Correlations between holo-transcobalamin II, holo-haptocorrin, and total B12 in serum samples from healthy subjects and patients

An examination of relationships between vitamin B12 status and functional measures of peripheral neuropathy in young adult vegetarians

Introduction

Prevalence rates for vitamin B12 deficiency in U.S. adult vegetarians may exceed 30%, which is concerning given the role for this vitamin in numerous nervous system functions, including the synthesis of myelin sheaths. Defective myelin synthesis and repair are directly linked to peripheral neuropathy; yet, few investigations have examined how physical indicators of peripheral neuropathy (e.g., hand dexterity, vibration sensitivity and balance) are impacted in individuals adhering to vegetarian diets. This feasibility research explored the relationships between peripheral neuropathy and vitamin B12 status using a cross-sectional study design. In addition, a small pilot trial was conducted for limited-efficacy testing of vitamin B12 supplementation for reducing peripheral neuropathy.

Methods

Healthy, able-bodied adults (n = 38; 19-40 years of age) reported exclusive adherence to a vegetarian or vegan diet for 3 years. Peripheral neuropathy was measured using a force plate for assessing balance, and a vibration sensitivity tester and pegboard tests to assess hand dexterity. Serum vitamin B12 and folate were measured using standard radioimmunoassay techniques.

Results

Twenty-six percent of the sample displayed deficient or marginal vitamin B12 status (serum vitamin B12 <221 pmol/L). Participants with adequate vitamin B12 status scored 10% higher on the Purdue pegboard assembly test and 20% higher on the left hand adjusted functional dexterity test in comparison to participants with marginal-to-deficient vitamin B12 status (p < 0.05).

Discussion

These data provide preliminary evidence that peripheral neuropathy can be detected in individuals with marginal-to-deficient vitamin B12 status.

The Regulation and Characterization of Mitochondrial-Derived Methylmalonic Acid in Mitochondrial Dysfunction and Oxidative Stress: From Basic Research to Clinical Practice

Methylmalonic acid (MMA) can act as a diagnosis of hereditary methylmalonic acidemia and assess the status of vitamin B12. Moreover, as a new potential biomarker, it has been widely reported to be associated with the progression and prognosis of chronic diseases such as cardiovascular events, renal insufficiency, cognitive impairment, and cancer. MMA accumulation may cause oxidative stress and impair mitochondrial function, disrupt cellular energy metabolism, and trigger cell death. This review primarily focuses on the mechanisms and epidemiology or progression in the clinical study on MMA.

Diagnostic and Therapeutic Perspectives Associated to Cobalamin-Dependent Metabolism and Transcobalamins’ Synthesis in Solid Cancers

Cobalamin or vitamin B12 (B12) is a cofactor for methionine synthase and methylmalonyl-CoA mutase, two enzymes implicated in key pathways for cell proliferation: methylation, purine synthesis, succinylation and ATP production. Ensuring these functions in cancer cells therefore requires important cobalamin needs and its uptake through the transcobalamin II receptor (TCII-R). Thus, both the TCII-R and the cobalamin-dependent metabolic pathways constitute promising therapeutic targets to inhibit cancer development. However, the link between cobalamin and solid cancers is not limited to cellular metabolism, as it also involves the circulating transcobalamins I and II (TCI or haptocorrin and TCII) carrier proteins, encoded by TCN1 and TCN2, respectively. In this respect, elevations of B12, TCI and TCII concentrations in plasma are associated with cancer onset and relapse, and with the presence of metastases and worse prognosis. In addition, TCN1 and TCN2 overexpressions are associated with chemoresistance and a proliferative phenotype, respectively. Here we review the involvement of cobalamin and transcobalamins in cancer diagnosis and prognosis, and as potential therapeutic targets. We further detail the relationship between cobalamin-dependent metabolic pathways in cancer cells and the transcobalamins’ abundancies in plasma and tumors, to ultimately hypothesize screening and therapeutic strategies linking these aspects.

The Origins of Salivary Vitamin A, Vitamin B12 and Vitamin D-Binding Proteins

Vitamin A- (retinol), vitamin B₁₂- (haptocorrin) and vitamin D-binding proteins are the major circulatory transporters of their respective ligands; they are also constituents of the salivary proteome, the origins of which, remain unclear. The aim of this study was to explore how these proteins enter saliva and their relationship (if any) with vitamin status. Firstly, the three vitamin-binding proteins were quantified in resting whole mouth saliva and chewing-stimulated saliva from healthy donors (n = 10) to determine if they enter the mouth by salivary secretion or from the circulation. Secondly paired whole mouth saliva and serum samples were analysed from healthy donors (n = 14) to determine the relationships between the vitamin-binding proteins and vitamin status. Salivary output of all three vitamin-binding proteins studied increased when secretion was stimulated, suggesting they are secreted by the salivary glands. Whilst retinol-binding protein and haptocorrin were secreted by all major salivary glands, vitamin D-binding protein was restricted to the mucus glands. Salivary vitamin-binding protein concentrations were not found to be indicative of systemic vitamin status.

