Studies on the riboflavin-binding capacity of the rat lens.

Previous studies have confirmed that riboflavin-binding protein exists in the rat lens. We now provide evidence that riboflavin-binding was the same at 4 degrees C and 37 degrees C, that there was a saturation curve when 14C-riboflavin was the substrate, that there was an ATP-dependent increase of binding, and that EGTA and PCMB did not affect riboflavin binding while it was decreased upon protease digestion and upon the addition of heavy metal compounds. Furthermore, binding was lower in the lenses of rats which had been fed a B2-deficient diet for 8 weeks than in animals receiving this diet for 4 weeks. When 14C-B2 butyrate rather than 14C-riboflavin was the substrate, binding was considerably higher in the lens homogenate and the single whole lens. Partial purification of riboflavin-binding protein using flavinyl agar-bead affinity chromatography confirmed the presence of a trace amount of riboflavin-binding protein in the bovine lens. Our results suggest that changes in the riboflavin-binding capacity of the lens may play a role in the regulation of absorption, transport and metabolism of riboflavin in the lens associated with the synthesis of ester forms of riboflavin.