Carbohydrate compositional effects on tissue distribution of chicken riboflavin-binding protein

Riboflavin-binding proteins (RBP) purified from chicken egg white, yolk and the serum of laying hens differ in their carbohydrate compositions reflecting tissue-specific modifications of a single gene product. All three are complex glycoproteins having more than twice as many N-acetylglucosamine residues (greater than 12) as mannose residues (approx. 6). Egg white RBP is distinctive in having only one sialic acid and two galactose residues. Serum RBP contains approx. five sialic acid and seven galactose residues. In addition there is one residue of fucose. The carbohydrate composition of yolk RBP indicated the hydrolysis, respectively, of one, one, two and 3 residues of sialic acid, fucose, galactose, and N-acetylglucosamine from its precursor, serum RBP. The effect of these differing levels of glycosylation on plasma clearance, ovarian uptake and tissue distribution of 125I-labeled riboflavin-binding proteins in laying hens were compared. 2 h after intravenous injection, 19% of the egg white RBP, 29% of the yolk RBP, and 37% of the serum RBP remained in circulation. The kinetics of plasma clearance was distinctly biphasic for each of the radioiodinated proteins. The initial rapid-turnover component (t1/2 = 13 min) ranged from 27% of the serum RBP sample to 48% of the egg white RBP sample. The remaining slow-turnover components were cleared with half-lives of 81 min (egg white RBP), 101 min (yolk RBP), and 121 min (serum RBP). 16 h after injection, only 4% of the egg white RBP was deposited in the yolk of developing oocytes while about 12% of the serum RBP and yolk RBP was deposited. This highly significant difference is apparently due to preferential, carbohydrate-dependent clearance of egg white RBP by the liver rather than preferential uptake of serum and yolk RBP by the ovarian follicle. We find no evidence for carbohydrate-directed uptake of riboflavin-binding protein by the ovarian follicle.