Vitamin B Supplementation and Nutritional Intake of Methyl Donors in Patients with Chronic Kidney Disease: A Critical Review of the Impact on Epigenetic Machinery

Cardiovascular morbidity and mortality are several-fold higher in patients with advanced chronic kidney disease (CKD) and end-stage renal disease (ESRD) than in the general population. Hyperhomocysteinemia has undoubtedly a central role in such a prominent cardiovascular burden. The levels of homocysteine are regulated by methyl donors (folate, methionine, choline, betaine), and cofactors (vitamin B6, vitamin B12,). Uremia-induced hyperhomocysteinemia has as its main targets DNA methyltransferases, and this leads to an altered epigenetic control of genes regulated through methylation. In renal patients, the epigenetic landscape is strictly correlated with the uremic phenotype and dependent on dietary intake of micronutrients, inflammation, gut microbiome, inflammatory status, oxidative stress, and lifestyle habits. All these factors are key contributors in methylome maintenance and in the modulation of gene transcription through DNA hypo- or hypermethylation in CKD. This is an overview of the epigenetic changes related to DNA methylation in patients with advanced CKD and ESRD. We explored the currently available data on the molecular dysregulations resulting from altered gene expression in uremia. Special attention was paid to the efficacy of B-vitamins supplementation and dietary intake of methyl donors on homocysteine lowering and cardiovascular protection.

Diagnostic Accuracy of Holotranscobalamin, Vitamin B12, Methylmalonic Acid, and Homocysteine in Detecting B12 Deficiency in a Large, Mixed Patient Population

Four biomarkers are commonly employed to diagnose B12 deficiency: vitamin B12 (B12), holotranscobalamin (HoloTC), methylmalonic acid (MMA), and homocysteine (Hcy). 4cB12, a combined index of the B12 status, has been suggested to improve the recognition of B12 deficiency. We aimed to evaluate the four different markers for detecting B12 deficiency, as determined by 4cB12. Within a large, mixed patient population, 11,833 samples had concurrent measurements of B12, HoloTC, MMA, and Hcy. 4cB12 was calculated according to the methods described by Fedosov. Diagnostic cutoffs as well as diagnostic accuracy for the detection of B12 deficiency were assessed with receiver operating characteristic (ROC) analysis. The median age was 56 years, and women accounted for 58.8% of the samples. Overall, the area under the curve (AUC) for the detection of subclinical B12 deficiency was highest for HoloTC (0.92), followed by MMA (0.91), B12 (0.9) and Hcy (0.78). The difference between HoloTC and B12 was driven by a significantly higher AUC for HoloTC (0.93) than for B12 (0.89), MMA (0.91), and Hcy in women 50 years and older (0.79; p < 0.05 for all). In the detection of subclinical B12 deficiency, there were no significant differences in the AUCs of HoloTC, B12, and MMA among men and women <50 years. In conclusion, in women < 50 years and in men, HoloTC, MMA, or Hcy do not appear superior to B12 for the detection of B12 deficiency. For women 50 years and older, HoloTC seems to be the preferred first-line marker for the detection of subclinical B12 deficiency.

Nutrition Assessment of B-Vitamins in Highly Active and Sedentary Women

Background: Female athletes and active women require adequate nutrition for optimal health and performance. Nutrition assessments are needed to identify potential nutrients of concern. Folate, vitamin B6, and vitamin B12 function in important pathways used during physical activity and female athletes may be at risk for poor status of these micronutrients. This cross-sectional study described a comprehensive nutrition assessment of the B-vitamins (folate, vitamin B6, and vitamin B12) using both dietary (food and dietary supplements) and biochemical assessments among highly active and sedentary women. Methods: Highly active (n = 29; age 20 ± 2 years; body mass index (BMI) 23.8 ± 3.5 kg/m²) and sedentary (n = 29; age 24 ± 3 years; BMI 22.6 ± 3.0 kg/m²) women were recruited for this study. Participants completed 7-day weighed food records and a fasting blood draw. Results: Although the highly active women reported higher intakes of energy (p < 0.01), folate (p < 0.01), vitamin B6 (p < 0.01), and vitamin B12 (p < 0.01), no significant differences were found between the groups for biomarkers of folate, vitamin B6, and vitamin B12. All of the highly active women had biomarkers within the desired reference ranges, suggesting good status. In general, most participants were able to meet the 1998 Recommended Daily Allowance (RDA) from food alone. For the women that reported using dietary supplements, micronutrient intakes met the 1998 RDA and in some cases, exceeded the Tolerable Upper Intake Level. Conclusion: This nutrition assessment documented good status for folate, vitamin B6, and vitamin B12 in the highly active women. Similar assessment approaches (food, dietary supplements, and biomarkers) should to completed with other nutrients of concern for the female athlete.

