Mechanism of foetal wastage following immunoneutralisation of riboflavin carrier protein in the pregnant rat

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.

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.

Isolation and characterisation of a biotin-binding protein from the pregnant-rat serum and comparison with that from the chicken egg-yolk

A biotin-binding protein exhibiting partial immunological cross-reactivity with the purified chicken egg-yolk biotin-binding protein has been detected, for the first time, in the sera of pregnant/estrogenised female rats but not of the normal males. This protein, purified by affinity chromatography on a biotin-AH-Sepharose was homogeneous by electrophoretic and immunological criteria. It was a glycoprotein of Mr 66,000 without any detectable subunites, had a pI 4.1, and specifically bound [14C]biotin. Several structural and functional features of the biotin-binding protein of rat and chicken were found to be similar. These included immunological cross-reactivity, acidic and glycoprotein nature, ability to tightly bind [14C]biotin, estrogen stimulation for their appearance in circulation, and the pattern of distribution of radioiodinated peptides upon proteolysis with trypsin.

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.

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.

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.

Biotin-binding proteins and biotin transport to oocytes

The eggs of chickens and other birds contain two proteins that bind biotin. Both are homotetrameric proteins of similar size. In contrast to the well-characterized egg white avidin, egg yolk biotin-binding protein has a very acidic isoelectric point, binds biotin with lower affinity, and is usually saturated with biotin. Like other egg yolk proteins, biotin-binding protein appears to be synthesized in the liver, transported by the blood stream to the ovary and deposited in the developing oocyte. Since the yolk of a chicken egg contains over 90% of the biotin in an egg and all of the biotin is bound to biotin-binding protein, the function of biotin-binding protein is undoubtedly to transport biotin to the egg for future use by the developing embryo. Avidin is produced by the oviduct and in the egg it is presumed to deter microbial growth around the oocyte by sequestering biotin. Among the eggs examined, those from turkeys have the lowest amount of biotin-binding protein and the highest amount of avidin. Furthermore, the majority of the biotin in turkey eggs can be bound to avidin in the egg white, suggesting a nutritional role for avidin in turkeys. An assay has been developed to conveniently measure apo- and holobiotin-binding proteins.

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.

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.