Application of a simple immunoadsorption assay for the measurement of saturated and unsaturated transcobalamin II and R-binders

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.

Holotranscobalamin, a marker of vitamin B-12 status: analytical aspects and clinical utility

Approximately one-quarter of circulating cobalamin (vitamin B-12) binds to transcobalamin (holoTC) and is thereby available for the cells of the body. For this reason, holoTC is also referred to as active vitamin B-12. HoloTC was suggested as an optimal marker of early vitamin B-12 deficiency >20 y ago. This suggestion led to the development of suitable assays for measurement of the compound and clinical studies that aimed to show the benefit of measurement of holoTC rather than of vitamin B-12. Today holoTC can be analyzed by 3 methods: direct measurement of the complex between transcobalamin and vitamin B-12, measurement of vitamin B-12 attached to transcobalamin, or measurement of the amount of transcobalamin saturated with vitamin B-12. These 3 methods give similar results, but direct measurement of holoTC complex is preferable in the clinical setting from a practical point of view. HoloTC measurement has proven useful for the identification of the few patients who suffer from transcobalamin deficiency. In addition, holoTC is part of the CobaSorb test and therefore useful for assessment of vitamin B-12 absorption. Clinical studies that compare the ability of holoTC and vitamin B-12 to identify individuals with vitamin B-12 deficiency (elevated concentration of methylmalonic acid) suggest that holoTC performs better than total vitamin B-12. To date, holoTC has not been used for population-based assessments of vitamin B-12 status, but we suggest that holoTC is a better marker than total vitamin B-12 for such studies.

Biomarkers of cobalamin (vitamin B-12) status in the epidemiologic setting: a critical overview of context, applications, and performance characteristics of cobalamin, methylmalonic acid, and holotranscobalamin II

Cobalamin deficiency is relatively common, but the great majority of cases in epidemiologic surveys have subclinical cobalamin deficiency (SCCD), not classical clinical deficiency. Because SCCD has no known clinical expression, its diagnosis depends solely on biochemical biomarkers, whose optimal application becomes crucial yet remains unsettled. This review critically examines the current diagnostic concepts, tools, and interpretations. Their exploration begins with understanding that SCCD differs from clinical deficiency not just in degree of deficiency but in fundamental pathophysiology, causes, likelihood and rate of progression, and known health risks (the causation of which by SCCD awaits proof by randomized clinical trials). Conclusions from SCCD data, therefore, often may not apply to clinical deficiency and vice versa. Although many investigators view cobalamin testing as unreliable, cobalamin, like all diagnostic biomarkers, performs satisfactorily in clinical deficiency but less well in SCCD. The lack of a diagnostic gold standard limits the ability to weigh the performance characteristics of metabolic biomarkers such as methylmalonic acid (MMA) and holotranscobalamin II, whose specificities remain incompletely defined outside their relations to each other. Variable cutoff selections affect diagnostic conclusions heavily and need to be much better rationalized. The maximization of reliability and specificity of diagnosis is far more important today than the identification of ever-earlier stages of SCCD. The limitations of all current biomarkers make the combination of ≥2 test result abnormalities, such as cobalamin and MMA, the most reliable approach to diagnosing deficiency in the research setting; reliance on one test alone courts frequent misdiagnosis. Much work remains to be done.

Neurologic presentations of nutritional deficiencies

Optimal functioning of the central and peripheral nervous system is dependent on a constant supply of appropriate nutrients. The first section of this review discusses neurologic manifestations related to deficiency of key nutrients such as vitamin B(12), folate, copper, vitamin E, thiamine, and others. The second section addresses neurologic complications related to bariatric surgery. The third sections includes neurologic presentations caused by nutrient deficiencies in the setting of alcoholism. The concluding section addresses neurologic deficiency diseases that have a geographic predilection.

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.

