Protein binding assays for an accurate differentiation of vitamin B12 from its inactive analogue. A study on edible cricket powder

Inactive analogues of vitamin B12 (cobalamin, Cbl) can mimic the active Cbl in food if using the traditional microbiological measurements. Thus, overestimated Cbl was recently revealed in edible insects employing immunoaffinity adsorption, HPLC-separation and mass spectrometry (https://doi.org/10.1016/j.foodchem.2021.129048). Here we demonstrate the utility of a convenient binding assay to evaluate Cbl in edible cricket powders. The assay employed the Cbl-specific protein intrinsic factor (IF) and the analogue-detecting protein haptocorrin. The excessive analogues had a weak affinity for IF, resulting in a modest overestimate of Cbl. This overestimate was corrected by a novel mathematical procedure, based on the ratio of analogue/Cbl in the sample and their relative affinities for IF. We found that 100 g of cricket powders contained 40-60 µg of analogues and 0.75-2.2 μg of Cbl. This result was confirmed by HPLC. A correct approach to Cbl-measurements is essential for nutritional assessment of any analogue-containing food.

Reference intervals and stability of haptocorrin and holotranscobalamin in Danish children and elderly

BACKGROUND

Haptocorrin (HC) and holotranscobalamin (holoTC) carry vitamin B12 (B12) in the circulation and can be useful biomarkers for evaluating B12 status. The concentration of both proteins depends on age, but data on reference intervals for children and the elderly are sparse. Similarly, not much is known about the effect of preanalytical factors.

METHODS

HC plasma samples from healthy elderly > 65 years (n = 124) were analysed, and both HC and holoTC were analysed in paediatric serum samples ≤ 18 years (n = 400). Furthermore, we investigated assay precision and stability.

RESULTS

HC and holoTC were effected by age. We established reference intervals for HC: 2-10 years, 369-1237 pmol/L; 11-18 years, 314-1128 pmol/L; 65-82 years, 242-680 pmol/L and for holoTC: 2-10 years, 46-206 pmol/L; 11-18 years, 30-178 pmol/L. Analytical coefficients of variations of 6.0-6.8% and 7.9-15.7% were found for HC and holoTC, respectively. HC were affected when stored at room temperature and by freeze/thaw. HoloTC was stable at room temperature and after delayed centrifugation.

CONCLUSION

We present novel 95% age-related reference limits for HC and HoloTC in children, and for HC both in children and elderly. Moreover, we found HoloTC to be fairly stable when stored, whereas HC was more vulnerable to preanalytical factors.

Age- and ethnicity-related reference intervals for serum vitamin B12

BACKGROUND

Age and ethnicity are known to influence serum vitamin B₁₂ (B₁₂) concentration, yet universal reference intervals (RIs) are typically applied by laboratories. Both lower and upper RI limits for B₁₂ are clinically relevant. Low values suggest deficiency leading to anemia and/or neurological impairment, while high values are not always an innocuous consequence of high B₁₂ intake but are associated with some cancers, autoimmune, liver, and renal diseases. This work aimed to establish age- and ethnicity-related RIs for B₁₂ using a modified indirect method based on Hoffmann's approach.

METHODS

A total of 72,091 anonymized B₁₂ results (Jan 2018-Nov 2019) were analyzed from an ethnically-diverse South-East London general practice patient population. Patients belonged to five ethnic groups: Asian, Black, White, Mixed, or Other. Multiple records for the same patient and results with missing ethnicity were excluded from the analysis of adult RIs. B₁₂ analyses were performed using ARCHITECT® (Abbott Diagnostics).

RESULTS

B₁₂ was significantly higher in Black compared with Asian and White adults. There were no differences in B₁₂ between Asian and White adults. Children (all ethnicities) between 2 and 5 years old had the highest B₁₂. Because of the small number of children (up to the age of 13) in each ethnic-related age category, all ethnic groups were combined to obtain age-related RIs. The children's RIs ranged from 159 to 1025 pmol/L for 0-1-year-olds to 276-1102 pmol/L for 2-5-year-olds. The RIs for Black and White/Asian people >13 years of age were 166-805 pmol/L and 134-511 pmol/L respectively.

CONCLUSIONS

The application of age- and ethnicity-appropriate RIs into diagnostic practice will provide a more accurate evaluation of B₁₂ status when using the B₁₂ test alone or in combination with other markers.

Diagnostic reliability of serum active B12 (holo-transcobalamin) in true evaluation of vitamin B12 deficiency: Relevance in current perspective

OBJECTIVE

Measurement of total vitamin B12 (vit B12) concentration raised concerns over early detection of vit B12 deficiency due to its clinical unreliability. In this present article we aimed to assess the efficacy of holo-transcobolamin (active vit B12) for true evaluation of vit B12 deficiency.

METHODS

This retrospective study included 100 participants referred for vit B12 assay. Serum total vit B12, active vit B12 and homocysteine were estimated.

