The nature of the biochemical lesion in avian renal riboflavinuria, IV. Comparison of the riboflavin-binding protein from homozygous (RdRd) and heterozygous (Rdrd) white leghorn albumen

Riboflavin binding proteins and flavin assimilation in insects

Recent studies on developmentally regulated hemolymph proteins in insects have shown that two proteins, a lipoprotein and a member of a hexamerin gene family, bind riboflavin. The biosynthesis, developmental regulation, and properties of these proteins are described and compared with the riboflavin-binding proteins and flavin distributions in vertebrates. The importance of riboflavin-binding proteins in insect development is discussed in relation to existing information and avenues for future research are presented.

Effect of riboflavin-binding protein deficiency on riboflavin metabolism in the laying hen

Normal chicken eggs contain substantial amounts of riboflavin, all of which is bound to a specific, high-affinity, riboflavin-binding protein (RfBP). Two hens, genetically unable to produce RfBP and thus unable to deposit sufficient riboflavin in their eggs, were compared to two normal hens with respect to the biological half-life of [14C]riboflavin, the tissue distribution of 14C-labeled flavins, and the relative contributions of tissue and dietary riboflavin to flavins deposited in the egg. The biological half-life of [14C]riboflavin was slightly but insignificantly less in the RfBP-deficient hens (11.5 +/- 1.7 days vs 15.1 +/- 3.3 days). The 14C-labeled flavin content of a variety of tissues 3 weeks after the intraperitoneal injection of 5 microCi of riboflavin was also very similar among the four hens. In contrast, the 14C-labeled flavin content of egg yolk, egg albumen, and blood plasma from RfBP-deficient birds was less than 10% of normal. For all hens, the specific radioactivity of flavins in yolk and albumen was similar to that in liver but less than that in heart. We conclude that riboflavin deposited in egg had equilibrated with the large hepatic flavin pool and was not derived preferentially from unlabeled dietary riboflavin. Other than the inability to deposit riboflavin in their eggs, hens of the mutant strain have normal riboflavin metabolism.

Solid phase enzyme-linked competitive binding assay for riboflavin

A new solid-phase enzyme-linked assay for riboflavin (vitamin B2) is described. The assay is based on the competition between analyte vitamin molecules and a glucose-6-phosphate dehydrogenase-3-carboxymethylriboflavin conjugate for a limited number of riboflavin-binding protein sites immobilized on Sepharose particles. Significant improvements in conjugate catalytic activity and thus detectability are achieved by optimizing the reaction conditions used to covalently link 3-carboxymethylriboflavin to the enzyme. Optimization experiments include studying the effects of reaction pH and organic solvent composition. Final assay detection limits and the sensitivity of the dose-response curves are dependent on the ratio of conjugate to binding protein sites utilized in an equilibrium assay protocol. Selectivity of the method correlates well with that predicted based on the known association constants of riboflavin-binding protein with flavin analogs. The assay is shown to offer adequate detection limits and selectivity for direct measurement of riboflavin in urine, infant formula, and vitamin capsules.

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.