Nutritional neuropathies

Optimal functioning of the central and peripheral nervous system is dependent on appropriate nutrients. Neurologic consequences of nutritional deficiencies are not restricted to underdeveloped countries. Multiple nutritional deficiencies can coexist. Obesity is of particular concern in the developed world. The rising rate of bariatric surgery are accompanied by neurologic complications related to nutrient deficiencies. Prognosis depends on prompt recognition and institution of appropriate therapy. This review discusses peripheral nervous system manifestations related to the deficiency of key nutrients, neurologic complications associated with bariatric surgery, and conditions that have a geographic significance associated with bariatric surgery and certain conditions that have a geographic predilection.

Low Folate and the Risk of Cognitive and Functional Deficits in the Very Old: The Monzino 80-plus Study

OBJECTIVE

To cross sectionally investigate the association of serum vitamin B(12) and folate concentrations with cognitive and functional ability in the very old in the general population.

METHODS

Serum vitamin B(12) and folate concentrations were assessed in 471 consenting subjects participating in the Monzino 80-plus study (mean age: 87.4 years), a door-to-door population-based survey among very old subjects living in Northern Italy. Cognitive and functional evaluations included Mini-Mental State Examination (MMSE), Instrumental Activities of Daily Living (IADL) and Spontaneous Behavior Interview-basic Activities of Daily Living (SBI-bADL).

RESULTS

MMSE, IADL and SBI-bADL scores were all significantly correlated with folate concentrations (respectively: r = 0.36, r = -0.39, r = -0.35; p < 0.0001), while no significant associations were found with vitamin B(12) concentrations. When entered into multiple linear regression analyses with several covariates, folate showed a highly significant, curvilinear association with both cognitive and functional scores (p < 0.0001). Subjects in low and middle folate tertiles had significantly higher odds ratios for dementia (p < 0.0001; adjusted ORs = 5.40 and 6.56, lower 95% CIs 2.53 and 3.11, higher 95% CIs 12.73 and 15.29).

CONCLUSIONS

Findings of this population-based study suggest that subclinical folate deficiency may represent a risk factor for the cognitive decline associated with aging that could contribute to AD as well as other dementia development.

Effect of Vitamin B12 Treatment on Haptocorrin

Background: Haptocorrin (HC) carries the major part of circulating cobalamin, but whether HC is altered on treatment with vitamin B12 remains unknown.

Methods: Our study included 3 populations: a population of vegan men (n = 174; vegan population), of whom 63 were treated daily with 5 mg of oral vitamin B12 for 3 months; a group of patients with a previous methylmalonic acid (MMA) concentration &gt;0.4 μmol/L (n = 140; population with suspected deficiency), of which 69 were treated with weekly vitamin B12 injections (1 mg) for 4 weeks; and a subgroup of participants in a vitamin B intervention study (n = 88; nondeficient population), of whom 45 were treated daily with 0.4 mg of oral vitamin B12 for 3 months. Total HC and holoHC were measured by ELISA. Cobalamin was measured by an intrinsic factor (IF)-based assay. Samples were collected at baseline and 3 months after start of treatment.

Results: Compared with baseline results for the 3 study populations, total HC and holoHC increased 30 pmol/L for every 100 pmol/L increase in cobalamin. After treatment with vitamin B12, holoHC (P &lt;0.0001) and total HC (P &lt;0.0001) increased significantly in the vegan population. Only holoHC increased in the population with suspected deficiency (P &lt;0.0001), whereas no alteration was observed in the nondeficient population.

Conclusions: The HC concentration is decreased in severely cobalamin-deficient individuals and increases on treatment. The concentration of cobalamin also relates significantly to the HC concentration in nondeficient individuals.

Glycosylation independent measurement of the cobalamin binding protein haptocorrin

BACKGROUND

Haptocorrin carries the major part of the circulating vitamin B12. The protein is heavily glycosylated and this may have implications for its measurement.

METHODS

We used two different ELISA assays. In one assay, we employed antibodies against native HC and no pre-treatment of samples or calibrators. In the other assay, we used antibodies raised against deglycosylated HC, and deglycosylated the samples and calibrators by treatment with neuraminidase and PNGase prior to analysis. Plasma samples from healthy donors were analysed.

