New insights into carrier binding and epithelial uptake of the erythropoietic nutrients cobalamin and folate

A Profound Vitamin B12 Deficiency in a Patient with Lofgren's Syndrome

Lofgren's syndrome is a unique manifestation of sarcoidosis presenting with erythema nodosum, bilateral hilar lymphadenopathy and migratory polyarthritis. A concurrent vitamin B12 deficiency is not well described and may be related to a rare gastrointestinal manifestation of sarcoid and Lofgren's syndrome. We describe a case of a 57-year-old male presented with migratory polyarthritis, erythemic nodules, edema of his legs and fever. His laboratory tests showed anemia with a profound vitamin B12 deficiency. Imaging demonstrated bilateral hilar adenopathy. Pathology revealed non-necrotizing granulomas consistent with sarcoidosis. The patient was started on prednisone and vitamin B12 supplements with improvement of his complaints and vitamin B12 levels. Sarcoidosis can manifest in many extrapulmonary organs, including the gastrointestinal tract, resulting in nutritional deficiencies, such as vitamin B12 deficiency. Treatment of these nutritional deficiencies includes treatment with steroids, as well as vitamin supplementation. We suggest this case to be a rare manifestation of gastrointestinal involvement in Lofgren syndrome; however, a biopsy from the GI tract was not performed to confirm the diagnosis. An informed consent was obtained from the patient. An institutional approval was not required for the publication of this case.

Vitamin B12 binding to mutated human transcobalamin, in-silico study of TCN2 alanine scanning and ClinVar missense mutations/SNPs

Many missense mutations/SNPs of the TCN2 gene (which yield Transcobalamin (TC)) were reported in the literature but no study is available about their effect on binding to vitamin B12(B12) at the structural level experimentally nor computationally. Predict the effect of TC missense mutations/SNPs on binding affinity to B12 and characterize their contacts to B12 at the structural level. TC-B12 binding energy difference from the wildtype (ΔΔGmut) was calculated for 378 alanine scanning mutations and 76 ClinVar missense mutations, repeated on two distinct X-ray structures of holoTC namely 2BB5 and 4ZRP. Destabilizing mutations then went through 100 ns molecular dynamics simulation to study their effect on TC-B12 binding at the structural level employing 2BB5 structure. Out of the studied 454 mutations (378 alanine mutations + 76 ClinVar mutations), 19 were destabilizing representing 17 amino acid locations. Mutation energy results show a neutral effect on B12 binding of several missense SNPs reported in the literature including I23V, G94S, R215W, P259R, S348F, L376S, and R399Q. Compared to the wildtype, all the destabilizing mutations have higher average RMSD-Ligand in the last 25% of the MD simulation trajectories and lower average hydrogen bond count while the other parameters vary. Previously reported TCN2 SNPs with an unknown effect on TC-B12 binding were found to have a neutral effect in the current study based on mutation energy calculations. Also, we reported 17 possible amino acids that destabilize TC-B12 binding upon mutation (four listed in ClinVar) and studied their structural effect computationally. Communicated by Ramaswamy H. Sarma.

Pernicious anemia: Pathophysiology and diagnostic difficulties

Pernicious anemia (PA) is the most common cause of vitamin B12 (cobalamin) deficiency anemia in the world. It is an autoimmune disease, comprising of salient features of autoimmune chronic atrophic gastritis (CAG) and cobalamin deficiency (CD). Although the anemia was first described as pernicious, it may well be controlled with vitamin B12 replacement. The onset and progression of PA is often insidious. Alternatively, patients may have no anemic symptoms since they become acclimatized to the subtle nature of the disease. Oftentimes, there is a possibility that the underlying disease may be missed unless a full blood count (FBC) is investigated, leading to hindrance in the treatment journey. Diagnostic challenges remain tangible for many practicing clinicians, since there is lack of reliable cobalamin assays to diagnose CD as well as clinical mimics, which simulate many other hematological conditions, such as myelodysplastic syndrome, acute leukemia, sideroblastic anemias, bone marrow failure states, thrombotic microangiopathy, and thromboembolism. Moreover, prompt recognition of the symptoms of CD is also vital, because some neurologic sequalae may become irreversible despite replenishing cobalamin. Herein, we discuss a literature review on the pathophysiology, challenging clinical presentations and diagnostic difficulties of PA. Since the cobalamin replacement therapy for PA is straightforward, it will not be discussed in this review.

