Occurrence and functional importance of a riboflavin-carrier protein in the pregnant rat

Absence of retbindin blocks glycolytic flux, disrupts metabolic homeostasis, and leads to photoreceptor degeneration

We previously reported a model of progressive retinal degeneration resulting from the knockout of the retina-specific riboflavin binding protein, retbindin (Rtbdn ^-/- ). We also demonstrated a reduction in neural retinal flavins as a result of the elimination of RTBDN. Given the role of flavins in metabolism, herein we investigated the underlying mechanism of this retinal degeneration by performing metabolomic analyses on predegeneration at postnatal day (P) 45 and at the onset of functional degeneration in the P120 retinas. Metabolomics of hydrophilic metabolites revealed that individual glycolytic products accumulated in the P45 Rtbdn ^-/- neural retinas along with the elevation of pentose phosphate pathway, while TCA cycle intermediates remained unchanged. This was confirmed by using ¹³C-labeled flux measurements and immunoblotting, revealing that the key regulatory step of phosphoenolpyruvate to pyruvate was inhibited via down-regulation of the tetrameric pyruvate kinase M2 (PKM2). Separate metabolite assessments revealed that almost all intermediates of acylcarnitine fatty acid oxidation, ceramides, sphingomyelins, and multiple toxic metabolites were significantly elevated in the predegeneration Rtbdn ^-/- neural retina. Our data show that lack of RTBDN, and hence reduction in flavins, forced the neural retina into repurposing glucose for free-radical mitigation over ATP production. However, such sustained metabolic reprogramming resulted in an eventual metabolic collapse leading to neurodegeneration.

Flavins Act as a Critical Liaison Between Metabolic Homeostasis and Oxidative Stress in the Retina

Derivatives of the vitamin riboflavin, FAD and FMN, are essential cofactors in a multitude of bio-energetic reactions, indispensable for lipid metabolism and also are requisites in mitigating oxidative stress. Given that a balance between all these processes contributes to the maintenance of retinal homeostasis, effective regulation of riboflavin levels in the retina is paramount. However, various genetic and dietary factors have brought to fore pathological conditions that co-occur with a suboptimal level of flavins in the retina. Our focus in this review is to, comprehensively summarize all the possible metabolic and oxidative reactions which have been implicated in various retinal pathologies and to highlight the contribution flavins may have played in these. Recent research has found a sensitive method of measuring flavins in both diseased and healthy retina, presence of a novel flavin binding protein exclusively expressed in the retina, and the presence of flavin specific transporters in both the inner and outer blood-retina barriers. In light of these exciting findings, it is even more imperative to shift our focus on how the retina regulates its flavin homeostasis and what happens when this is disrupted.

Comparative Quantitative Phosphoproteomic Analysis of the Chicken Egg during Incubation Based on Tandem Mass Tag Labeling

Protein phosphorylation plays an important role in protein structure and function. To investigate the role of egg protein phosphorylation in chicken embryonic development, a comparative and quantitative phosphoproteomic analysis of fertilized chicken egg white and yolk was performed during incubation. Overall, 215 phosphosites mapped onto 205 phosphopeptides corresponding to 100 phosphoproteins were identified. Among these phosphoproteins, 123 phosphosites from 62 egg proteins were found significantly changed (p < 0.05) at day 12 during incubation. Furthermore, GO analysis suggested that these differentially phosphorylated proteins were associated with various molecular functions, primarily including binding, molecular function regulator, and transport activity. Such findings in this study improved our understanding of the protein molecular functions involved in chicken embryonic development from a protein phosphorylation perspective.

