The carbohydrates of the isoforms of three avian riboflavin-binding proteins

The carbohydrate chains of nine isoforms of chicken egg-white riboflavin-bi nding protein (RfBP) and six isoforms each of quail egg-white and yolk RfBP have been structurally characterized. The two N-glycosylation sites, Asn36 and Asn147, of the most abundant isoform of each of the three proteins wer e analyzed in further detail leading to the identification of different gly cosylation patterns. In both chicken and quail egg-white RfBP the carbohydr ates attached to position 36 had a lower degree of branching and, in the ca se of the quail protein, this site was only partially glycosylated. A very heterogeneous mixture of complex structures was characteristic of the other glycosylation site. Analysis of the two Sites in quail yolk RfBP confirmed this result which agrees with what has been established for hen yolk RfBP. The presence in the three proteins of a highly heterogeneous mixture of di fferently branched glycans suggests that the differences in isoelectric poi nts, which is a peculiarity of the different isoforms, are probably indeed due to differences in carbohydrate structure.