Holotranscobalamin (HoloTC, Active-B12) and Herbert's model for the development of vitamin B12 deficiency: a review and alternative hypothesis

The concentration of total vitamin B12 in serum is not a sufficiently sensitive or specific indicator for the reliable diagnosis of vitamin B12 deficiency. Victor Herbert proposed a model for the staged development of vitamin B12 deficiency, in which holotranscobalamin (HoloTC) is the first indicator of deficiency. Based on this model, a commercial immunoassay has been controversially promoted as a replacement for the total vitamin B12 test. HoloTC is cobalamin (vitamin B12) attached to the transport protein transcobalamin, in the serum, for delivery to cells for metabolism. Although there have been many published reports supporting the claims for HoloTC, the results of some studies were inconsistent with the claim of HoloTC as the most sensitive marker of vitamin B12 deficiency. This review examines the evidence for and against the use of HoloTC, and concludes that the HoloTC immunoassay cannot be used to measure vitamin B12 status any more reliably than total vitamin B12, or to predict the onset of a metabolic deficiency, because it is based on an erroneous hypothesis and a flawed model for the staged development of vitamin B12 deficiency. The author proposes an alternative model for the development of vitamin B12 deficiency.

Role of serum holotranscobalamin (holoTC) in the diagnosis of patients with low serum cobalamin. Comparison with methylmalonic acid and homocysteine

Plasma holotranscobalamin (holoTC) transports active cobalamin. Decreased levels of holoTC have been considered to be the earliest marker of cobalamin (Cbl) deficiency. In this work, holoTC was evaluated in low or borderline serum Cbl (LB12) and a concordance analysis was carried out with methylmalonic acid (MMA) and homocysteine (Hcy). Levels of Cbl, holoTC, MMA, and Hcy were investigated in a reference group in 106 patients with LB12 (≤200 pmol/l) and in 27 with folate deficiency (FOL). HoloTC levels were evaluated by an automated immunoassay (Active B12, Abbott Lab, Abbott Park, IL, USA). Lower levels of holoTC were observed in both LB12 and FOL groups (reference group vs LB12; p < 0.0001. Reference group vs FOL; p = 0.002). HoloTC levels were lower in LB12 than in FOL (p = 0.001). In LB12, concordance between Hcy and MMA was 82.1 % (chi-square test, p < 0.001; Kappa Index, 0.64, p < 0.0001). Concordance between Hcy and holoTC was 62 % (chi-square test, p = 0.006; Kappa index, 0.245, p = 0.006). Concordance between holoTC and MMA was 55.6 % (p = 0.233). Some cases with LB12 and elevated MMA did not show decreased holoTC. By contrast, MMA and Hcy were not increased in some patients with low holoTC and LB12. In conclusion, levels of holoTC were decreased in LB12 and FOL. In LB12 patients, holoTC concordance with MMA was poor. MMA/Hcy levels were not increased in a significant number of subjects with LB12 and low holoTC. This profile was found in iron deficiency. The significance of these changes remains to be clarified.

Vitamin A and vitamin B-12 concentrations in relation to mortality and morbidity among children born to HIV-infected women

Vitamin A supplementation starting at 6 months of age is an important child survival intervention; however, not much is known about the association between vitamin A status before 6 months and mortality among children born to HIV-infected women. Plasma concentrations of vitamins A and B-12 were available at 6 weeks of age (n = 576 and 529, respectively) for children born to HIV-infected women and they were followed up for morbidity and survival status until 24 months after birth. Children in the highest quartile of vitamin A had a 49% lower risk of death by 24 months of age compared to the lowest quartile (HR: 0.51, 95% CI: 0.29-0.90; P-value for trend = 0.01). Higher vitamin A levels were protective in the sub-groups of HIV-infected and un-infected children but this was statistically significant only in the HIV-uninfected subgroup. Higher vitamin A concentrations in plasma are protective against mortality in children born to HIV-infected women.