Gestational folate deficiency alters embryonic gene expression and cell function

Folic acid is a nutrient essential for embryonic development. Folate deficiency can cause embryonic lethality or neural tube defects and orofacial anomalies. Folate receptor 1 (Folr1) is a folate binding protein that facilitates the cellular uptake of dietary folate. To better understand the biological processes affected by folate deficiency, gene expression profiles of gestational day 9.5 (gd9.5) Folr1^-/- embryos were compared to those of gd9.5 Folr1^+/+ embryos. The expression of 837 genes/ESTs was found to be differentially altered in Folr1^-/- embryos, relative to those observed in wild-type embryos. The 837 differentially expressed genes were subjected to Ingenuity Pathway Analysis. Among the major biological functions affected in Folr1^-/- mice were those related to 'digestive system development/function', 'cardiovascular system development/function', 'tissue development', 'cellular development', and 'cell growth and differentiation', while the major canonical pathways affected were those associated with blood coagulation, embryonic stem cell transcription and cardiomyocyte differentiation (via BMP receptors). Cellular proliferation, apoptosis and migration were all significantly affected in the Folr1^-/- embryos. Cranial neural crest cells (NCCs) and neural tube explants, grown under folate-deficient conditions, exhibited marked reduction in directed migration that can be attributed, in part, to an altered cytoskeleton caused by perturbations in F-actin formation and/or assembly. The present study revealed that several developmentally relevant biological processes were compromised in Folr1^-/- embryos.

An update on vitamin B12-related gene polymorphisms and B12 status

Background

Vitamin B12 is an essential micronutrient in humans needed for health maintenance. Deficiency of vitamin B12 has been linked to dietary, environmental and genetic factors. Evidence for the genetic basis of vitamin B12 status is poorly understood. However, advancements in genomic techniques have increased the knowledge-base of the genetics of vitamin B12 status. Based on the candidate gene and genome-wide association (GWA) studies, associations between genetic loci in several genes involved in vitamin B12 metabolism have been identified.

Objective

The objective of this literature review was to identify and discuss reports of associations between single-nucleotide polymorphisms (SNPs) in vitamin B12 pathway genes and their influence on the circulating levels of vitamin B12.

Methods

Relevant articles were obtained through a literature search on PubMed through to May 2017. An article was included if it examined an association of a SNP with serum or plasma vitamin B12 concentration. Beta coefficients and odds ratios were used to describe the strength of an association, and a P < 0.05 was considered as statistically significant. Two reviewers independently evaluated the eligibility for the inclusion criteria and extracted the data.

Results

From 23 studies which fulfilled the selection criteria, 16 studies identified SNPs that showed statistically significant associations with vitamin B12 concentrations. Fifty-nine vitamin B12-related gene polymorphisms associated with vitamin B12 status were identified in total, from the following populations: African American, Brazilian, Canadian, Chinese, Danish, English, European ancestry, Icelandic, Indian, Italian, Latino, Northern Irish, Portuguese and residents of the USA.

Conclusion

Overall, the data analyzed suggests that ethnic-specific associations are involved in the genetic determination of vitamin B12 concentrations. However, despite recent success in genetic studies, the majority of identified genes that could explain variation in vitamin B12 concentrations were from Caucasian populations. Further research utilizing larger sample sizes of non-Caucasian populations is necessary in order to better understand these ethnic-specific associations.

