Abstract
Riboflavin (Vitamin B_2) is essential for the normal cellular functions. From 1960's, many in vivo and in vitro studies have indicated that riboflavin transporter (s) plays important roles in the absorption, distribution, excretion and reabsorption. Recently, we identified novel mammalian riboflavin transporter RFT1 using our rat kidney mRNA expression database. Human RFT1 consists of 448 amino acids and was predicted to have 10 putative membrane-spanning domains. Subsequently, its homolog RFT2 was identified and showed 42.9% identity with RFT1. It has 11 putative membrane-spanning domains. RFT1 is mainly expressed in the placenta and small intestine, and RFT2 in the testis and small intestine, Transfection of RFT1 or RFT2 into culture cells increased the cellular uptake of riboflavin. Riboflavin transport by RFT1 and RFT2 was independent of Na^+. RFT2 was extracellular pH-sensitive, although RFT1 was not. Further studies will clarify the physiological roles of RFT1 and RFT2 in riboflavin homeostasis.
