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 3 are complex glycoproteins having more than twice as many N-acetylglucosamine residues (> 12) as mannose residues (.apprx. 6). Egg white RBP is distinctive in having only 9 sialic acid and 2 galactose residues. Serum RBP contains .apprx. 5 sialic acid and 7 galactose residues. In addition there is 1 residue of fucose. The carbohydrate composition of yolk RBP indicates the hydrolysis, respectively, of 1, 1, 2 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. Two hours after i.v. 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 [half-life] = 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). Sixteen hours 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. No evidence was found for carbohydrate-directed uptake of RBP by the ovarian follicle.