Effect of Vitamin B12 Treatment on Haptocorrin

The Role of Methyl Donors of the Methionine Cycle in Gastrointestinal Infection and Inflammation

Vitamin deficiency is well known to contribute to disease development in both humans and other animals. Nonetheless, truly understanding the role of vitamins in human biology requires more than identifying their deficiencies. Discerning the mechanisms by which vitamins participate in health is necessary to assess risk factors, diagnostics, and treatment options for deficiency in a clinical setting. For researchers, the absence of a vitamin may be used as a tool to understand the importance of the metabolic pathways in which it participates. This review aims to explore the current understanding of the complex relationship between the methyl donating vitamins folate and cobalamin (B12), the universal methyl donor S-adenosyl-L-methionine (SAM), and inflammatory processes in human disease. First, it outlines the process of single-carbon metabolism in the generation of first methionine and subsequently SAM. Following this, established relationships between folate, B12, and SAM in varying bodily tissues are discussed, with special attention given to their effects on gut inflammation.

Relationship between Vitamin B12 and Cobalt Metabolism in Domestic Ruminant: An Update

Cobalt, as a trace element, is essential for rumen microorganisms for the formation of vitamin B12. In the metabolism of mammals, vitamin B12 is an essential part of two enzymatic systems involved in multiple metabolic reactions, such as in the metabolism of carbohydrates, lipids, some amino acids and DNA. Adenosylcobalamin and methylcobalamin are coenzymes of methylmalonyl coenzyme A (CoA) mutase and methionine synthetase and are essential for obtaining energy through ruminal metabolism. Signs of cobalt deficiency range from hyporexia, reduced growth and weight loss to liver steatosis, anemia, impaired immune function, impaired reproductive function and even death. Cobalt status in ruminant animals can be assessed by direct measurement of blood or tissue concentrations of cobalt or vitamin B12, as well as the level of methylmalonic acid, homocysteine or transcobalamin in blood; methylmalonic acid in urine; some variables hematological; food consumption or growth of animals. In general, it is assumed that the requirement for cobalt (Co) is expressed around 0.11 ppm (mg/kg) in the dry matter (DM) diet; current recommendations seem to advise increasing Co supplementation and placing it around 0.20 mg Co/kg DM. Although there is no unanimous criterion about milk production, fattening or reproductive rates in response to increased supplementation with Co, in some investigations, when the total Co of the diet was approximately 1 to 1.3 ppm (mg/kg), maximum responses were observed in the milk production.

What is the cobalamin status among vegetarians and vegans in Australia?

Water-soluble vitamin B12 (cobalamin) plays a vital role in normal blood function and neurological functioning. Clinical and subclinical B12 deficiency has been notably reported in vegans, vegetarians, the elderly and metformin-treated diabetics. Currently, the prevalence of cobalamin deficiency among vegans and vegetarians in Australia is lacking; data on dietary intake including supplements and nutritional status are also limited. The increasing multiculturalism of Australia has seen an influx of imported foods, of which some may contain considerable vitamin B12. However, values for such foods are not included in the food composition databases. This review highlights the need to update the food composition database with culturally diverse foods containing vitamin B12. Moreover, the need for assessing dietary intakes and status using the most current best evidence and best practice on nutritional indicators (biochemical and functional biomarkers) to estimate the risk of deficiency and/or depletion is discussed.

Vitamin B12 , folate, and the methionine remethylation cycle-biochemistry, pathways, and regulation