Determination of serum methylmalonic acid by alkylative extraction and liquid chromatography coupled to tandem mass spectrometry

Despite the new advances in bioanalytical techniques, the analysis of low-molecular-weight organic acids in complex matrices is still a challenge. Although new strategies applying liquid chromatography-tandem mass spectrometry (LC-MS/MS) seem to be promising, sample preparation methodologies hamper its application in most clinical laboratories. The quantitation of methylmalonic acid (MMA) in biological matrices is an emblematic example due to its low concentration, the need for derivatization to increase its molecular weight, and the presence of the physiologically more abundant isomer succinic acid. Here we present a new strategy for rapid and sensitive MMA quantitation by combining alkylative extraction and LC-MS/MS. Alkylative extraction conditions were optimized to allow endogenous detection of MMA using only 50 microL of serum with a short sample preparation procedure. The formation of a unique ion from the MMA dipentafluorobenzyl derivative in negative atmospheric pressure chemical ionization (APCI) allowed its detection with high sensitivity and with no interference from succinic acid, a more abundant physiologically present isomer.

Elevated plasma homocysteine and low vitamin B-6 status in nonsupplementing older women with rheumatoid arthritis

OBJECTIVE

The purpose of this study was to determine if nonsupplementing older women (aged >or=55 years) with rheumatoid arthritis had higher plasma homocysteine and lower B-vitamin status compared to healthy controls. Elevated plasma homocysteine, a risk factor for cardiovascular disease, may help explain why individuals with rheumatoid arthritis have an increased risk of cardiovascular disease.

METHODS

Older, free-living women were classified as rheumatoid arthritis (n=18) or healthy control (n=33). Participants were not using B-vitamin supplements. Fasting blood samples were measured for pyridoxal 5'phosphate (PLP) (the metabolically active coenzyme form of vitamin B-6), folate, red blood cell folate, vitamin B-12, transcobalamin II, homocysteine, C-reactive protein, and lipid concentrations. Participants completed 7-day weighed food records, the Stanford Health Assessment Questionnaire (HAQ), and a visual analog pain scale.

RESULTS

PLP concentrations were lower in the rheumatoid arthritis vs healthy control participants (4.93+/-3.85 vs 11.35+/-7.11 ng/mL [20+/-16 vs 46+/-29 nmol/L]; P<0.01) whereas plasma homocysteine was higher in the rheumatoid arthritis group (1.63+/-0.74 vs 1.15+/-0.38 mg/L [12.1+/-5.5 vs 8.5+/-2.8 micromol/L]; P=0.02). Red blood cell folate concentrations were lower in the rheumatoid arthritis vs healthy control participants [414+/-141 vs 525+/-172 ng/mL [938+/-320 vs 1,190+/-390 nmol/L]; P=0.02). No significant differences were found for plasma folate, vitamin B-12, and transcobalamin II. An inverse correlation was found between PLP concentrations and the HAQ disability index (r=-0.37; P<0.01). A positive correlation was found between homocysteine concentrations and the HAQ disability index (r=0.36; P=0.01). Total cholesterol and low-density lipoprotein cholesterol levels were lower in the rheumatoid arthritis group (cholesterol 191+/-43 vs 218+/-33 mg/dL [4.95+/-1.11 vs 5.65+/-0.85 mmol/L]; P=0.02; low-density lipoprotein cholesterol 110+/-36 vs 137+/-29 mg/dL [2.85+/-0.93 vs 3.55+/-0.75 mmol/L]; P<0.01). No significant differences were seen between groups for protein (g/day), fat (g/day), cholesterol (mg/day), folate (microg/day), vitamin B-12 (microg/day), and vitamin B-6 (mg/day) dietary intakes.

CONCLUSIONS

Poor vitamin B-6 status and elevated plasma homocysteine concentrations were seen in older women with rheumatoid arthritis compared to healthy controls and may contribute to their increased risk of cardiovascular disease.

Long-term ovo-lacto vegetarian diet impairs vitamin B-12 status in pregnant women

A well-planned vegetarian diet has been stated to be adequate during pregnancy. The aim of the present study was to compare serum vitamin B-12 and homocysteine concentrations in pregnant women (n = 109) consuming vegetarian and Western diets and to evaluate the adequacy of current dietary reference intakes of vitamin B-12 for these women. Pregnant women adhering to vegetarian diets for at least 3 y, with subgroups of ovo-lacto vegetarians (OLVs; n = 27), low-meat eaters (LME, n = 43), and women eating an average Western diet (control group, n = 39), were recruited. Dietary vitamin B-12 intake, serum vitamin B-12, and plasma total homocysteine (tHcy) concentrations were measured in wk 9-12, 20-22, and 36-38 of pregnancy. During pregnancy serum vitamin B-12 concentrations of ovo-lacto vegetarians (P < 0.001) and low-meat eaters (P = 0.050) were lower than those of the control group. We observed the combination of low serum vitamin B-12 concentrations and elevated plasma tHcy in 22% of ovo-lacto vegetarians, in 10% of low-meat eaters, and in 3% of controls (P = 0.003). In OLVs, serum vitamin B-12 predicted 60% of the plasma tHcy variation (P < 0.001), but in LMEs and controls only <10% (NS). Serum vitamin B-12 concentrations increased and plasma tHcy decreased sharply with increasing dietary intake of vitamin B-12 toward a cutoff point of 3 mug/d. Pregnant women consuming a long-term predominantly vegetarian diet have an increased risk of vitamin B-12 deficiency. Current recommended dietary intakes urgently need reevaluation.

