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

Abstract

Possible regulation of riboflavin (RF) intestinal uptake process by dietary substrate level is not known and was examined in this study. RF uptake was examined using intestinal brush border membrane vesicles (BBMV). 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. 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.