In vitro kinetics of the intestinal transport of riboflavin in rats.

Transmural intestinal transport of riboflavin was studied in a concentration range between 0.033 and 10.0 microM by an in vitro perfusion technique by using everted jejunal segments of rats. The transport was found to have a dual characteristic: at low, i.e., physiologically relevant concentrations, a carrier-mediated saturable component predominated; at higher concentrations this component was increasingly obscured by simple diffusion as the prevailing transport mechanism. The transport constant KT of the saturable component was calculated to be 0.54 microM. The Q10-value was 2.31 for a low concentration (0.322 microM) and 1.44 for a high concentration (10.0 microM) of the substrate. When Na+ was partially replaced by Li+ or choline+ in the media or when ouabain was added to the serosal medium, the saturable component of the transport was completely abolished. The substrate analogue lumiflavin reduced the transport rate of riboflavin at low substrate concentration only. Generally, the results indicate that it is essential to consider the physiologically relevant concentration of micronutrients in studies concerning the mechanism of their intestinal transport.