Vitamin B₁₂ (cobalamin, Cbl) is a nutrient essential to human health. Due to its complex structure and dual cofactor forms, Cbl undergoes a complicated series of absorptive and processing steps before serving as cofactor for the enzymes methylmalonyl-CoA mutase and methionine synthase. Methylmalonyl-CoA mutase is required for the catabolism of certain (branched-chain) amino acids into an anaplerotic substrate in the mitochondrion, and dysfunction of the enzyme itself or in production of its cofactor adenosyl-Cbl result in an inability to successfully undergo protein catabolism with concomitant mitochondrial energy disruption. Methionine synthase catalyzes the methyl-Cbl dependent (re)methylation of homocysteine to methionine within the methionine cycle; a reaction required to produce this essential amino acid and generate S-adenosylmethionine, the most important cellular methyl-donor. Disruption of methionine synthase has wide-ranging implications for all methylation-dependent reactions, including epigenetic modification, but also for the intracellular folate pathway, since methionine synthase uses 5-methyltetrahydrofolate as a one-carbon donor. Folate-bound one-carbon units are also required for deoxythymidine monophosphate and de novo purine synthesis; therefore, the flow of single carbon units to each of these pathways must be regulated based on cellular needs. This review provides an overview on Cbl metabolism with a brief description of absorption and intracellular metabolic pathways. It also provides a description of folate-mediated one-carbon metabolism and its intersection with Cbl at the methionine cycle. Finally, a summary of recent advances in understanding of how both pathways are regulated is presented.

Effect of 8-week oral supplementation with 3-µg cyano-B12 or hydroxo-B12 in a vitamin B12-deficient population

PURPOSE

We compare the effect of 8-week oral supplementation with cyano-B12 (currently used in vitamin pills) and hydroxo-B12 (predominant form in the diet) in a population with nutritional vitamin B12 deficiency.

METHODS

Fifty-one healthy Indian adults with baseline serum cobalamin < 200 pmol/L were supplied for 8 weeks with daily oral supplements of 3-µg cyano-B12 (n = 15), 3-µg hydroxo-B12 (n = 16), or a placebo (n = 20). Blood at baseline, and each following week, was examined for total cobalamin, holotranscobalamin, methylmalonic acid, and homocysteine.

RESULTS

The study groups did not differ at baseline and were characterized by [median (range)] serum cobalamin [128 (68-191) pmol/L], holotranscobalamin [16 (6-41) pmol/L], methylmalonic acid [0.8 (0.3-1.7) µmol/L], homocysteine [17.9 (8.5-100.9) µmol/L], and a combined indicator of B12 status 4cB12 of - 1.65 (- 0.64 to - 4.07). The group supplemented with cyano-B12 showed a higher increase in total serum cobalamin than the group treated with hydroxo-B12, while other biomarkers changed comparably in the two groups. After 8 weeks of treatment, the biomarker values of the supplemented groups (pooled) differed significantly from the placebo group. Yet, the vitamin B12 status was still poor [cobalamin: 168 (87-302) pmol/L; holotranscobalamin: 19 (8-45) pmol/L; methylmalonic acid: 0.7 (0.2-1.7) µmol/L; homocysteine: 17.2 (2.6-96.8) µmol/L; 4cB12 = - 1.34 (- 0.33 to - 3.3)].

CONCLUSION

8-week supplementation with 3-µg cyano-B12 elevated serum cobalamin more than 3 µg hydroxo-B12, but all other biomarkers changed similarly in both groups. Supplementation with 3 µg vitamin B12 did not reverse the low status in individuals with nutritional vitamin B12 deficiency.

CLINICAL TRIAL REGISTRY OF INDIA

REF/2017/02/013343.

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.

Cobalamin analogues in humans: a study on maternal and cord blood

BACKGROUND

Haptocorrin (HC) carries cobalamin analogues (CorA), but whether CorA are produced in the body is unknown. All cobalamins (Cbl) to the foetus are delivered by the Cbl-specific protein transcobalamin (TC), and therefore analysis of cord serum for CorA may help to clarify the origin of CorA.

METHODS

HC-CorA were quantified in paired samples of cord serum from newborns and serum from mothers (n = 69).

RESULTS

The CorA-concentration was higher in cord serum (median = 380, range: 41-780 pmol/L) than in serum from the mothers (median = 160, range: 64-330 pmol/L), (p<0.0001). HPLC-analysis showed CorA-peaks with retention times of 13.5, 14,5 and 16.5 min in samples from both the mother and cord serum. The peak with retention time 16.5 min constituted 24% (mother) and 45% (cord serum) of the total amount CorA, and eluted as does dicyanocobinamide.