Vitamin Carrier Proteins During Embryonic Development in Birds and Mammals

Egg maturation in oviparous vertebrates involves the hepatic synthesis, secretion, and deposition in the developing oocyte of several maternal proteins with specific nutrient carrier function. Thus, in the chicken, adequate yolk deposition of riboflavin, thiamin, etc. is obligatorily mediated by carrier proteins specific to each vitamin. Like vitellogenin, these are oestrogen-inducible specific gene products. Despite differences in patterns of embryonic development in mammals vis-à-vis oviparous species, immunologically and biochemically similar maternal vitamin carriers participate in the transplacental transport and fetal accumulation of these vitamins during gestation in the rat. The rodent riboflavin and thiamin carrier proteins are also oestrogen-induced maternal proteins of hepatic origin. Their functional importance in fetal development was established by in vivo passive immunoneutralization of the endogenous proteins, which precipitated fetal wastage leading to pregnancy termination, due to curtailment of the vitamin supply to the fetuses. Similarly, active immunization of female rats with the vitamin carrier proteins led to early fetal resorption without interference with maternal health, cyclicity and fecundity. The discovery of similar gestation-specific carrier proteins in higher mammals and humans suggests that carrier-mediated vitamin delivery mechanisms ensuring embryonic growth have been conserved during evolution.

Recognition, Cointernalization, and Recycling of an Avian Riboflavin Carrier Protein in Human Placental Trophoblasts

Absorption of riboflavin (RF) across membrane barriers is essential to cellular oxidation reduction processes. Riboflavin carrier protein (RCP), a 37-kDa secretory protein, is proposed to play an important role in RF absorption, although information on the mammalian ortholog remains unclear. This study alludes to the existence of a mammalian RF carrier protein and further characterizes its carrier role and fate using avian RCP in human placental trophoblast (BeWo), another mammalian cell line, monkey kidney (COS-1), and the avian control, chicken hepatic (LMH/2A) cells. The presence of RCP and its involvement in RF internalization was analyzed by immunofluorescence and immunobinding assays using chicken RCP (cRCP) antibodies. In the presence of anti-cRCP, cellular RF uptake is significantly decreased (5% of control) in BeWo cells. Kinetic analyses of intracellular accumulation of (125)I-cRCP revealed a J(max) and K(m) of 28.56 +/- 2.70 pmol/mg protein/min and 142.43 +/- 82.16 nM, respectively, in BeWo cells and 75.14 +/- 7.6 pmol/mg protein/min and 104.37 +/- 23.96 nM in the species-specific control, LMH/2A cells. Subcellular fractionation studies revealed colocalization of both radiolabeled RF and cRCP within endosomal and lysosomal fractions, further elucidating RCP's role in trafficking RF through the cell. Following intracellular release of RF from the carrier complex, the protein is either subject to lysosomal breakdown or is conserved via recycling mechanisms for continued RF sequestration and uptake. In summary, mammalian placental trophoblasts exhibit specific carrier protein dependence that sequesters and essentially mediates RF internalization via the proposed receptor-mediated endocytic pathway.

Status of immunodiagnosis and immunocontraceptive vaccines in India

The article focuses on the Indian initiative of making kits for diagnosis of various infectious and non-infectious diseases as well as reproductive hormones and hormones in various other endocrine disorders. Indigenous diagnostic kits for the detection of various infections such as filariasis, typhoid, amebiasis, Japanese encephalitis, hepatitis, HIV, dengue, leishmaniasis, malaria, rabies, toxoplasmosis, rotavirus, and group A streptococci have been developed. Agreements to transfer the know-how of some of these leads to industries have been signed. The know-how of enzyme-linked immunosorbent assay (ELISA) for detection of hepatitis C has been successfully transferred to industry and is being commercially produced. For detection of HIV-1 and HIV-2, indigenous diagnostic kits based on three different formats, namely ELISA, Western blot and rapid test have been developed and are being commercially produced by Indian industries. The factors influencing the successful transfer of laboratory-scale diagnostic assays from academia to industry and their commercial exploitation have been discussed. Indian scientists have made seminal contributions in exploring the possibility to develop an effective and safe contraceptive vaccine to control the increasing human population of India. Achieving contraception by means of vaccine is a novel approach, which entails generation of a specific antibody response against antigens critically involved in the process of mammalian reproduction. In India, three major programs on contraceptive vaccines based on the beta-subunit of human chorionic gonadotrophin ((beta)hCG) for women, ovine follicle stimulating hormone (oFSH) for men, and riboflavin carrier protein for both males and females have been initiated. The work at the National Institute of Immunology, New Delhi on contraceptive vaccine for women, based on (beta)hCG, has demonstrated, for the first time, that it is feasible to regulate fertility by such an approach. Basic research being carried out to achieve immunocontraception by interfering at sperm-oocyte interaction level has been briefly discussed. These developments are still at the research stage. In addition to advances in the area of contraceptive vaccines, a non-steroidal contraceptive oral pill has been developed by Central Drug Research Institute, Lucknow, commercially produced by two Indian pharmaceutical companies and has been incorporated in the National Family Welfare Program. Another interesting approach for fertility regulation in male has been developed in India, which involves vas occlusion with styrene maleic anhydride (SMA) and is currently undergoing clinical trials in human subjects.

