Riboflavin Homeostasis in the Central Nervous System

The mechanisms by which riboflavin, which is not synthesized in mammals, enters and leaves brain, CSF, and choroid plexus were investigated by injecting [ 14 C]riboflavin intravenously or intraventricularly. Tracer amounts of [ 14 C]riboflavin with or without FMN were infused intravenously at a constant rate into normal, starved, or probenecid‐pretreated rabbits. At 3 h, [ 14 C]riboflavin readily entered choroid plexus and brain, and, to a much lesser extent, CSF. Over 85% of the [ 14 C]riboflavin in brain and choroid plexus was present as [ 14 C]FMN and [ 14 C]FAD. The addition of 0.2 mmol/kg FMN to the infusate markedly depressed the relative entry of [ 14 C]riboflavin into brain, choroid plexus, and, less so, CSF, whereas starvation increased the relative entry of [ 14 C]riboflavin into brain and choroid plexus. After intraventricular injection (2 h), most of the [ 14 C]riboflavin was extremely rapidly cleared from CSF into blood. Some of the [ 14 C]riboflavin entered brain, where over 85% of the 14 C was present as [ 14 C]FMN plus [ 14 C]FAD. The addition of 1.2 3 μmol FAD (which was rapidly hydrolyzed to riboflavin) to the injectate decreased the clearance of [ 14 C]riboflavin from CSF and the phosphorylation of [ 14 C]riboflavin in brain. Probenecid in the injectate also decreased the clearance of [ 14 C]riboflavin from CSF. These results show that the control of entry and exit of riboflavin is the mechanism, at least in part, by which total riboflavin levels in brain cells and CSF are regulated. Penetration of riboflavin through the blood‐brain barrier, saturable efflux of riboflavin from CSF, and saturable entry of riboflavin into brain cells are three distinct parts of the homeostatic system for total riboflavin in the central nervous system.