CONCLUSION

Our results support that CorA in the human body are derived from Cbl.

Genetic disorders of vitamin B₁₂ metabolism: eight complementation groups--eight genes

Vitamin B12 (cobalamin, Cbl) is an essential nutrient in human metabolism. Genetic diseases of vitamin B12 utilisation constitute an important fraction of inherited newborn disease. Functionally, B12 is the cofactor for methionine synthase and methylmalonyl CoA mutase. To function as a cofactor, B12 must be metabolised through a complex pathway that modifies its structure and takes it through subcellular compartments of the cell. Through the study of inherited disorders of vitamin B12 utilisation, the genes for eight complementation groups have been identified, leading to the determination of the general structure of vitamin B12 processing and providing methods for carrier testing, prenatal diagnosis and approaches to treatment.

Thermolability of mutant MMACHC protein in the vitamin B12-responsive cblC disorder

Methylmalonic aciduria and homocystinuria, cblC type, is the most common inborn error of cellular vitamin B12 metabolism. We previously showed that the protein carrying the mutation responsible for late-onset cblC (MMACHC-R161Q), treatable with high dose OHCbl, is able to bind OHCbl with wild-type affinity, leaving undetermined the disease mechanism involved [Froese et al., Mechanism of responsiveness, Mol. Genet. Metab. (2009).]. To assess whether the mutation renders the protein unstable, we investigated the thermostability of the wild-type and mutant MMACHC proteins, either unbound or bound to different cobalamins (Cbl), using differential scanning fluorimetry. We found that MMACHC-wt and MMACHC-R161Q are both very thermolabile proteins in their apo forms, with melting temperatures (T(m)) of 39.3+/-1.0 and 37.1+/-0.7 degrees C, respectively; a difference confirmed by unfolding of MMACHC-R161Q but not MMACHC-wt by isothermal denaturation at 35 degrees C over 120 min. However, with the addition of OHCbl, MMACHC-wt becomes significantly stabilized (Delta T(m max)=8 degrees C, half-maximal effective ligand concentration, AC(50)=3 microM). We surveyed the effect of different cobalamins on the stabilization of the wild-type protein and found that AdoCbl was the most stabilizing, exerting a maximum increase in T(m) of approximately 16 degrees C, followed by MeCbl at approximately 13 degrees C, each evaluated at 50 microM cofactor. The other cobalamins stabilized in the order (CN)(2)Cbi>OHCbl>CNCbl. Interestingly, the AC(50)'s for AdoCbl, MeCbl, (CN)(2)Cbi and OHCbl were similar and ranged from 1-3 microM, which compares well with the K(d) of 6 microM for OHCbl [Froese et al., Mechanism of responsiveness, Mol. Genet. Metab. (2009).]. Unlike MMACHC-wt, the mutant protein MMACHC-R161Q is only moderately stabilized by OHCbl (Delta T(m max)=4 degrees C). The dose-response curve also shows a lower effectivity of OHCbl with respect to stabilization, with an AC(50) of 7 microM. MMACHC-R161Q showed the same order of stabilization as MMACHC-wt, but each cobalamin stabilized this mutant protein less than its wild-type counterpart. Additionally, MMACHC-R161Q had a higher AC(50) for each cobalamin form compared to MMACHC-wt. Finally, we show that MMACHC-R161Q is able to support the base-off transition for AdoCbl and CNCbl, indicating this mutant is not blocked in that respect. Taken together, our results suggest that protein stability, as well as propensity for ligand-induced stabilization, contributes to the disease mechanism in late-onset cblC disorder. Our results underscore the importance of cofactor stabilization of MMACHC and suggest that even small increases in the concentration of cobalamin complexed with MMACHC may have therapeutic benefit in children with the late-onset, vitamin responsive cblC disease.

