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

Abstract

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.

FIGURE 1.

Cobalamin [vitamin B-12 (B12)] and its binding proteins in human plasma. The figure shows the relation between the total concentration of the vitamin B-12 binding proteins in plasma and the distribution of vitamin B-12 and its analogs on the proteins. The average concentrations used are as follows—total transcobalamin: 1000 pmol/L [holotranscobalamin (HoloTC): 100 pmol/L; apotranscobalamin (ApoTC): 900 pmol/L]; total haptocorrin: 450 pmol/L [vitamin B-12 bound to haptocorrin (B12HC): 200 pmol/L; vitamin B-12 analogs bound to haptocorrin (AnaHC): 200 pmol/L; apohaptocorrin (ApoHC): 50 pmol/L]; total vitamin B-12: 300 pmol/L.

FIGURE 2.

Markers of cobalamin [vitamin B-12 (B12)] status. The figure shows the relation between vitamin B-12 deficiency markers. Holotranscobalamin (holoTC) transports vitamin B-12 into the cells by binding to a specific receptor, CD320. In the cell, vitamin B-12 acts as a coenzyme for 2 enzymes. Methionine synthetase (5-methyltetrahydropteroyl-l-glutamate:l-homocysteine-S-methyltransferase; EC 2.1.1.13), present in the cytoplasm, converts homocysteine (tHcy) to methionine. Methylmalonyl–coenzyme A (-CoA) mutases (methylmalonyl-CoA CoA-carbonylmutase; EC 5.4.99.2), which are present in the mitochondria, are involved in the conversion of methylmalonyl-CoA to succinyl-CoA. A decrease in holoTC results in a decrease in the cellular uptake of vitamin B-12. Once the cells become deficient, methylmalonic acid (MMA) and tHcy accumulate in the bloodstream; total vitamin B-12 concentration decreases.

FIGURE 3.

Holotranscobalamin (holoTC) and cobalamin (vitamin B-12) in relation to methylmalonic acid (MMA). The figure shows combined data from Hvas and Nexo (32) and Clarke et al (56). The figure includes 1842 data sets divided into tertiles in accordance with elevating MMA values. The x axis denotes the mean MMA value in each group. The upper panel shows the fractional change in holoTC (•) and haptocorrin-bound vitamin B-12 (total vitamin B-12 minus holoTC values; ○). To calculate the fractional change, we divided the mean for each group by the mean for the group with the lowest MMA values. The lower panel shows the absolute values for holoTC (•), haptocorrin-bound vitamin B-12 (○), and total vitamin B-12 (×). The figure depicts the mean (±SEM) for each group.