Impact of genetic polymorphisms in key enzymes of homocysteine metabolism on the pathophysiology of sickle cell anemia

This work aimed at studying a possible influence of methylenetetrahydrofolate reductase (MTHFR; c. 677C>T) and cystathionine β-synthase (CBS; 844ins68) polymorphisms on overall oxidative status of sickle cell anemia (SCA) patients and on routine markers, correlating them with hydroxycarbamide (HC) treatment. We evaluated 95 unrelated and diagnosed SCA patients. All patients received a prophylactic treatment with folic acid of 5mg/day, while 41 (43.2%) of them were under hydroxycarbamide (HC) treatment (average dose: 22mg/kg/day). MTHFR and CBS polymorphisms were identified by Polymerase Chain Reaction. Biochemical parameters were measured using spectrophotometric and chromatographic methods. Routine markers were developed by specialized laboratory. We did not find any effect of 677T and "I" allele combination on the biomarkers evaluated. On the other hand, MTHFR 677T mutation was related to a depletion of antioxidant capacity, according to the decreased catalase activity and a reduction about 30% of glutathione levels. Moreover, the presence of the insertion was related to about 23% less biomolecule oxidation levels and lower monocytes count, but about 14% higher lactate dehydrogenase activity. These findings may contribute to highlight that the MTHFR and CBS polymorphisms involvement in SCA pathophysiology is likely to be far more complex than it was explored to date.

Association of the transcobalamin II gene 776C→G polymorphism with Alzheimer’s type dementia: dependence on the 5, 10-methylenetetrahydrofolate reductase 1298A→C polymorphism genotype

Background

Decreased serum concentrations of vitamin B12 are associated with Alzheimer’s type dementia. The transcobalamin II gene (TCN2) 776C→G polymorphism affects transcobalamin II function as a carrier of vitamin B12 and might modify its availability. The association of the TCN2 776C→G polymorphism with Alzheimer’s type dementia is unclear and was investigated in the present study.

Methods

Case–control study including 27 individuals diagnosed with Alzheimer’s type dementia and 28 healthy controls. Serum concentrations of vitamin B12, homocysteine and other analytes were determined and the presence of TCN2 776C→G and 5, 10-methylenetetrahydrofolate reductase 1298A→C polymorphisms genotypes was ascertained by polymerase chain reaction–restriction fragment length polymorphism.

Results

Serum concentrations of vitamin B12 were lower while those of homocysteine were higher in patients than in controls ( P < 0.05). The frequency of individuals carrying at least one 5, 10-methylenetetrahydrofolate reductase 1298C allele was higher (59% versus 32%) while frequency of individuals harbouring at least one TCN2 776G allele was lower (58% versus 86%) in patients than in controls ( P < 0.05). Univariate logistic regression showed negative association of TCN2 776CG genotype with Alzheimer’s type dementia (OR = 0.17 versus CC genotype, P < 0.02). Multivariate logistic regression identified TCN2 776C→G polymorphism as independent predictor of Alzheimer’s type dementia together with higher concentrations of homocysteine, cholesterol and uric acid and lower concentrations of oestradiol. Association of TCN2 776C→G polymorphism with Alzheimer’s type dementia was observed for individuals carrying the 5,10-methylenetetrahydrofolate reductase 1298AA genotype but not the AC or CC genotypes, indicating interaction between the two polymorphisms.

Conclusions

The TCN2 776C→G polymorphism is negatively associated with Alzheimer’s type dementia, suggesting a protective role against the disease in subjects with the 5, 10-methylenetetrahydrofolate reductase 1298AA genotype.

Physiological and molecular aspects of cobalamin transport

Minute doses of a complex cofactor cobalamin (Cbl, vitamin B12) are essential for metabolism. The nutritional chain for humans includes: (1) production of Cbl by bacteria in the intestinal tract of herbivores; (2) accumulation of the absorbed Cbl in animal tissues; (3) consumption of food of animal origin. Most biological sources contain both Cbl and its analogues, i.e. Cbl-resembling compounds physiologically inactive in animal cells. Selective assimilation of the true vitamin requires an interplay between three transporting proteins - haptocorrin (HC), intrinsic factor (IF), transcobalamin (TC) - and several receptors. HC is present in many biological fluids, including gastric juice, where it assists in disposal of analogues. Gastric IF selectively binds dietary Cbl and enters the intestinal cells via receptor-mediated endocytosis. Absorbed Cbl is transmitted to TC and delivered to the tissues with blood flow. The complex transport system guarantees a very efficient uptake of the vitamin, but failure at any link causes Cbl-deficiency. Early detection of a negative B12 balance is highly desirable to prevent irreversible neurological damages, anaemia and death in aggravated cases. The review focuses on the molecular mechanisms of cobalamin transport with emphasis on interaction of corrinoids with the specific proteins and protein-receptor recognition. The last section briefly describes practical aspects of recent basic research concerning early detection of B12-related disorders, medical application of Cbl-conjugates, and purification of corrinoids from biological samples.

