Uptake of Riboflavin by the Human Red Blood Cells

Abstract

The mechanism of riboflavin uptake by the human red blood cells was investigated in vitro. The cells were incubated in the physiological saline con taining riboflavin (FR). The uptake of FR showed plateau within 30 min during the time course of the uptake. Apparently the rate of the uptake showed temperature dependency, but no saturation kinetics was found. It was revealed that approximately 75% of ^<14>C-riboflavin transported was converted to FMN and/or FAD within 15 min after incubation and this ratio of FMN plus FAD remained constant until 60 min of the incubation. This result, together with the temperature dependency of the uptake, suggests that two enzymes, flavokinase and FAD pyrophosphorylase, may participate in the uptake of riboflavin. This assumption was supported by the fact that these enzymes were detected in the cytoplasm but not in the erythrocyte membrane fraction. The localization of these enzyme activities also ruled out a possibility of the involvement of group translocation in riboflavin uptake by the red cells. These results suggest that riboflavin passes through the erythrocyte membrane without conformational change and then metabolized to FMN and FAD in the cytoplasm.