Uptake of riboflavin across the brush border membrane of rat intestine: regulation by dietary vitamin levels.

BACKGROUND: Possible regulation of riboflavin (RF) intestinal uptake process by dietary substrate level is not known and was examined in this study.
METHODS: RF uptake was examined using intestinal brush border membrane vesicles (BBMV).
RESULTS: Oversupplementation with RF caused significant decrease (P < 0.05; down-regulation) in the uptake of 3 mumol/L RF compared with control. This effect was not due to differences in the relative purity of the intestinal BBMV preparations and appeared to be specific for RF. This down-regulation was mediated through a significant (P < 0.01) decrease in the Vmax of the RF uptake process with no significant change in the apparent Michaelis constant (Km). In contrast, RF deficiency caused a significant (P < 0.01) enhancement (up-regulation) in the uptake of 3 mumol/L RF compared with pair-fed control. Again the enhancement was not due to differences in the relative purity of the BBMV preparations and appeared to be specific for RF. This up-regulation was mediated via a significant (P < 0.01) increase in the Vmax of the RF uptake process with no significant change in the apparent Km.
CONCLUSIONS: These findings show that the RF intestinal uptake process is regulated by the level of the vitamin in the diet (and/or body stores) and that the regulation is mediated via changes in the number (and/or activity) of the RF uptake carriers with no change in their affinity.