Investigation of a vitamin B12 conjugate as a PET imaging probe

Nutrient demand is a fundamental characteristic of rapidly proliferating cells. Vitamin B12 is vital for cell proliferation; thus neoplastic cells have an increased demand for this essential nutrient. In this study we exploited the vitamin B12 uptake pathway to probe the nutritional demand of proliferating cells with a radiolabeled B12 derivative in various preclinical tumor models. We describe the synthesis and biological evaluations of copper-64-labeled B12 -ethylenediamine-benzyl-1,4,7-triazacyclononane-N,N',N''-triacetic acid (B12 -en-Bn-NOTA-(64) Cu), the first example of a B12 derivative for positron emission tomography (PET) imaging. Small-animal imaging and pharmacological evaluation show high tumor uptake ranging from 2.20 to 4.84% ID g(-1) at 6 h post-administration. Competition studies with excess native B12 resulted in a 95% decrease in tumor accumulation, indicating the specificity of this radiopharmaceutical for B12 endocytotic transport proteins. These results show that a vitamin B12 PET radiopharmaceutical has potential utility for non-invasive imaging of enhanced nutrient demand in proliferating cells.

Three family members with elevated plasma cobalamin, transcobalamin and soluble transcobalamin receptor (sCD320)

BACKGROUND

Plasma cobalamin is requested in order to diagnose cobalamin deficiency and low levels confirm a deficient state. Here, we present three family members with unexpected high levels of cobalamin.

METHODS

We included a patient referred for cobalamin measurement due to neurological symptoms, her son and her daughter. Mother and son both suffered from myotonic dystrophy type II, while the daughter tested negative for this disease. Blood samples were analyzed for cobalamin, haptocorrin, transcobalamin, holoTC, and sCD320. We employed gel filtration and antibody precipitation for further characterization. The protein coding region of the TCN2 gene, encoding transcobalamin, was sequenced.

RESULTS

The patient, her {son} and [daughter] all had cobalamin levels above the measurement range of the routine method employed (>1476 pmol/L). Total transcobalamin and (holoTC) were 5980 (1500), {5260 (2410)} and [5630 (1340)] pmol/L, which is well above the upper reference limits of 1500 (160) pmol/L. The sCD320 concentration was also well above the upper reference limit of 97 arb.u.: 1340, {1510} and [1090] arb.u. Haptocorrin levels were within the reference range and no signs of cobalamin deficiency were found. DNA sequencing of the TCN2 gene revealed several known polymorphisms not associated with highly elevated transcobalamin levels. Upon gel filtration, sCD320 eluted as a larger molecule than previously reported. By incubation with anti-transcobalamin antibodies, we precipitated both transcobalamin and part of sCD320.

CONCLUSIONS

The high cobalamin levels were mainly explained by high levels of holoTC, possibly caused by complex formation with its soluble receptor, sCD320. The family occurrence points to a genetic explanation.