Longitudinal Concentrations of Vitamin B12 and Vitamin B12-binding Proteins during Uncomplicated Pregnancy

Background: Because reference values for vitamin B12 concentrations and vitamin B12-binding capacities for pregnant women have not been established, the reference values for nonpregnant women are often applied to assess vitamin B12 status. The aim of the present study was to describe ranges of biochemical indices of vitamin B12 status, including red blood cell (RBC) vitamin B12, saturated and unsaturated cobalamin-binding proteins, and binding capacities in all trimesters of uncomplicated pregnancy.

Methods: A total of 39 healthy pregnant women with long-term daily intake of vitamin B12 &gt;2.6 μg/day and uncomplicated pregnancies participated in the study throughout their pregnancies. RBCs and serum vitamin B12, holo-haptocorrin, unsaturated cobalamin-binding proteins, unsaturated and total vitamin B12-binding capacities, total homocysteine (tHcy), and RBC count were assessed in weeks 9–12, 20–22, and 36–38 of gestation.

Results: Significant changes in vitamin B12 status occurred in the course of pregnancy. Serum vitamin B12 concentrations and percentage of saturation of vitamin B12-binding proteins decreased steadily throughout pregnancy. In the third trimester, 35% of the participants had serum vitamin B12 concentrations &lt;150 pmol/L and 68.6% had &lt;15% saturation of total vitamin B12-binding capacities, but no women had RBC vitamin B12 concentrations &lt;148 pmol/L. However, the decrease in these indices was not associated with reduced hemoglobin concentrations or RBC count or with increased tHcy concentrations.

Conclusions: Our findings suggest that the reference values for vitamin B12 status in nonpregnant women may not be applicable to pregnant women.

Plasma total transcobalamin I. Ethnic/racial patterns and comparison with lactoferrin

Plasma total transcobalamin (TC) I levels were measured in 434 healthy volunteers by radioimmunoassay (RIA). The results were analyzed for demographic patterns and were compared with lactoferrin, cobalamin, homocysteine, and chemistry panel results. Plasma TC I was higher in blacks than in other ethnic/racial groups and higher in women than in men. TC I levels did not correlate with lactoferrin levels. Lactoferrin showed significant ethnic differences also, but, unlike TC I, its levels were highest in whites. TC I levels correlated with cobalamin but not homocysteine levels. Neither TC I nor lactoferrin correlated with chemistry panel results, including creatinine, total protein, albumin, lactate dehydrogenase, and alkaline phosphatase levels. The demonstration with an RIA that directly measures total TC I that plasma levels are significantly higher in blacks than in other groups may explain the well-known higher cobalamin levels in blacks. Surprisingly, plasma lactoferrin, which has the same cellular sources as TC I, does not correlate with plasma TC I levels and shows dissimilar demographic patterns; lactoferrin levels are highest in whites. These findings suggest that regulation and/or secretion of these 2 proteins differ even though their localization and expression patterns in myeloid precursors are similar.

Relationship between methylmalonic acid and cobalamin in uremia

Relationship between methylmalonic acid and cobalamin in uremia.

BACKGROUND

To evaluate the requirement for routine supplementation with vitamin B12 and to study the effect of a change from injection to oral B12 supplementation, we examined the relationship between cobalamin and methylmalonic acid in plasma from 67 patients on chronic hemodialysis, all in regular therapy with intramuscular cobalamin injections (1 mg) every third month.

METHODS

Starting just before one cobalamin injection, blood samples were collected once a month during a nine-month withdrawal from regular cobalamin substitution to a final three-month period with cyanocobalamin tablets (1 mg) administered once daily.

RESULTS

Plasma cobalamin was above the lower reference limit in all subjects, and from a peak value one month after the regular injection, the cobalamin concentration during the withdrawal period decreased to a level below the point of origin, followed by a significant rise after cyanocobalamin tablets. The methylmalonic acid concentrations were above the reference interval. In the withdrawal period, the concentrations significantly increased further, followed by a significant decrease after oral cyanocobalamin substitution.

CONCLUSION

We demonstrated a within-patient inverse relationship between the concentrations of methylmalonic acid and cobalamin in plasma from these uremic patients. Despite the fact that only two of the patients developed subnormal plasma cobalamin values, we demonstrated a B12 depletion during the withdrawal period. Treatment with cyanocobalamin tablets once daily was found efficient, but the oral doses should possibly be increased.

