Requirement of sequential hydrolysis by CD73 and ALP for uptake of vitamin B₂ into cells

Abstract

Extracellular hydrolysis of flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) to riboflavin is thought to be important for cellular uptake of vitamin B2 because FAD and FMN are hydrophilic and do not pass the plasma membrane. However, it is not clear whether FAD and FMN are hydrolyzed by cell surface enzymes for vitamin B2 uptake. Here, we show that in human cells, FAD, a major form of vitamin B2 in plasma, is hydrolyzed by CD73 (also called ecto-5′ nucleotidase) to FMN, then FMN is hydrolyzed by alkaline phosphatase to riboflavin, which is efficiently imported into cells. This process is impaired on the surface of glycosylphosphatidylinositol (GPI)-deficient cells due to lack of these GPI-anchored enzymes. During culture of GPI-deficient cells with FAD or FMN, hydrolysis of these forms of vitamin B2, intracellular levels of vitamin B2, vitamin B2-dependent pyridoxal 5′-phosphate formation, and mitochondrial functions were significantly decreased compared with those in GPI-restored cells. These results suggest that inefficient uptake of vitamin B2 might account for mitochondrial dysfunction seen in some cases of inherited GPI deficiency.