Chromatographic investigation on the binding site characteristics of quail egg-white riboflavin binding protein as a chiral stationary phase

Recently we described the use of riboflavin binding protein extracted from quail egg-white, as a new HPLC chiral stationary phase. In this study we show the further results obtained with the use of high-performance affinity chromatography to provide a better understanding of the chiral recognition mechanism for the observed enantioselectivity and to gain a deeper knowledge into the binding site that has been recently characterised by X-ray crystallography for chicken egg-white. High-performance affinity chromatography provides information on the potential protein structural changes occurring upon its immobilisation and enables competitive binding studies as well as the assessment of binding constants through frontal analysis experiments.

Immunological relatedness of chicken and human riboflavin carrier protein

Polyclonal antibodies (pAbs) to riboflavin carrier protein (RCP) purified from human amniotic fluid were raised in two rabbits. These pAbs apart from recognizing the parent antigen, reacted significantly with chicken RCP as well. When all the 212 octapeptides overlapping by one amino acid residue and covering the entire primary amino acid sequence of chicken RCP immobilized on polyethylene rods were probed with rabbit anti-human RCP antibodies, it was found that these antibodies recognize five epitopic sequences (residues 37-42, 73-76, 133-140, 174-176 and 200-207) in chicken RCP primary structure common to both the rabbits. In addition, the antisera from the two individual animals also recognized additional epitopes characteristic to that particular animal. The reactivities of these antigenic determinants have been confirmed with polyclonal anti-human RCP sera by using synthetic peptides harbouring these chicken RCP sequences in inhibition ELISA. This high level of immunological similarities among linear epitopes observed may reflect the degree of conservation of local conformations among linear antigenic determinants shared by the two evolutionarily distant homologous proteins.

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

The carbohydrate chains of nine isoforms of chicken egg-white riboflavin-binding 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 were analyzed in further detail leading to the identification of different glycosylation patterns. In both chicken and quail egg-white RfBP the carbohydrates attached to position 36 had a lower degree of branching and, in the case 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 differently branched glycans suggests that the differences in isoelectric points, which is a peculiarity of the different isoforms, are probably indeed due to differences in carbohydrate structure.

Identification and mapping of linear antigenic determinants of chicken riboflavin carrier protein

All of the possible 212 overlapping octapeptides covering the amino acid sequence of chicken egg white riboflavin carrier protein (RCP) was synthesized on polyethylene rods. To identify the peptides constituting the sequential antigenic epitopes on chicken RCP, a PEPSCAN ELISA was employed where antibodies raised against SDS-treated, reduced and carboxymethylated RCP (RCM-RCP) in mice, rats (n = 4), rabbits and monkeys (n = 3) were used. By this approach, we could identify five sequences on chicken RCP commonly recognized by all the antisera from these species which were spanning the residues 10-17, 42-49, 134-141, 172-179 and 200-207. Analysis of the data for each individual antiserum indicated inter- and intra-species variations with N- and C-terminal shifts in peak reactivity. The present study offers clues for development of potential immunocontraceptive vaccines.

Crystal structure of chicken riboflavin-binding protein

The crystal structure of chicken egg white riboflavin-binding protein, determined to a resolution of 2.5 A, is the prototype of a family that includes other riboflavin- and folate-binding proteins. An unusual characteristic of these molecules is their high degree of cross-linking by disulfide bridges and, in the case of the avian proteins, the presence of stretches of highly phosphorylated polypeptide chain. The structure of chicken egg white riboflavin-binding protein is characterized by a ligand-binding domain and a phosphorylated motif. The ligand-binding domain has a fold that appears to be strongly conditioned by the presence of the disulfide bridges. The phosphorylated motif, essential for vitamin uptake, is made up of two helices found before and after the flexible phosphorylated region. The riboflavin molecule binds to the protein with the isoalloxazine ring stacked in between the rings of Tyr75 and Trp156. This geometry and the proximity of other tryptophans explain the fluorescent quenching observed when riboflavin binds to the protein.