Cobalamin Deficiency in General Practice. Assessment of the Diagnostic Utility and Cost-Benefit Analysis of Methylmalonic Acid Determination in Relation to Current Diagnostic Strategies

Diagnosing cobalamin deficiency is often difficult. We investigated the diagnostic strategies that 224 general practitioners used to assess cobalamin status and the criteria on which they based their decisions to supplement patients. From all serum cobalamin analyses carried out at a single laboratory during 1993, individuals with serum cobalamin concentrations &lt;300 pmol/L were identified, and one patient per general practitioner was included. When serum methylmalonic acid (s-MMA) values &gt;0.376 μmol/L were used as the “reference standard” for cobalamin deficiency, the serum cobalamin assay had a diagnostic sensitivity of 0.40 and a specificity of 0.98. With the same reference standard, the diagnostic accuracy of the physicians’ decision to supplement patients had the same specificity but a higher sensitivity (0.51). Cost-benefit analysis indicated that measurement of s-MMA can be recommended in patients with serum cobalamin &gt;60–90 pmol/L and &lt;200–220 pmol/L, depending on its diagnostic accuracy.

Total serum homocysteine in senile dementia of Alzheimer type

OBJECTIVE

The main hypothesis was that subtle vitamin B12 deficiencies occur more commonly in senile dementia of Alzheimer type (SDAT) that in healthy elderly individuals, and may be revealed by elevated total serum homocysteine (tHcy). A subsidiary hypothesis was that such deficiencies would be nutritionally independent as determined by retinol binding protein (RBP).

DESIGN

A prospective case-controlled survey.

SETTING

A Welsh urban psychogeriatric assessment centre and local general practice.

PATIENTS

Thirty patients, aged 65 or over, seen consecutively in 1994 with features compatible with DSM-III-R criteria for primary degenerative dementia of Alzheimer type and 30 cognitively intact age-matched control subjects.

MEASURES

Diagnosis was assessed using the CAMDEX. Cognitive scores were evaluated with the CAMCOG scale for patients and MMSE scores for control subjects. THcy was measured using high performance liquid chromatography (HPLC), and RBP assayed by a radial immunodiffusion method.

RESULTS

Patients had a highly significant elevation of tHcy compared with control (p < 0.0001). Multiple regression highlighted the interrelated effects of tHcy and total serum cobalamin on cognitive scores. RBP did not differ between groups. Macrocytosis was absent, and neutrophil hypersegmentation uncommon, in hyperhomocysteinaemic patients.

CONCLUSIONS

SDAT patients have significantly elevated tHcy. This is independent of RBP determined nutritional status. 'Classical' haematological changes of cobalamin or folate deficiency are poor predictors of tHcy in these patients. Aberrant cobalamin tissue delivery appears to contribute to SDAT cognitive decline. Relative contributions of other tHcy determinants require further investigation.

Age-related changes in cobalamin (vitamin B12) handling. Implications for therapy

Cobalamin (vitamin B12) deficiency is more common in the elderly than in younger patients. This is because of the increased prevalence of cobalamin malabsorption in this age group, which is mainly caused by (autoimmune) atrophic body gastritis. Cobalamin supplementation is affordable and nontoxic, and it may prevent irreversible neurological damage if started early. Elderly individuals with cobalamin deficiency may present with neuropsychiatric or metabolic deficiencies, without frank macrocytic anaemia. An investigation of symptoms and/or signs includes the diagnosis of deficiency as well as any underlying cause. Deficiency states can still exist even when serum cobalamin levels are higher than the traditional lower reference limit. Cobalamin-responsive elevations of serum methylmalonic acid (MMA) and homocysteine are helpful laboratory tools for the diagnosis. The health-related reference ranges for homocysteine and MMA appear to vary with age and gender. Atrophic body gastritis is indirectly diagnosed by measuring serum levels of gastrin and pepsinogens, and it may cause dietary cobalamin malabsorption despite a normal traditional Schilling's test. The use of gastroscopy may also be considered to diagnose dysplasia, bacterial overgrowth and intestinal villous atrophy in healthy patients with atrophic body gastritis or concomitant iron or folic acid deficiency. Elderly patients respond to cobalamin treatment as fully as younger patients, with complete haematological recovery and complete or good partial resolution of neurological deficits. Chronic dementia responds poorly but should, nevertheless, be treated if there is a metabolic deficiency (as indicated by elevated homocysteine and/or MMA levels). Patients who are at risk from cobalamin deficiency include those with a gastrointestinal predisposition (e.g. atrophic body gastritis or previous partial gastrectomy), autoimmune disorders [type 1 (insulin-dependent) diabetes mellitus and thyroid disorders], those receiving long term therapy with gastric acid inhibitors or biguanides, and those undergoing nitrous oxide anaesthesia. To date, inadequate cobalamin intake has not proven to be a major risk factor. Intervention trials of cobalamin, folic acid and pyridoxine (vitamin B6) in unselected elderly populations are currently under way.

