RIBOFLAVIN-BINDING PROTEIN INDUCES EARLY DEATH OF CHICKEN EMBRYOS

Maternal riboflavin-binding protein (RfBP) mediates the deposition of riboflavin in avian eggs. A strain of Single-Comb White Leghorn chicke n, genetically unable to synthesize RfBP, produces eggs deficient in r iboflavin. Embryos in these eggs die on or about 13 of incubation, wit h severe hypoglycemia and impaired fatty acid oxidation. Injection of free riboflavin prior to incubation allows these embryos to survive an d hatch normally. The egg whites of eggs from the RfBP-deficient strai n were injected with different relative amounts of riboflavin and RfBP to determine their effects on the developing embryo. Injection of pro tein-bound riboflavin resulted in a substantial number of the embryos (27%) surviving until the end of the experiment (d 19) with an average weight of 13.7 g, demonstrating that some embryos were able to use th e bound riboflavin. In control groups injected with riboflavin alone, 10 of 17 embryos survived until d 19 with an average weight 18.5 g, wh ereas those injected with saline died between d 10 and 13 with an aver age weight of 3.8 g. A fourth group, injected with apo-RfBP, produced embryos with an average weight of 0.24 g. This early death suggests th at the protein scavenges the small amount of riboflavin normally avail able to the early embryo. Further studies showed that titration of RfB P-binding sites with exogenous riboflavin lessens this effect. These s tudies show that although riboflavin-binding protein is essential for the deposition of riboflavin in eggs, its presence in excess can dimin ish the availability of riboflavin to the chicken embryo.