Antigenic determinants at the carboxy terminus of chicken egg white riboflavin carrier protein (RCP): epitope mapping and antibody-mediated pregnancy curtailment in rodents

The epitopic core sequences recognized by three monoclonal antibodies raised to chicken riboflavin carrier protein (RCP) were mapped to the C-terminal tail-end of the protein using the pepscan method. A 21-residue synthetic peptide corresponding to residues 200-219 of the protein and comprising the regions corresponding to the antibodies was synthesized. Administration of polyclonal antibodies specific to this peptide led to termination of early pregnancy in mice. Also, active immunization of rats with the peptide-purified protein derivative conjugate inhibited establishment of pregnancy. These results demonstrate the functional importance of the C-terminal 200-219 region of chicken RCP.

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.

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.

Termination of pregnancy in common marmosets (Callithrix jacchus) following administration of antiserum to chicken riboflavin carrier protein

The presence of protein(s) which is immunologically and functionally similar to the well characterised avian riboflavin carrier protein has been demonstrated in female common marmosets during pregnancy. The protein(s) that interacts with [14C]riboflavin has an immunological homology to chicken riboflavin carrier protein. The termination of pregnancy in four out of six marmosets following administration of antiserum to chicken riboflavin carrier protein supports the hypothesis that vitamin carrier protein ensures an uninterrupted supply of the vitamin to the growing embryo and that immunoneutralization of maternal riboflavin carrier protein leads to fetal death and termination of pregnancy.

Effects of antibodies against chicken riboflavin carrier protein on fetal hepatic cell ultrastructure

Day 12 pregnant mice administered antibodies to cRCP exhibited riboflavin deprival, which resulted in progressive alteration in the fetal hepatic cell structure, which eventually led to fetal wastage and termination of pregnancy. These changes were evident as early as 1 h following antiserum treatment. Three hours following treatment, other degenerative changes such as disorganization of glycogen particles and endoplasmic reticulum and degranulation of mitochondria were observed. Nuclear pycnosis, and increased myelin figures, accumulation of lysosomes, all indicative of autolytic changes, occurred 6 h following treatment.

Characterization of antibodies to chicken riboflavin carrier protein. Immunoneutralizing ability of antibodies to a sequence-specific region of the protein

The properties of antibodies generated in rabbits against native riboflavin carrier protein (cRCP), riboflavin carrier protein that had been denatured/renatured by SDS treatment (SDS-RCP) or disulphide-bond-reduced then S-carboxymethylated (Carb-RCP) were studied. SDS-RCP could displace native RCP in radioimmunoassay (r.i.a.), whereas Carb-RCP could not. By using antibodies raised in five different rabbits against native cRCP, 125I-labelled Carb-RCP could bind between 0 and 30% of the native antibodies. Antibodies raised against native RCP appear to be largely directed towards specific conformational determinants of RCP. Carb-RCP displaced native RCP in an r.i.a. using antibodies raised against SDS-RCP. SDS denaturation presumably unmasks cryptic epitopes in native RCP. Carb-RCP was a weak immunogen and elicited, presumably, antibodies to sequential epitope/epitopes. When injected into pregnant mice the antibodies caused neutralization of RCP, leading to termination of pregnancy, indicating highly conserved sequential epitopes in chicken and rodent RCP. Antibodies raised against Carb-RCP or native RCP reacted with CNBr fragments of native RCP, further confirming the presence of sequence-specific antibodies elicited by Carb-RCP.

Estrogen modulation of riboflavin carrier protein in the bonnet monkey (Macaca radiata)

Circulatory concentrations of riboflavin carrier protein (RCP) were quantitated in bonnet macaques by employing a heterologous radioimmunoassay involving 125I-labelled chicken RCP and its antiserum. The levels of monkey RCP in the serum seem to be governed by the estrogenic status of the animals. An increase in concentration of serum estradiol in the adult females during the menstrual cycle and early pregnancy could be correlated with enhanced serum RCP levels. Estadiol-17 beta administered to both immature female and male monkeys, specifically brought about elevated levels of RCP with a slower time course of response in males than in females. These results could be a reflection of a more rapid decline of both circulatory estrogen and RCP concentrations in male serum. Repeated administration of estradiol-17 beta to male animals led to prolonged elevated levels of RCP following estrogen administration. Thus, it would appear that the evolutionary conservation of RCPs from the aves to the primates encompasses not only their physicochemical similarities but also extends to the estrogenic modulation of their elaboration.