Limited value of serum holo-transcobalamin II measurements in the differential diagnosis of macrocytosis

AIM

To study the value of serum holo-transcobalamin II (holo-TCII) measurements in the differential diagnosis of macrocytosis.

METHODS

Holo-TCII concentrations were measured in serum samples from 50 healthy non-vegetarian subjects and 30 patients with macrocytosis, using a technique based on the adsorption of holo-TCII with amorphous, precipitated silica. Deoxyuridine (dU) suppression tests were performed on the bone marrow cells of all the patients. Haematological diagnoses were made using standard criteria.

RESULTS

The causes of macrocytosis were cobalamin (Cbl) deficiency due to pernicious anaemia or following partial gastrectomy (10 patients), dietary folate deficiency with/without Cb1 deficiency (four patients), chronic alcoholism (four patients), myelodysplastic syndrome (five patients), treatment with methotrexate or azathioprine (three patients), and congenital dyserythropoietic anaemia (CDA) (four patients). Undetectable or low holo-TCII concentrations were found in all patients with Cb1 deficiency and in some or all patients from each of the other diagnostic categories. There was also no correlation between the dU suppressed value and the holo-TCII concentration: all 15 patients with high dU suppressed values and nine of 15 with normal dU suppressed values, including four patients with CDA, had low holo-TCII concentrations.

CONCLUSIONS

Measurements of serum holo-TCII concentrations by the silica adsorption method are not of value in the differential diagnosis of macrocytosis. The finding of low serum holo-TCII concentrations in patients with macrocytosis due to causes other than Cb1 deficiency may result not only from a state of negative Cb1 balance but also from other factors, such as increased utilisation of holo-TCII as a consequence of erythroid hyperplasia.

Transcobalamin from cow milk: isolation and physico-chemical properties

The concentration of endogenous cobalamin (Cbl) in cow milk was 3.3 nM while the Cbl-binding capacity was 0.05 nM. Both endogenous and newly added Cbl showed similar quantitative distribution between a 280 kDa protein complex (45%) and a 43 kDa Cbl-binder (55%). Long time incubation, as well as urea treatment, was accompanied by a slow release of the 43 kDa Cbl-binder from the 280 kDa fraction. No other Cbl-binding proteins appeared after these procedures. The 43 kDa binder from cow milk, depleted of the ligand by urea treatment, reacted with Cbl even in the presence of a B12-analogue cobinamide (Cbi) at the ratio Cbl:Cbi = 1:40. The stokes radius of the binder changed from 2.7 nm for the Cbl-free protein to 2.5 nm for the Cbl-saturated form and the Cbl-saturated binder was able to displace human transcobalamin (TC) from the TC-receptor. The interaction between the protein and Cbl was significantly suppressed at pH 2.0. The N-terminal sequence of the purified 43 kDa Cbl-binder revealed homology with TC from human and rabbit plasma. In conclusion we have shown that TC is the main Cbl-binding protein in cow milk. This is surprising, since previous studies on human and rat milk have shown another Cbl-binder, apo-haptocorrin, to be the dominating Cbl-binding protein.

Cobalamin

Cobalamin (vitamin B12) is an essential nutrient derived exclusively from bacterial sources. It is an essential cofactor for three known enzymatic reactions. Untreated deficiency, caused by either the autoimmune disease pernicious anemia or nutritional lack, results in a macrocytic anemia and/or subacute combined degeneration of the spinal cord and is eventually fatal. Cobalamin in serum is bound to two proteins, transcobalamin and haptocorrin. The former is responsible for the essential delivery of cobalamin to most tissues. Inadequate tissue availability of cobalamin results in increased concentration of methylmalonic acid and homocyst(e)ine due to inhibition of methylmalonyl-CoA mutase and methionine synthase, respectively. Strict vegetarians have long been known to be at risk of cobalamin deficiency, which develops insidiously over many years. It is now clear that a significant number of the elderly and HIV-positive individuals are also at increased risk of deficiency. Any individual with reduced ability to split cobalamin from food-protein may also become deficient even though intrinsic factor is present. Diagnosis of cobalamin deficiency has frequently relied on total serum cobalamin and the Schilling test. Newer approaches such as analysis of methylmalonic acid, homocyst(e)ine, holotranscobalamin, anti-intrinsic factor antibodies, and serum gastrin may provide more cost-effective testing, as well as identify those with a covert deficiency.