Structural basis for universal corrinoid recognition by the cobalamin transport protein haptocorrin

Cobalamin (Cbl; vitamin B12) is an essential micronutrient synthesized only by bacteria. Mammals have developed a sophisticated uptake system to capture the vitamin from the diet. Cbl transport is mediated by three transport proteins: transcobalamin, intrinsic factor, and haptocorrin (HC). All three proteins have a similar overall structure but a different selectivity for corrinoids. Here, we present the crystal structures of human HC in complex with cyanocobalamin and cobinamide at 2.35 and 3.0 Å resolution, respectively. The structures reveal that many of the interactions with the corrin ring are conserved among the human Cbl transporters. However, the non-conserved residues Asn-120, Arg-357, and Asn-373 form distinct interactions allowing for stabilization of corrinoids other than Cbl. A central binding motif forms interactions with the e- and f-side chains of the corrin ring and is conserved in corrinoid-binding proteins of other species. In addition, the α- and β-domains of HC form several unique interdomain contacts and have a higher shape complementarity than those of intrinsic factor and transcobalamin. The stabilization of ligands by all of these interactions is reflected in higher melting temperatures of the protein-ligand complexes. Our structural analysis offers fundamental insights into the unique binding behavior of HC and completes the picture of Cbl interaction with its three transport proteins.

Effect of the vitamin B12-binding protein haptocorrin present in human milk on a panel of commensal and pathogenic bacteria

Background

Haptocorrin is a vitamin B12-binding protein present in high amounts in different body fluids including human milk. Haptocorrin has previously been shown to inhibit the growth of specific E. coli strains, and the aim of the present study was to elucidate whether the antibacterial properties of this protein may exert a general defense against pathogens and/or affect the composition of the developing microbiota in the gastrointestinal tracts of breastfed infants.

Findings

The present work was the first systematic study of the effect of haptocorrin on bacterial growth, and included 34 commensal and pathogenic bacteria to which infants are likely to be exposed. Well-diffusion assays addressing antibacterial effects were performed with human milk, haptocorrin-free human milk, porcine holo-haptocorrin (saturated with B-12) and human apo-haptocorrin (unsaturated). Human milk inhibited the growth of S. thermophilus and the pathogenic strains L. monocytogenes LO28, L. monocytogenes 4446 and L. monocytogenes 7291, but the inhibition could not be ascribed to haptocorrin. Human apo-haptocorrin inhibited the growth of only a single bacterial strain (Bifidobacterium breve), while porcine holo-haptocorrin did not show any inhibitory effect.

Conclusions

Our results suggest that haptocorrin does not have a general antibacterial activity, and thereby contradict the existing hypothesis implicating such an effect. The study contributes to the knowledge on the potential impact of breastfeeding on the establishment of a healthy microbiota in infants.

Mouse transcobalamin has features resembling both human transcobalamin and haptocorrin

In humans, the cobalamin (Cbl) -binding protein transcobalamin (TC) transports Cbl from the intestine and into all the cells of the body, whereas the glycoprotein haptocorrin (HC), which is present in both blood and exocrine secretions, is able to bind also corrinoids other than Cbl. The aim of this study is to explore the expression of the Cbl-binding protein HC as well as TC in mice. BLAST analysis showed no homologous gene coding for HC in mice. Submaxillary glands and serum displayed one protein capable of binding Cbl. This Cbl-binding protein was purified from 300 submaxillary glands by affinity chromatography. Subsequent sequencing identified the protein as TC. Further characterization in terms of glycosylation status and binding specificity to the Cbl-analogue cobinamide revealed that mouse TC does not bind Concanavalin A sepharose (like human TC), but is capable of binding cobinamide (like human HC). Antibodies raised against mouse TC identified the protein in secretory cells of the submaxillary gland and in the ducts of the mammary gland, i.e. at locations where HC is also found in humans. Analysis of the TC-mRNA level showed a high TC transcript level in these glands and also in the kidney. By precipitation to insolubilised antibodies against mouse TC, we also showed that >97% of the Cbl-binding capacity and >98% of the Cbl were precipitated in serum. This indicates that TC is the only Cbl-binding protein in the mouse circulation. Our data show that TC but not HC is present in the mouse. Mouse TC is observed in tissues where humans express TC and/or HC. Mouse TC has features in common with both human TC and HC. Our results suggest that the Cbl-binding proteins present in the circulation and exocrine glands may vary amongst species.

