Chemical modifications of riboflavin-binding protein: effects on function and antigenicity

A comparison of two dissimilar monoclonal antibodies that are directed to a common epitope in the reduced and carboxymethylated chicken riboflavin carrier protein

Two monoclonal antibodies, D2A1 and D5G3 were elicited by immunization with a preparation of chicken egg riboflavin carrier protein which had been reduced and carboxymethylated. Epitopes recognised by the monoclonal antibodies were mapped using the Pepscan method. Epitopic determinants for D2A1 as well as D5G3 were identified within a region spanning 13 amino acids (residues 170-182) in the primary sequence of riboflavin carrier protein. Interestingly, these monoclonal antibodies, despite sharing a common epitope exhibited a marked difference in their binding to native (folded) riboflavin carrier protein versus reduced carboxymethylated (unfolded) riboflavin carrier protein. Both monoclonal antibodies bound reduced carboxymethylated riboflavin carrier protein to comparable extents in solid phase (ELISA and immunoblots) and liquid phase (radioimmunoassay) assays. However, while D5G3 could bind native riboflavin carrier protein in solid and liquid phase assays, D2A1 showed negligible binding to the native structure. Alanine substituted peptide analogs of the epitope in question defined the crucial amino acids of the epitope needed for binding to the two antibodies.

A monoclonal antibody recognizing the C-terminal region of chicken egg white riboflavin carrier protein terminates early pregnancy in mice

In order to identify the functionally relevant epitopes on chicken riboflavin carrier protein, we have raised monoclonal antibodies to the vitamin carrier. One of these, 6B2C12, was found to interact specifically with a synthetic oligopeptide corresponding to the C-terminal 17 amino acid residues of the chicken egg white riboflavin carrier protein, which is missing in part in the egg yolk riboflavin carrier protein. This epitope is conserved through evolution in mammals including humans. Administration of the ascites fluid of 6B2C12 to pregnant mice intraperitoneally, resulted in the termination of pregnancy indicating that this epitope is involved in or closely associated with the transplacental transport of the vitamin from the maternal circulation to the growing fetus.

Interaction of riboflavin binding protein with riboflavin, quinacrine, chlorpromazine and daunomycin

1. circular dichroism and fluorescence measurements were used to study the reversible unfolding of riboflavin-riboflavin binding protein complex. Both methods showed that the complex was unfolded according to the two-state model. 2. The results suggest that riboflavin was strongly bound in the hydrophobic cleft of the protein and that it could not be dislodged by TFE, MeOH or SDS without major unfolding of the unique tertiary structure of the protein. 3. In addition, it has been also shown that quinacrine, chlorpromazine and daunomycin did not form stereospecific complexes with riboflavin binding protein.

Immunological characterization of riboflavin carrier proteins using monoclonal antibodies

Monoclonal antibodies to chicken riboflavin carrier protein have been produced by fusing immunized mouse spleen cells with myeloma SP2/O-Ag 14. The three different monoclonal antibodies specifically bound 125I-labelled chicken riboflavin carrier protein and were characterized with respect to their affinities to bind the antigen, subclass and isotype. These three monoclonal antibodies had similar affinities for holo-, apo- and SDS-denatured riboflavin carrier protein but were unable to recognize the reduced and carboxymethylated protein indicating that they were directed to specific conformational epitopes on the native avian protein. Succinylation of the vitamin carrier protein while still retaining flavin binding characteristics totally abolished the cross-reactivity with all the three monoclonal antibodies indicating that lysine residues were involved at the antigenic sites of the protein. This shows that the antigenic loci may be distinct from the flavin binding sites in the protein. All three antibodies were able to recognize riboflavin carrier protein present in the sera of pregnant rats, monkeys and humans indicating that the epitopes to which they are directed are conserved throughout evolution. These antibodies can therefore be effectively used for radioimmunoassays and further studies on the functional aspects of this protein in higher mammals.

Studies on the modification of Escherichia coli ribosomal protein L7/L12 by succinic anhydride

Lysine modification by increasing quantities of succinic anhydride in the Escherichia coli ribosomal protein L7/L12 produces loss of its ability in reconstitution of elongation-factor-G-dependent GTP hydrolysis and polyphenylalanine synthesis activities, showing lower antigenicity and loss of antigenic determinants.

The role of oligosaccharide in transport of egg yolk riboflavin-binding protein to the egg

The carbohydrate portion of chicken egg yolk riboflavin-binding protein was examined to determine its role in the biological activity of the protein. Yolk RBP was found to contain 5--6 mannose, five galactose, 12 N-acetylglucosamine and four sialic acid residues. Specific modifications of the oligosaccharide moiety were performed which included removal of sialic acid by mild acid hydrolysis, oxidation of galactose by galactose oxidase, and removal of N-acetylglucosamine and galactose residues by a mixture of glycosidases from Aspergillus niger. All of the modified proteins retained the ability to bind riboflavin although their capacities were lower than that of native yolk RBP. Circular dichroism of the modified yolk RBP samples showed changes in the near ultraviolet, but molar ellipticities in the far ultraviolet displayed only minor variations indicating no gross structural changes. All samples cross-reacted with RBP-specific antiserum. The plasma half-life of 125I-labeled yolk RBP was 62 min. Each of the modified samples was cleared more rapidly from the blood than native yolk RBP. Removal of sialic acid decreased the half-life of yolk RBP by 31%, while the other modifications decreased the half-life by as much as 60%. During a 10-day period following injection of 125I-labeled yolk RBP, 5.9% of the labeled protein was recovered from egg yolk. Relative to native yolk RBP, the transport of asialo-yolk RBP was decreased by 82%. The other modifications resulted in even less transport to the egg, the lowest being glycosidase-treated asialo-yolk RBP which was decreased by over 99%. By comparison of samples with similar clearance times, a positive correlation was made between sialic acid and ovarian transport.

Avian riboflavinuria--XI. Immunological quantitation of cross-reacting liver proteins from normal, heterozygous, and mutant hens

A genetic defect, avian riboflavinuria, was discovered in a strain of Single Comb White Leghorn chickens and has been attributed to the absence of functional riboflavin-binding protein (RBP). The ratio of functional RBP in blood, egg yolk, and egg white was 2 (RdRd) : 1 (Rdrd) : 0 (rdrd). The present study on non-riboflavin-binding, cross-reacting proteins (CRPs) from RdRd, Rdrd, and rdrd hens involved partial purification and immunochemical quantitation using antiserum to RBP. Immunoreactivities (microgram/g liver) of CRPs were found to be 2.6 (RdRd) : 1.3 (Rdrd) : 0.02 (rdrd). Reciprocal cross-reactions were observed with rabbit sera directed toward both RdRd CRP and RBP. Reaction of the CRPs with antiglycopeptide serum (specific for beta-linked galactose) showed that they were glycosylated. CRPs from RdRd and Rdrd hens had relative antigenicities similar to that of RBP (Krel approximately or equal to 1), while rdrd CRP had a significantly lower antigenicity (Krel = 0.003). The molecular weights of the CRPs as determined by SDS-polyacrylamide gel electrophoresis were as follows: RdRd = 31,6000, Rdrd = 30,900, and rdrd = 27,500. The molecular weight of egg yolk RBP was 34,7000 by the same method. The conclusion is drawn that the rd gene codes for a nonfunctional mutant protein, possibly an "altered precursor", that is different from RBP.