indicated the presence of saturable uptake in duodenum, jejunum and ileum. Studies in jejunum at low riboflavin concentrations demonstrated saturable uptake [Km = 0.154-0.177 mumol/L, Vmax = 19.6-25.8 pmol/(100 mg dry tissue.min)]. In contrast, uptake was linear with respect to higher concentrations of vitamin (10-50 mumol/L). Uptake at low (0.1 mumol/L) but not high (20 mumol/L) riboflavin concentrations was inhibited by 50 mumol/L lumiflavin, anoxia, 5 mmol/L indoacetamide, Na(+)-free buffer and low temperature (Q10 = 1.64). Conclusions are as follows: 1) Saturable uptake of riboflavin occurs throughout the rat small intestine; 2) uptake by the jejunal mucosa is competitively inhibited and is consistent with a transport carrier located in the brush border membrane; 3) saturable uptake is energy-dependent and may be directly or indirectly driven by a Na+ gradient; and 4) riboflavin is also taken up by rat intestinal mucosa by a nonsaturable, energy-independent mechanism consistent with simple, passive diffusion.