Embryonic development in the reduced folate carrier knockout mouse is modulated by maternal folate supplementation

BACKGROUND

The reduced folate carrier (RFC1) is a ubiquitously expressed integral membrane protein that mediates delivery of 5-methyltetrahydrofolate into mammalian cells. In this study, embryonic/fetal development is characterized in an RFC1 knockout mouse model in which pregnant dams receive different levels of folate supplementation.

METHODS

RFC1(+/-) males were mated to RFC1(+/-) females, and pregnant dams were treated with vehicle (control) or folic acid (25 or 50 mg/kg) by daily subcutaneous injection (0.1 mL/10 g bwt), beginning on E0.5 and continuing throughout gestation until the time of sacrifice.

RESULTS

Without maternal folate supplementation, RFC1 nullizygous embryos die shortly postimplantation. Supplementation of pregnant dams with 25 mg/kg/day folic acid prolongs survival of mutant embryos until E9.5-E10.5, but they are developmentally delayed relative to wild-type littermates, display a marked absence of erythropoiesis, severe neural tube and limb bud defects, and failure of chorioallantoic fusion. Fgfr2 protein levels are significantly reduced or absent in the extraembryonic membranes of RFC1 nullizygous embryos. Maternal folate supplementation with 50 mg/kg/day results in survival of 22% of RFC1 mutants to E18.5, but they develop with multiple malformations of the eyelids, lungs, heart, and skin.

CONCLUSIONS

High doses of daily maternal folate supplementation during embryonic/fetal development are necessary for early postimplantation embryonic viability of RFC1 nullizygous embryos, and play a critical role in chorioallantoic fusion, erythropoiesis, and proper development of the neural tube, limbs, lungs, heart, and skin.

New Derivatives of Vitamin B12 Show Preferential Targeting of Tumors

Rapidly growing cells show an increased demand for nutrients and vitamins. The objective of our work is to exploit the supply route of vitamin B12 to deliver new derivatives of this vital vitamin to hyperproliferative cells. To date, radiolabeled (57Co and 111In) vitamin B12 derivatives showed labeling of tumor tissue but also undesired high accumulation of radioactivity in normal tissue. By abolishing the interaction of a tailored vitamin B12 derivative to its transport protein transcobalamin II and therefore interrupting transcobalamin II receptor and megalin mediated uptake in normal tissue, preferential accumulation of a radiolabeled vitamin in cancer tissue could be accomplished. We identified transcobalamin I on tumors as a possible new receptor for this preferential accumulation of vitamin-mediated targeting. The low systemic distribution of radioactivity and the high tumor to blood ratio opens the possibility of a more successful clinical application of vitamin B12 for imaging or therapy. [Cancer Res 2008;68(8):2904–11]

Microarray analysis of E9.5 reduced folate carrier (RFC1; Slc19a1) knockout embryos reveals altered expression of genes in the cubilin-megalin multiligand endocytic receptor complex

BACKGROUND

The reduced folate carrier (RFC1) is an integral membrane protein and facilitative anion exchanger that mediates delivery of 5-methyltetrahydrofolate into mammalian cells. Adequate maternal-fetal transport of folate is necessary for normal embryogenesis. Targeted inactivation of the murine RFC1 gene results in post-implantation embryolethality, but daily folic acid supplementation of pregnant dams prolongs survival of homozygous embryos until mid-gestation. At E10.5 RFC1-/- embryos are developmentally delayed relative to wildtype littermates, have multiple malformations, including neural tube defects, and die due to failure of chorioallantoic fusion. The mesoderm is sparse and disorganized, and there is a marked absence of erythrocytes in yolk sac blood islands. The identification of alterations in gene expression and signaling pathways involved in the observed dysmorphology following inactivation of RFC1-mediated folate transport are the focus of this investigation.