RESULTS

The ELISA against native HC showed a high detection limit (71 pmol/l) and a poor linearity for serial dilutions of samples. The ELISA against deglycosylated HC showed a detection limit of 1.6 pmol/l, an excellent linearity between 1.6 and 100 pmol/l (r(2) = 0.99) and an inaccuracy of 5% for concentrations ranging from 250 to 840 pmol/l. The 95% reference interval was 240-680 pmol/l (n = 148). The concentration of HC showed a strong association to plasma cobalamins (p < 0.0001).

CONCLUSIONS

An ELISA against native HC does not ensure an equimolar measurement of HC, while this is the case when a glycosylation independent assay is employed. Using this assay, a very strong correlation between total plasma HC and cobalamins in healthy donors is obtained.

Influence of cobalamin deficiency compared with that of cobalamin absorption on serum holo-transcobalamin II

BACKGROUND

Cobalamin attached to transcobalamin II (TC II), known as holo-TC II, is the active cobalamin fraction taken up by tissues. Holo-TC II is also the form in which absorbed cobalamin enters the circulation from the ileum. Therefore, holo-TC II has been proposed variously as a marker of cobalamin adequacy, cobalamin absorption, or both, including even its advocacy as a surrogate Schilling test. Such claims carry conflicting diagnostic implications because metabolic adequacy and absorption are not identical.

OBJECTIVE

The objective was to examine metabolic and absorptive influences on holo-TC II.

DESIGN

Treated patients with pernicious anemia (PA), who have abnormal absorption but a normal metabolic status, were chosen as the model to differentiate between the effects of the 2 cobalamin-related characteristics. Serum holo-TC II and indexes of cobalamin metabolism in 23 treated patients were compared with those of 6 untreated PA patients (abnormal absorption and metabolic status) and 33 control subjects (normal absorption and metabolic status).

RESULTS

Holo-TC II, which correlated directly with cobalamin and inversely with homocysteine, was significantly higher in treated PA patients in metabolic remission than in untreated PA patients (74 +/- 59 compared with 9 +/- 6 pmol/L) and was significantly lower than in control subjects (105 +/- 58 pmol/L), although the latter difference was small and the values overlapped greatly.

CONCLUSIONS

Metabolic cobalamin status is a major determinant of serum holo-TC II. Absorption status may have mild influence as well, although other explanations remain possible. Serum holo-TC II cannot be used clinically to diagnose cobalamin malabsorption because of overlap with normal values. The influences on holo-TC II are complex and require careful analysis.

Mild transcobalamin I (haptocorrin) deficiency and low serum cobalamin concentrations

BACKGROUND

Low cobalamin concentrations are common, but their causes are often unknown. Transcobalamin I/haptocorrin (TC I/HC) deficiency, viewed as a rare cause, has not been examined systematically in patients with unexplained low serum cobalamin.

METHODS

Total TC I/HC was measured by RIA in three subgroups of 367, 160, and 38 patients with different categories of low cobalamin concentrations and three comparison subgroups of 112, 281, and 119 individuals with cobalamin concentrations within the reference interval. Additional studies, including family studies, were done in selected patients found to have low TC I/HC concentrations.

RESULTS

Low TC I/HC concentrations suggestive of mild TC I/HC deficiency occurred in 54 of 367 (15%) patients with low cobalamin identified by clinical laboratories and 24 of 160 (15%) patients whose low cobalamin was unexplained after absorption and metabolic evaluation, but in only 2 of 38 patients with malabsorptive causes of low cobalamin concentrations (5%). The prevalence was only 3% (8 of 281 plasma samples) to 5% (6 of 112 sera) in patients with cobalamin concentrations within the reference interval and 3% (4 of 119) in healthy volunteers. Three patients with low cobalamin (0.6%) had severe TC I/HC deficiency with undetectable TC I/HC. Presumptive heterozygotes for severe TC I/HC deficiency in two families had the findings of mild TC I/HC deficiency; mild deficiency was also found in at least three of seven studied families of patients with mild TC I/HC deficiency.

