Current perspectives on the cellular uptake and trafficking of riboflavin. 
The role of riboflavin in cell maintenance and growth, and the mechanism by which it is absorbed into various human tissues and cell lines has been extensively studied over the past decade.  Evidence suggests two absorption mechanisms, a saturable-active component that dominates at near physiological vitamin concentrations and a passive component that is revealed at oversupplemented riboflavin conditions.  Various transport modulator studies consistently suggest a highly riboflavin specific, temperature-dependent active transport mechanism that is regulated by the Ca2+/calmodulin pathway.  The PKA and PKG pathways have also been implicated in absorption regulation.  The long-standing model that riboflavin absorption involves a carrier-mediated transporter has recently been challenged through studies suggesting a receptor-mediated endocytic component.  The presence of a soluble, human riboflavin binding protein in the transport stratagem has been shown to play an important role in fetal development.  The relationship of this binding protein with the riboflavin specific membrane bound protein, though currently not well defined, may involve a protein-protein interaction that plays a primary role in this proposed receptor-mediated component.