RESULTS

Affymetrix microarray analysis of the relative gene expression profiles in whole E9.5 RFC1-/- vs. RFC1+/+ embryos identified 200 known genes that were differentially expressed. Major ontology groups included transcription factors (13.04%), and genes involved in transport functions (ion, lipid, carbohydrate) (11.37%). Genes that code for receptors, ligands and interacting proteins in the cubilin-megalin multiligand endocytic receptor complex accounted for 9.36% of the total, followed closely by several genes involved in hematopoiesis (8.03%). The most highly significant gene network identified by Ingenuitytrade mark Pathway analysis included 12 genes in the cubilin-megalin multiligand endocytic receptor complex. Altered expression of these genes was validated by quantitative RT-PCR, and immunohistochemical analysis demonstrated that megalin protein expression disappeared from the visceral yolk sac of RFC1-/- embryos, while cubilin protein was widely misexpressed.

CONCLUSION

Inactivation of RFC1 impacts the expression of several ligands and interacting proteins in the cubilin-amnionless-megalin complex that are involved in the maternal-fetal transport of folate and other nutrients, lipids and morphogens such as sonic hedgehog (Shh) and retinoids that play critical roles in normal embryogenesis.

Haptocorrin in humans

BACKGROUND

Evolutionary haptocorrin is the youngest of the cobalamin-binding proteins. It evolved by duplication of the intrinsic factor gene and has been identified in most mammals examined. Its ability to bind both cobalamin and analogues is well established, but apart from that, our knowledge concerning its function and its distribution in adult and foetal life is limited. In this study, we present data on the tissue expression of haptocorrin and on the relation between analogues on haptocorrin and vitamin B(12) status in humans.

METHODS

Polyclonal antibodies towards haptocorrin were used to study the localisation in foetal and adult tissues by immunohistochemistry. Positive immunoreactions were primarily observed in exocrine glands, the gastrointestinal tract and the respiratory system. ELISA was used for measurement of holo- and total haptocorrin in blood samples from individuals diagnosed with vitamin B(12) deficiency, based on measurement of methylmalonic acid (micromol/L) as evident (>0.75, n=61), suspected (0.29-0.75, n=155) or not present (<0.29, n=170). Cobalamins and holotranscobalamin were measured in the same individuals.

RESULTS

Holohaptocorrin was considerably higher than holohaptocorrin-cobalamins (cobalamins minus holotranscobalamin). The median (25th-75th percentile, pmol/L) for holohaptocorrin analogues (holohaptocorrin minus holohaptocorrin-cobalamins) was higher in deficient [200 (130-240)] compared to the non-deficient [140 (80-200)] individuals (analysis of variance and Tukey's multiple comparison test, p<0.01).

CONCLUSIONS

Our results indicate that haptocorrin is widely distributed also in foetal tissues and suggest analogues to accumulate on haptocorrin in vitamin B(12)-deficient individuals, a result that warrants further studies employing methods directly measuring cobalamins and analogues attached to haptocorrin.

A novel superfamily containing the beta-grasp fold involved in binding diverse soluble ligands

Background

Domains containing the β-grasp fold are utilized in a great diversity of physiological functions but their role, if any, in soluble or small molecule ligand recognition is poorly studied.

Results

Using sensitive sequence and structure similarity searches we identify a novel superfamily containing the β-grasp fold. They are found in a diverse set of proteins that include the animal vitamin B12 uptake proteins transcobalamin and intrinsic factor, the bacterial polysaccharide export proteins, the competence DNA receptor ComEA, the cob(I)alamin generating enzyme PduS and the Nqo1 subunit of the respiratory electron transport chain. We present evidence that members of this superfamily are likely to bind a range of soluble ligands, including B12. There are two major clades within this superfamily, namely the transcobalamin-like clade and the Nqo1-like clade. The former clade is typified by an insert of a β-hairpin after the helix of the β-grasp fold, whereas the latter clade is characterized by an insert between strands 4 and 5 of the core fold.

Conclusion

Members of both clades within this superfamily are predicted to interact with ligands in a similar spatial location, with their specific inserts playing a role in the process. Both clades are widely represented in bacteria suggesting that this superfamily was derived early in bacterial evolution. The animal lineage appears to have acquired the transcobalamin-like proteins from low GC Gram-positive bacteria, and this might be correlated with the emergence of the ability to utilize B12 produced by gut bacteria.

Reviewers

This article was reviewed by Andrei Osterman, Igor Zhulin, and Arcady Mushegian.