CONCLUSIONS

Mild TC I/HC deficiency is frequently associated with low cobalamin, is often familial, and its biochemical phenotype appears identical to the heterozygous state of severe TC I/HC deficiency. Severe TC I/HC deficiency also appears to be more common than suspected. Both diagnoses should be considered in all patients with unexplained low serum cobalamin.

RIA for serum holo-transcobalamin: method evaluation in the clinical laboratory and reference interval

BACKGROUND

Decreased serum holo-transcobalamin (holoTC) could be the earliest marker of cobalamin (Cbl) deficiency, but there has been no method suitable for routine use. We evaluated a new commercial holoTC RIA, determined reference values, and assessed holoTC concentrations in relation to other biochemical markers of Cbl deficiency.

METHODS

The reference population consisted of 303 individuals 22-88 years of age, without disease or medication affecting Cbl or homocysteine metabolism. In elderly individuals (>or=65 years), normal Cbl status was further confirmed by total homocysteine (tHcy; <19 micro mol/L) and methylmalonic acid (MMA; <0.28 micro mol/L) concentrations within established reference intervals. HoloTC in Cbl deficiency was studied in a population of 107 elderly individuals with normal renal function. The Cbl deficiency was graded as potential (total Cbl <or=150 pmol/L or tHcy >or=19 micro mol/L), possible (total Cbl <or=150 pmol/L and either tHcy >or=19 micro mol/L or MMA >or=0.45 micro mol/L), and probable (tHcy >or=19 micro mol/L and MMA >or=0.45 micro mol/L).

RESULTS

The intra- and between-assay imprecision (CV) for the holoTC RIA were 4-7% and 6-8%, respectively. A 95% central reference interval for serum holoTC was 37-171 pmol/L. All participants (n = 16) with probable Cbl deficiency, 86% of those with possible, and 30% of those with potential Cbl deficiency had holoTC below the reference limit (<37 pmol/L). The holoTC correlated with total Cbl (r(s) = 0.80; P <0.0001) and inversely with MMA (r(s) = -0.52; P <0.0001). HoloTC concentrations were significantly (P = 0.01) higher in women than in men.

CONCLUSIONS

The new holoTC RIA is precise and simple to perform. Low holoTC is found in individuals with biochemical signs of Cbl deficiency, but the sensitivity and specificity of low holoTC in diagnosis of Cbl deficiency need to be further evaluated.

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.

Cobalamin binding proteins in patients with HIV infection

P-Cobalamins have been reported to be decreased in patients with HIV infection. Because of this, we found it of interest to examine both cobalamin-saturated binding proteins (holo-transcobalamin, holo-TC and holo-haptocorrin, holo-HC) and cobalamin unsaturated binding proteins (apo-transcobalamin, apo-TC and apo-haptocorrin, apo-HC). The results are given as range and (median). Eighteen male HIV-infected patients with plasma cobalamins below 200 pmol/l were studied. We found low concentrations of holo-TC (37-88 (47.5) pmol/l) and holo-HC (64-184 (135.5) pmol/l). The concentration of apo-TC and apo-HC was increased (480-1730 (1025) pmol/l; 70-800 (235) pmol/l). It is concluded that, in HIV-infected patients, low plasma cobalamin does not reflect a low concentration of transcobalamin or haptocorrin. In 20 HIV-infected patients and 31 patients with malignant haematological diseases, the TC isopeptide patterns were determined. In the HIV group, an increased frequency of TC isopeptide X was found and the overall distribution of TC isopeptides was significantly different from the reference population (p less than 0.05). There was no difference between the group of patients with malignant haematological diseases and the reference group.

