Biochemical and immunological aspects of riboflavin carrier protein.
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Date
1988-03
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Abstract
Riboflavin carrier protein which is obligatorily involved in yolk deposition of the
vitamin in the chicken egg, is a unique glycophosphoprotein present in both the yolk and
white compartments. The yolk and egg white proteins are products of a single estrogeninducible
gene expressed in the liver and the oviduct respectively of egg laying birds. Despite
the fact that the carbohydrate composition of the yolk and white riboflavin carrier proteins
differ presumably due to differential post-translational modification, the proteins are
immunologically similar and have identical amino acid sequence (including a cluster of 8
phosphoser residues towards the C-terminus) except at the carboxy terminus where the yolk
riboflavin carrier protein lacks 13 amino acids as a consequence of proteolytic cleavage
during uptake by oocytes. The protein is highly conserved throughout evolution all the way
to humans in terms of gross molecular characteristics such as molecular weight and
isoelectric point, and in immunological properties, preferential affinity for free riboflavin
and estrogen inducibility at the biosynthetic locus viz., liver. Obligatory involvement of the
mammalian riboflavin carrier protein in transplacental flavin transport to subserve fetal
vitamin nutrition during gestation is revealed by experiments using pregnant rodent or subhuman
primate models wherein immunoneutralisation of endogenous maternal riboflavin
carrier protein results in fetal wastage followed by pregnancy termination due to selective
yet drastic curtailment of vitamin efflux into the fetoplacental unit. Using monoclonal
antibodies to chicken riboflavin carrier protein, it could be shown that all the major
epitopes of the avian riboflavin carrier protein are highly conserved throughout evolution
although the relative affinities of some of the epitopes for different monoclonal antibodies
have undergone progressive changes during evolution. Using these monoclonal antibodies,
an attempt is being made to map the different epitopes on the riboflavin carrier protein
molecule with a view to delineate the immunodominant regions of the vitamin carrier to
understand its structure-immunogenicity relationship.
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Keywords
Riboflavin carrier protein, evolutionary conservation, transplacental transport, immunoneutralisation, monoclonal antibodies, epitope analysis
Citation
Adiga P R, Visweswariah S S, Karande A, Kuzhandhaivelu N. Biochemical and immunological aspects of riboflavin carrier protein. Journal of Biosciences. 1988 Mar; 13(1): 87-104.