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Effect of TCN2 776C>G on vitamin B 12 cellular availability in end-stage renal disease patients. Background Transcobalamin II is a serum protein that transports vitamin B 12 from the intestine to the tissues. This complex, holo-transcobalamin II, may reflect vitamin B 12 availability in the body. Conflicting data exist with regard to the effect of a polymorphism in the gene coding for transcobalamin II, TCN2 776C>G, on transcobalamin II levels in the general population, which in turn may affect holo-transcobalamin II, vitamin B 12 , as well as total homocysteine (tHcy) plasma levels. The effect of TCN2 776C>G on vitamin B 12 cellular availability in dialysis patients is unknown. Methods We examined the effect of TCN2 776C>G on holo-transcobalamin II, vitamin B 12 , and tHcy plasma concentrations in 120 dialysis patients. Results Holo-transcobalamin II levels were normal or supranormal in all patients and showed a linear association with albumin ( r = 0.205, P = 0.025) and with vitamin B 12 ( r = 0.778, P = 0.001), but not with age, creatinine, body mass index, tHcy, ln-tHcy, vitamin B 6 , plasma folate, and red blood cell folate concentration. TCN2 776C>G showed no effect on holo-transcobalamin II, vitamin B 12 , and tHcy concentration [one-way analysis of variance (ANOVA), post-hoc Scheffe test]. Multiple linear regression analyses showed that albumin and B 12 are independently associated with holo-transcobalamin II, whereas TCN2 776C>G and MTHFR 677C>T had no effect. Independent predictors of ln-tHcy included creatinine, red blood cell folate, and the MTHFR 677TT genotype. There was also an effect of the TCN2 776CC genotype on ln-tHcy levels in this multivariate analysis, however, that deserves cautious interpretation because there was no effect of TCN2 genotypes by ANOVA and Scheffe test [median ln-tHcy concentrations according to TCN2 genotypes (μmol/L): CC, 3.22; CG, 3.30; GG, 3.23]. Conclusion TCN2 776C>G does not influence holo-transcobalamin II or vitamin B 12 levels, and has no major effect on tHcy concentrations of end-stage renal disease patients.

At one time, diagnosing cobalamin deficiency was fairly simple, usually involving a patient with clinical problems, and was usually settled by determining whether the serum cobalamin concentration was low or not. As reviewed elsewhere (1), things began to change after sensitive metabolic tests were introduced and as attention extended to the cobalamin status of asymptomatic persons. Metabolic studies confirmed that most low cobalamin concentrations in asymptomatic patients and seemingly healthy persons represented subclinical cobalamin insufficiency, but ∼30–40% of these cobalamin concentrations did not represent insufficiency and thus could be considered “falsely low”. The diagnostic reliability of serum cobalamin was further challenged by metabolic demonstrations of “falsely normal” cobalamin concentrations; these too occurred most often, but not exclusively, in asymptomatic persons. The search continues for the optimal test to diagnose deficiency because the metabolic tests also have disadvantages. Increased plasma total homocysteine is too nonspecific; methylmalonic acid determination, although posing fewer problems of specificity, is complex and expensive; and the deoxyuridine suppression test is too unwieldy for practical use. Interest has been drawn to the possible benefit of measuring only the cobalamin attached to transcobalamin (TC; also called TC II; the TC-cobalamin complex is called holo-TC or holo-TC II) rather than the total cobalamin content of plasma. The concept, suggested by Lindemans et al. in 1983 (2), is simple and attractive: holo-TC contains the biologically available cobalamin because only TC promotes specific uptake of its cobalamin by all cells. The much larger fraction of serum cobalamin carried by haptocorrin (HC; also called TC I, R binder, or sometimes cobalophilin) is considered metabolically inert because no cellular receptors exist for holo-HC (also called holo-TC I). However, the physiologic cycle of holo-TC is quite complex: ( a ) ileal holo-TC enters the portal circulation, carrying cobalamin absorbed from the gut (3); ( b ) some of the …

We describe here a competitive inhibition radioassay of vitamin B12 in serum and erythrocytes, including differential radioassays to measure total corrinoids, “true” vitamin B12 (cobalamin), and vitamin B12 analogues in the assay of Lau et al.13 as modified by Kolhouse et al.15, in which the ligands are pure IF and R-binders. We also describe a method to determine holo-TC II (i.e., vitamin B12 on TC II) and holo-haptocorrin (i.e., vitamin B12 on TC I and TC III) by adsorbing TC II from haptocorrin (TC I and III) with microfine precipitate of silica (Quso). This procedure adds only one step to the vitamin B12 assay, and delineates negative vitamin B12 balance of patients at a very early stage of developing deficiency, well before the total serum vitamin B12 level falls below the laboratory range of normal.

AIMS--To study the correlations between total vitamin B12(B12), holo-haptocorrin, and holo-transcobalamin II (holo-TCII) concentrations in human sera; the association between reduced holo-TCII concentrations and macrocytosis attributable to B12 deficiency. METHODS--Serum samples from 38 healthy volunteers, 113 patients with normal total serum B12 concentrations and 93 patients with low total serum B12 were studied. Holo-TCII was removed from whole serum by adsorption with amorphous precipitated silica, and both whole serum and adsorbed serum were assayed for B12 using the Becton Dickinson vitamin B12 [57Co] radioassay kit. RESULTS--In all three groups of subjects studied there were strong correlations between the logarithms of the total serum B12 and the holo-haptocorrin concentrations with regression coefficients between 0.884 and 0.967. By contrast, the correlations between the logarithms of the total serum B12 and holo-TCII concentrations were weaker, especially in the patients with normal or low total serum B12, for whom the regression coefficients were 0.491 and 0.391, respectively. Analysis of the clinical records of a proportion of the patients studied indicated that there were many more patients with low holo-TCII concentrations than with haematological disturbances related to B12 deficiency. CONCLUSIONS--The total serum B12 concentration is a relatively poor indicator of holo-TCII concentrations and, therefore, of the ability of serum to deliver B12 to tissues. Additional information regarding B12 values can therefore be gleaned from measuring holo-TCII concentrations in the serum. Low holo-TCII concentrations, however, are an early sign of negative B12 balance and are frequently unassociated with haematological abnormalities caused by B12 deficiency.