Mechanism of vitamin B12-responsiveness in cblC methylmalonic aciduria with homocystinuria

Patients with the cblC vitamin B(12) (cobalamin, cbl) disorder are defective in the intracellular synthesis of adenosylcobalamin and methylcobalamin and have combined homocystinuria and methylmalonic aciduria. While other vitamin B(12) disorders are treatable with high dose cyanocobalamin (CNCbl) or hydroxocobalamin (OHCbl), cblC patients respond well to OHCbl but not to CNCbl. Patient mutations were introduced into recombinant MMACHC (cblC) protein and the binding of CNCbl and OHCbl was examined. Three mutations were analyzed: G147D, associated with early onset, vitamin B(12) unresponsive disease; R161Q, associated with late onset disease that is highly responsive to OHCbl; and H122A, selected to test the hypothesis that H122 is central to a proposed vitamin B(12) binding motif on MMACHC. We report here that wild-type MMACHC binds both OHCbl and CNCbl with similar, tight affinity (K(d)=5.7 microM). We also report that MMACHC binds CNCbl in the base-off form, with the dimethylbenzimidazole (DMB) base of cobalamin displaced from coordination with the cobalt. In this form, wild-type MMACHC is able to reductively decyanate CNCbl to cob(II)alamin requiring only the presence of NADPH and FAD. We demonstrate that MMACHC with the G147D mutation is unable to bind either CNCbl or OHCbl, providing a straight forward explanation for the absence of response to either vitamin form. However, we show that MMACHC containing the R161Q mutation binds OHCbl with wild-type affinity, but is disturbed in binding CNCbl and has impaired decyanation. Finally, we show that H122A has reduced binding, but like R161Q, it binds OHCbl more tightly than CNCbl, suggesting that this histidine is not absolutely required for binding. These studies suggest that the ability of mutant MMACHC to respond to vitamin therapy depends on its ability to bind the vitamin with significant affinity, and for CNCbl, also on its ability to bind in the base-off form to facilitate reductive decyanation. These studies emphasize the continued use of OHCbl with cblC patients for maximum therapeutic effect.

Genomic mutations associated with mild and severe deficiencies of transcobalamin I (haptocorrin) that cause mildly and severely low serum cobalamin levels

Transcobalamin (TC) I deficiency, like the function of TC I itself, is incompletely understood. It produces low serum cobalamin levels indistinguishable from those of true cobalamin deficiency. Diagnosis is especially elusive when TC I deficiency is mild. To provide new, more substantive definition, the TCN1 gene was examined in two well-characterised families that included members with both severe and mild TC I deficiencies. A severely deficient proposita with undetectable TC I levels displayed compound heterozygosity for two mutations, each causing a premature stop codon. Relatives in both families who had mildly low or low-normal plasma levels of TC I and cobalamin were heterozygous for one or the other of these mutations. An unrelated patient with mild TC I deficiency and unknown familial TC I and cobalamin status was then tested and found to be similarly heterozygous for one of the mutations. The two nonprivate mutations identify a genetic basis for TC I deficiency for the first time. They also add new approaches to studying mild and severe TC I deficiency and to reducing confusion of its low cobalamin levels with those of cobalamin deficiency and its often dramatically different prognosis and management.

An update on cobalamin deficiency in adults

Cobalamin (vitamin B12) deficiency is particularly common in the elderly (>65 years of age), but is often unrecognized because of its subtle clinical manifestations; although they can be potentially serious, particularly from a neuropsychiatric and hematological perspective. In the general population, the main causes of cobalamin deficiency are pernicious anemia and food-cobalamin malabsorption. Food-cobalamin malabsorption syndrome, which has only recently been identified, is a disorder characterized by the inability to release cobalamin from food or its binding proteins. This syndrome is usually caused by atrophic gastritis, related or unrelated to Helicobacter pylori infection, and long-term ingestion of antacids and biguanides. Besides these syndromes, mutations in genes encoding endocytic receptors involved in the ileal absorption and cellular uptake of cobalamin have been recently uncovered and explain, at least in part, the hereditary component of megaloblastic anemia. Management of cobalamin deficiency with cobalamin injections is currently well codified, but new routes of cobalamin administration (oral and nasal) are being studied, especially oral cobalamin therapy for food-cobalamin malabsorption.

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.