Isolation and partial characterisation of human riboflavin carrier protein and the estimation of its levels during human pregnancy

Human cord serum contains protein(s) capable of binding to [14C]-riboflavin. Riboflavin-bound protein cross-reacts with anti-serum to chicken riboflavin carrier protein (cRCP). The carrier protein was isolated using affinity chromatography on a riboflavin AH Sepharose column. Bulk isolation and purification was also attempted by a combination of ion exchange, gel filtration and gel permeation chromatography on HPLC. The protein so isolated had a molecular weight of 36,000 +/- 2,000 daltons with an isoelectric point of 4.1. The levels of RCP in maternal serum during pregnancy were monitored using a sensitive heterologous radioimmunoassay system, using cRCP as standard and anti serum to cRCP. The levels of the protein increased after 4 months and remained significantly elevated up to 8 months. Although the level of the protein in the maternal serum remained low until 4 months, its level in amniotic fluid was elevated 2-3-fold as compared to that in serum.

Pregnancy suppression in the bonnet monkey by active immunisation with chicken riboflavin carrier protein

Five healthy female bonnet monkeys (Macaca radiata) of proven fertility and normal menstrual cyclicity have been actively immunised with the purified chicken egg white riboflavin carrier protein (cRCP). All the immunised animals exhibited specific antibodies to cRCP and the immunopotencies of their sera varied from 200 to 840 micrograms/ml at equivalent point. A definite fraction of antibodies in these sera specifically recognised the purified and 125I-labelled monkey RCP. Immunisation per se had no adverse effect on the animals' menstrual cyclicity, circulating levels of estrogen and progesterone and the riboflavin status as reflected by glutathione reductase activities and the total flavin contents of the erythrocytes. The fertility of these animals was monitored for a period extending up to 3 years after primary immunisation. Four out of the five animals exhibited termination of pregnancy once or more than once depending on their antibody titers. Towards the end of the study period, when the immune response was poor, all the animals delivered normal babies at term. Circulating anti-cRCP antibodies were monitored by 125I-labelled cRCP binding. The results show that pregnancy termination, owing to immuno-neutralisation of monkey RCP, occurred only in animals which had sufficiently high antibody titers. If the titers fell below a critical threshold level the pregnancies were carried to term.

Purification of a circulatory riboflavin carrier protein from pregnant bonnet monkey (M. radiata): comparison with chicken egg vitamin carrier

A specific protein exhibiting immunological cross-reactivity with chicken egg-white riboflavin carrier protein was detected by radioimmunoassay in the pregnancy sera of bonnet monkeys (Macaca radiata). This protein, which is capable of binding [14C]riboflavin, was purified by gel filtration on Sephacryl S-300, fast protein liquid chromatography on a Mono Q anion exchanger and chromatofocusing on Mono P columns. The isolated primate carrier protein was similar to its avian counterpart in terms of physicochemical characteristics, such as isoelectric point (pI less than or equal to 4), electrophoretic mobility, molecular weight (approx. 36,000) and ligand binding. These findings may account for the extensive immunological cross-reactivity observed between the two proteins and suggest that the two vitamin carriers may have similar function in terms of embryonic vitamin nutrition.

Flavin radicals: chemistry and biochemistry

The redox properties of free and protein-bound flavin are discussed extensively. The interaction of one and two-electron reduced flavin with oxygen is emphasized.

