Riboflavin metabolism in the single lens of rat

To study the mode of riboflavin metabolism in the lens in vivo, the uptake of 14C-labeled riboflavin and synthesis of ester forms from 14C-riboflavin were assayed in the single whole lens of the rats. 14C-riboflavin uptake increased in proportion to the substrate concentration in the incubation medium, reaching a plateau within 60 min. At 4 degrees C the uptake was lowest, at 30, 35 and 40 degrees C it was similar. Neither uncoupler (2,4-dinitrophenol and monoiodoacetate) nor ouabain inhibited the uptake. Within 15 min, approximately 35% of transported 14C-riboflavin was converted to ester forms of riboflavin; this ratio remained constant until 120 min of incubation. Characteristically, 2-3% of 14C-riboflavin was bound to lens protein. 2,4-Dinitrophenol or monoiodoacetate inhibited the synthesis of ester forms of riboflavin, but ouabain did not. No synthesis of ester forms of riboflavin was observed in the lens capsule with epithelium. These results suggest that two enzymes, flavokinase and flavin adenine dinucleotide (FAD) pyrophosphorylase which convert riboflavin to flavin mononucleotide and FAD, respectively, exist in the lens of the rat. These enzymes may participate in riboflavin uptake and riboflavin may pass through the lens capsule by simple diffusion without undergoing a conformational change and then may be metabolized to ester forms of riboflavin with subsequent partial binding to lens protein.