Identification and Functional Characterization of Rat Riboflavin Transporter 2

Abstract

We have newly identified rat riboflavin transporter 2 (rRFT2) and its human orthologue (hRFT2), and carried out detailed functional characterization of rRFT2. The mRNA of rRFT2 was highly expressed in jejunum and ileum. When transiently expressed in human embryonic kidney (HEK) 293 cells, rRFT2 could transport riboflavin efficiently. Riboflavin transport mediated by rRFT2 was Na⁺-independent but moderately pH-sensitive, being more efficient in acidic conditions than in neutral and basic conditions. Kinetic analysis indicated that rRFT2-mediated riboflavin transport was saturable with a Michaelis constant (K_m) of 0.21 μM. Furthermore, it was specifically and strongly inhibited by lumiflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), and to a lesser extent by amiloride. Such ability to transport riboflavin in a specific manner, together with its high expression in the small intestine, indicates that RFT2 may play a role in the intestinal absorption of riboflavin.