Transcobalamin II and the membrane receptor for the transcobalamin II-cobalamin complex

Transcobalamin II is a plasma protein that binds vitamin B12 (cobalamin) as it is absorbed in the terminal ileum and distributes it to tissues. The circulating transcobalamin II-cobalamin complex binds to receptors on the plasma membrane of tissue cells and is then internalized by receptor-mediated endocytosis. A number of genetic abnormalities are characterized either by a failure to express transcobalamin II or by synthesis of an abnormal protein. These disorders result in cellular cobalamin deficiency and megaloblastic anaemia. In this chapter we review the structural and functional properties of transcobalamin II, the receptor for the transcobalamin-cobalamin complex and the clinical disorders that are associated with perturbation of circulating transcobalamin II. In addition, we provide emerging data about the molecular genetics of transcobalamin II which has emanated from our own and other laboratories.

Inherited errors of cobalamin metabolism and their management

Cobalamins are essential biological compounds structurally related to haemoglobin and the cytochromes. Although the basic cobalamin molecule is only synthesized by micro-organisms, all mammalian cells can convert this into the coenzymes adenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl). AdoCbl is the major form in cellular tissues, where it is retained in the mitochondria. MeCbl predominates in blood plasma and certain other body fluids such as breast milk; in cells MeCbl is found in the cytosol. Inherited disorders of cobalamin metabolism are single gene defects, transmitted as recessive traits. They affect absorption, transport or intracellular metabolism of cobalamin. At least 12 different mutations are known, including defects or deficiencies of IF, IF-receptor and TCII, MM-CoA mutase and of the various reductases and synthases required for synthesis of AdoCbl and MeCbl. These have been designated cblA to cblG. Abnormalities are detectable by urine and plasma assays of methylmalonic acid and homocysteine, and plasma and erythrocyte analysis of cobalamin coenzymes, which can reveal deficiencies of MeCbl or AdoCbl. Fibroblast studies discriminate between closely similar defects. In man, AdoCbl is required in only two reactions: the catabolic isomerization of MM-CoA to succinyl-CoA and interconversion of alpha- and beta-leucine. MeCbl is required in the anabolic transmethylation of homocysteine to methionine. Intestinal absorption of cobalamin requires the glycoproteins TCI and IF from the stomach and IF-cobalamin receptors in the ileum. Cobalamin is transported to cells bound to a polypeptide, TCII, is captured by surface receptors and absorbed by endocytosis. The complex is then split in the lysosomes, cobalamin is released and the coenzymes are synthesized. In plasma, 80-90% of the cobalamin is bound to TCI, whose function is uncertain. Megaloblastic anaemia at birth or in the first few weeks of life is a rare but serious event. Myelopathy and developmental delay, with or without seizures may also occur without anaemia. If urine and light-protected blood samples are collected and sent to an appropriate metabolic unit, an inborn error of cobalamin metabolism, including TCII deficiency in which the serum B12 may be normal, can quickly be diagnosed. IF deficiency or Imerslund-Gräsbeck disease usually presents with signs of cobalamin deficiency within the first year of life and can be diagnosed by absorption studies. Current treatment involves dietary protein restriction and/or parenteral OHCbl and the prognosis is very variable. Since lack of MeCbl leads to depressed DNA synthesis affecting rapidly dividing cells in the brain and elsewhere, treatment with this coenzyme should be considered at the earliest stage in appropriate cases.(ABSTRACT TRUNCATED AT 400 WORDS)

How to diagnose cobalamin deficiency

Cobalamin deficiency must be suspected in all patients with unexplained neuropsychiatric symptoms or unexplained anemia. Special attention should be paid to patients at risk of developing cobalamin deficiency such as elderly people, vegetarians, HIV-infected patients, patients with gastrointestinal diseases and patients with autoimmunity or a family history of pernicious anemia. The assays aimed to answer the question: does this patient suffer from cobalamin deficiency, include analysis of P--cobalamins and analyses of the metabolites that accumulate upon cellular cobalamin deficiency, P--methylmalonate and P--homocysteine. P--cobalamins or especially a fraction of P--cobalamins, P--TC cobalamins are markers for latent cobalamin deficiency. An increased concentration of P--methylmalonate that decreases upon injection of cobalamin indicates overt metabolic cobalamin deficiency. The same holds for P--homocysteine but this analysis is less specific than P--methylmalonate. We suggest that either assay of P--cobalamins or P--methylmalonate is employed as screening test for cobalamin deficiency, and that further tests are performed only if the initial test in combination with the clinical picture gives an unclear answer. Once cobalamin deficiency has been diagnosed, the cause for the deficiency should be sought and the patient should be treated for life. Cobalamin absorption tests such as the Schilling test are considered of limited use. Gastric atrophy is likely to be present in patients with increased P--gastrin or decreased P--pepsinogen A. However, this condition can be diagnosed also by upper gastrointestinal endoscopy.