Effect of riboflavin-binding protein deficiency on riboflavin metabolism in the laying hen

Normal chicken eggs contain substantial amounts of riboflavin, all of which is bound to a specific, high-affinity, riboflavin-binding protein (RfBP). Two hens, genetically unable to produce RfBP and thus unable to deposit sufficient riboflavin in their eggs, were compared to two normal hens with respect to the biological half-life of [¹⁴C]riboflavin, the tissue distribution of ¹⁴C-labeled flavins, and the relative contributions of tissue and dietary riboflavin to flavins deposited in the egg. The biological half-life of [¹⁴C]riboflavin was slightly but insignificantly less in the RfBP-deficient hens (11.5 ± 1.7 days vs 15.1 ± 3.3 days). The ¹⁴C-labeled flavin content of a variety of tissues 3 weeks after the intraperitoneal injection of 5 μCi of riboflavin was also very similar among the four hens. In contrast, the ¹⁴C-labeled flavin content of egg yolk, egg albumen, and blood plasma from RfBP-deficient birds was <10% of normal. For all hens, the specific radioactivity of flavins in yolk and albumen was similar to that in liver but less than that in heart. We conclude that riboflavin deposited in egg had equilibrated with the large hepatic flavin pool and was not derived preferentially from unlabeled dietary riboflavin. Other than the inability to deposit riboflavin in their eggs, hens of the mutant strain have normal riboflavin metabolism.