RESULTS

Our study showed 59% of the total participants with vit B12 deficiency (185 ± 64.62 pg/ml) and 18% with hyper-cobalaminemia (1666.9 ± 367.13 pg/ml) based on their total vit B12 concentrations. A comparative study on total vit B12 and active vit B12 was done which reflected a striking disparity in results. Active vit B12 reported 28.8% patients with vit B12 deficiency (19.8 ± 17.48 pg/ml) and only 16.6% patients with hyper-cobalaminemia (224.14 ± 10 pg/ml). Active vit B12 appeared to be more sensitive (82.35% vs 65%) and specific (46.6% vs. 43.8%) diagnostic marker compared to total vit B12. Pearson Correlation study indicated a strong positive correlation (r = 0.695 at p < 0.01) hence justified use of the two methods.

CONCLUSION

We claim that active vit B12 is a much more reliable biomarker than total vit B12 for early diagnosis of vit B12 deficiency.

Vitamin B12 deficiency

Of the water-soluble vitamins, vitamin B12 (B12) has the lowest daily requirement. It also has several unique properties including a complex pathway for its absorption and assimilation requiring intact gastric and terminal small intestinal function, an enterohepatic pathway, and several dedicated binding proteins and chaperons. The many causes of B12 deficiency include malabsorption and defects in cellular delivery and uptake, as well as limited dietary intake. B12 is required as a cofactor for only two reactions in humans, the cytosolic methionine synthase reaction and the mitochondrial methymalonyl CoA mutase reaction. Disruption of either of these reactions gives rise to B12 deficiency. Although more common with advancing age, because of the higher prevalence of malabsorptive disorders in the elderly, B12 deficiency is widely distributed across all age groups particularly where food insecurity occurs. The consequences and severity of B12 deficiency are variable depending on the degree of deficiency and its duration. Major organ systems affected include the blood, bone marrow and nervous system. Megaloblastic anemia results from a defect in thymidine and therefore DNA synthesis in rapidly dividing cells. Nervous system involvement is varied, some of which results from defective myelin synthesis and repair. Cognitive impairment and psychosis may also occur. Diagnosis of B12 deficiency rests on clinical suspicion followed by laboratory testing, which consists of a panel of tests, that together provide clinically reliable predictive indices. B12 metabolism and deficiency is closely intertwined with folate, another B-vitamin. This chapter explores the various aspects of a unique and fascinating micronutrient.

Vitamin B12 absorption and malabsorption

Vitamin B12 is assimilated and transported by complex mechanisms that involve three transport proteins, intrinsic factor (IF), haptocorrin (HC) and transcobalamin (TC) and their respective membrane receptors. Vitamin deficiency is mainly due to inadequate dietary intake in vegans, and B12 malabsorption is related to digestive diseases. This review explores the physiology of vitamin B12 absorption and the mechanisms and diseases that produce malabsorption. In the stomach, B12 is released from food carrier proteins and binds to HC. The degradation of HC by pancreatic proteases and the pH change trigger the transfer of B12 to IF in the duodenum. Cubilin and amnionless are the two components of the receptor that mediates the uptake of B12 in the distal ileum. Part of liver B12 is excreted in bile, and undergoes an enterohepatic circulation. The main causes of B12 malabsorption include inherited disorders (Intrinsic factor deficiency, Imerslund-Gräsbeck disease, Addison's pernicious anemia, obesity, bariatric surgery and gastrectomies. Other causes include pancreatic insufficiency, obstructive Jaundice, tropical sprue and celiac disease, bacterial overgrowth, parasitic infestations, Zollinger-Ellison syndrome, inflammatory bowel diseases, chronic radiation enteritis of the distal ileum and short bowel. The assessment of B12 deficit is recommended in the follow-up of subjects with bariatric surgery. The genetic causes of B12 malabsorption are probably underestimated in adult cases with B12 deficit. Despite its high prevalence in the general population and in the elderly, B12 malabsorption cannot be anymore assessed by the Schilling test, pointing out the urgent need for an equivalent reliable test.

Competitive chemiluminescent enzyme immunoassay for vitamin B12 analysis in human milk

Recent discoveries of matrix interferences by haptocorrin (HC) in human milk and serum show that past analyses of vitamin B12 in samples with high HC content might have been inaccurate (Lildballe et al., 2009; Carmel & Agrawal, 2012). We evaluated two competitive enzyme-binding immunoassays for serum/plasma (IMMULITE and SimulTRAC-SNB) for B12 analysis in human milk. B12-recovery rates (United States Environmental Protection Agency, 2007) were determined to be 78.9 ± 9.1% with IMMULITE and 225 ± 108% (range 116-553%) using SimulTRAC-SNB, most likely due to the presence of excess HC. HC-interferences were not observed with the IMMULITE assay, rendering previously reported mandatory HC-removal (Lildballe et al., 2009) unnecessary. Linearity continued at low B12-concentrations (24-193 pM; r(2)>0.985). Milk B12 concentrations from Bangladeshi women (72-959 pM) were significantly lower than those from California (154-933 pM; p<0.0001) showing IMMULITE's robustness against the complex milk matrix and its ability to measure low milk B12 concentrations.

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.