Variations in riboflavin binding by human plasma: identification of immunoglobulins as the major proteins responsible

Riboflavin binding by plasma proteins from healthy human subjects was examined by equilibrium dialysis using a physiological concentration of [2-14C]riboflavin (0.04 microM). Binding ranged from 0.080 to 0.917 pmole of riboflavin/mg of protein (with a mean +/- SD of 0.274 +/- 0.206), which corresponded to 4.14 to 49.4 pmole/ml of plasma (15.5 +/- 11.0) (N = 34). Males and females yielded similar results. Upon fractionation of plasma by gel filtration, the major riboflavin-binding components eluted with albumin and gamma-globulins. Albumin was purified and found to bind riboflavin only very weakly (Kd = 3.8 to 10.4 mM), although FMN and photochemical degradation products (e.g., lumiflavine and lumichrome) were more tightly bound. Binding in the gamma-globulin fraction was attributed to IgG and IGA because the binding protein(s) and immunoglobulins copurified using various methods were removed by treatment of plasma with protein A-agarose, and were coincident upon immunoelectrophoresis followed by autoradiography to detect [2-14C]riboflavin. Differences among the plasma samples correlated with the binding recovered with the immunoglobulins. Binding was not directly related to the total IgG or IgA levels of subjects. Hence, it appears that the binding is due to a subfraction of these proteins. These findings suggest that riboflavin-binding immunoglobulins are a major cause of variations in riboflavin binding in human circulation, and may therefore affect the utilization of this micronutrient.

Enzymic basis of deranged foetal flavin-nucleotide metabolism consequent on immunoneutralization of maternal riboflavin carrier protein in the pregnant rat

A comparison of the kinetic and other parameters of enzymes of flavin-nucleotide metabolism in the whole foetus vis-à-vis the maternal liver in the pregnant rat revealed relatively lower activities of foetal flavokinase and FAD pyrophosphorylase. Passive immunoneutralization of the maternal riboflavin carrier protein suppresses foetal FAD pyrophosphorylase rather selectively. Additionally, although the activities of foetal nucleotide pyrophosphatase and FMN phosphatase were unchanged owing to immunoneutralization, higher activities of these enzymes in the whole foetus as compared with the maternal liver may be responsible for the drastic depletion of FAD levels that precipitates foetal degeneration.

Mechanism of foetal wastage following immunoneutralization of riboflavin carrier protein in the pregnant rat: disturbances in flavin coenzyme levels

Immunoneutralization of maternal RCP results in a greater than 90% decrease in the content and the incorporation of [2-14C]riboflavin into embryonic FAD as well as a percentage redistribution of both embryonic FMN and riboflavin. This is unaccompanied by any discernible changes in flavin distribution pattern in the maternal liver. Embryonic alpha-glycerophosphate dehydrogenase and NADPH-cytochrome c reductase register significant decreases in activities in the RCP antiserum-treated rats. These alterations readily explain the arrest of foetal growth culminating in pregnancy termination in the antiserum-treated animals.

Highly phosphorylated region of chicken riboflavin-binding protein: chemical characterization and 31P NMR studies

Phosphate residues on chicken riboflavin-binding protein (RBP) have been implicated in the recognition and deposition of the protein-vitamin complex in egg yolk by the ovary. We demonstrate that all of the phosphate of RBP is linked to serine and that most if not all of the phosphoserine residues are contained in a single tryptic peptide having a composition SerP8, Glx6, His2, Leu2, Ala, Met, Lys. Despite differences in the tissue of synthesis, RBPs isolated from egg yolk and egg white yield phosphopeptides virtually identical in their amino acid composition and 101-MHz 31P NMR spectra. This implies that posttranslational phosphorylation of RBP is the same in liver and oviduct. The 31P NMR spectrum of the phosphopeptide is quite different from that of the phosphoprotein. The 31P NMR spectra of egg yolk and egg white RBP are quite similar but not identical and are unaffected by the binding of riboflavin. Optimal resolution of seven phosphorus resonances was obtained near pH 7.0. The titration behavior of all the phosphoserine residues is similar. We propose that this highly anionic peptide, exposed at the surface of RBP, is the principal determinant for the uptake of RBP by the vitelline membrane of the ovarian follicle.

Pregnancy suppression by active immunization against gestation-specific riboflavin carrier protein

A riboflavin carrier protein isolated from chickens cross-reacts with a gestation-specific rodent carrier for riboflavin. Active immunization of female rats of proved fertility with the purified chicken carrier protein completely yet reversibly suppressed early pregnancy without impairing implantation per se. Concurrently there were no discernible adverse effects on maternal health in terms of weight gain, vitamin status, and fertility.