Separation of transcobalamin II isoproteins by means of chromatofocusing

Transcobalamin II, the principal cobalamin-binding protein in human plasma, expresses a genetic polymorphism. Four more or less common alleles, denoted by X, S, M and F, have been defined earlier by means of gel electrophoretic techniques followed by autoradiography. This technique is less suitable for the analysis of individual samples and requires long exposure times. This paper describes the analysis of transcobalamin II phenotypes by means of fast protein liquid chromatofocusing. This technique has the advantage that the results of the analysis of several samples can be obtained within a day, and it also seems applicable to the preparative separation of transcobalamin II isoproteins. The sequence of elution of the isoproteins was in complete accordance with the banding pattern obtained by electrophoretic separation. The characteristic doublet bands found with polyacrylamide gel electrophoresis were less obvious in the chromatofocusing elution pattern.

Application of heparin-conjugated Sepharose for the measurement of cobalamin-saturated and unsaturated transcobalamin II

The cobalamin-binding plasma protein transcobalamin II has a high affinity for the anticoagulant heparin. This phenomenon has been exploited in a new method for the quantification of cobalamin-saturated (holo-) and unsaturated (apo-) transcobalamin II in human plasma. Transcobalamin II is adsorbed from human plasma to heparin-conjugated Sepharose under suitable conditions and either cobalamin from adsorbed holo-transcobalamin II is measured by a radioisotope dilution assay or apo-transcobalamin II is determined by measuring the adsorbed unsaturated cobalamin-binding capacity with radioactive cobalamin. The assay results for apo- and holo-transcobalamin II are similar (r = 0.99 and 1.0, respectively) to those obtained with the established radioimmunosorbent assay using specific rabbit anti-human transcobalamin II-conjugated Sepharose. The assay cannot be carried out in heparin-anticoagulated plasma, because the free heparin competes with the immobilized heparin for the binding of transcobalamin II. The amount of heparin in plasma from patients being treated with subcutaneous or intravenous heparin is too low to interfere significantly with the measurement of transcobalamin II. Also the presence of circulating anti-transcobalamin II antibodies, as occur in some rare patients after frequent intramuscular injections of cobalamin, does not influence the assay.

Variant-specific differences in human unsaturated transcobalamin II

Electrophoresis and subsequent autoradiography of 57Co-cobalamin (57 Co-Cbl)-labeled serum show intensity differences between the genetic variants of human transcobalamin II (TC2), suggesting differences in the unsaturated (apo-) TC2 concentration. In order to distinguish between variant-specific differences in the Cbl binding affinity and those in the total-TC2 concentration, techniques were developed to determine total, apo-, and holo-TC2. Prolonged incubation at 37 degrees C with a 20-fold excess of 57Co-Cbl resulted in an almost complete exchange of endogenously bound Cbl, which allowed determination of the total TC2. The holo-TC2 concentration of both gene products in TC2 heterozygotes could be estimated by comparison of the labeling levels of apo- and total TC2, using densitometric quantification of the autoradiographs. By means of ion-exchange chromatography, TC2 could be separated from other Cbl-binding proteins, permitting a simple quantitative assay of apo- and total TC2, the results of which correlate fairly well with those measured by an immunoadsorption assay. The results obtained in the present investigation indicate that the variant-specific variation in the apo-TC2 concentration is caused by differences in the total-TC2 concentration rather than in the Cbl binding affinity.

Elevated serum vitamin B12 in cystic fibrosis

In 62 patients with cystic fibrosis the serum vitamin B12 concentration ranged from 160-2600 pmol/l with a mean of 1 105 pmol/l. Both vitamin B12-binding proteins in the serum, transcobalamin II and R-binders, carried increased amounts of vitamin B12, but showed relatively normal levels of unsaturated vitamin B12-binding capacity. This combination is rather typical for hepatic dysfunction, although the recurrent pulmonary infections might exert an upward effect on plasma R-binder concentration through increased turnover of myeloid cells. A significant positive correlation between transcobalamin II-vitamin B12 and serum alkaline phosphatase suggests that transcobalamin II-bound vitamin B12 might be an early indicator of focal biliary cirrhosis, which is known to occur in these patients.