Active-B12 (holotranscobalamin) ELISA

Active-B12 (holotranscobalamin) ELISA

The Active-B₁₂ (Holotranscobalamin) ELISA is an enzyme immunoassay (ELISA) for the quantitative determination of holotranscobalamin (HoloTC) in human serum. HoloTC (vitamin B₁₂ bound to transcobalamin) is used as an aid in the diagnosis and treatment of vitamin B₁₂ deficiency.

Introduction

Three binding proteins are involved in the transport of vitamin B₁₂ around the body - Intrinsic Factor (IF), transcobalamin (TC) and haptocorrin (HC). These binding proteins ensure the efficient uptake of the very small amounts of vitamin B₁₂ available from the diet. When TC and HC bind vitamin B₁₂ the resulting complexes are known as holotranscobalamin (HoloTC) and holohaptocorrin (HoloHC) to distinguish them from the proteins carrying no vitamin. The major fraction in the circulation, HoloHC, represents 70-90% of vitamin B₁₂ in the blood but is biologically inert. HoloTC represents only 10-30% of vitamin B₁₂ circulating in the blood but is the only form of vitamin B₁₂ that can be taken up by cells in the body. The TC protein alone transports vitamin B₁₂ from its site of absorption in the ileum to tissues and cells. The vitamin is then internalised as the HoloTC (vitamin B₁₂ bound to transcobalamin) complex via a specific receptor-mediated uptake. This process delivers vitamin B₁₂ into the cells of the body and provides the vitamin as a co-enzyme for essential cellular functions such as DNA synthesis. As HoloTC has a shorter circulating half-life compared to HoloHC the earliest change that occurs on entering negative vitamin B₁₂ balance is very likely to be a decrease in serum HoloTC concentration (1).
The measurement of Total Serum B₁₂ suffers from some limitations; in particular, most of the measured cobalamin is bound to biologically inert HC. Several studies have been published which conclude that HoloTC would be a better indicator of vitamin B₁₂ status than Total Serum B₁₂ (2,3). As expected, HoloTC levels are low in patients with biochemical signs of vitamin B₁₂ deficiency (4). Low values have been reported in vegetarians (5), vegans (6), and in populations with a low intake of vitamin B₁₂ (7). Notably, low levels of HoloTC but not Total B₁₂ in serum were reported in patients with Alzheimer’s disease compared to levels in a healthy control group (8). HoloTC levels reflect vitamin B₁₂ status, independent of recent absorption of the vitamin (9).

Improved diagnosis of Vitamin B₁₂ deficiency

An independent study showed that Active-B₁₂ (HoloTC) is a more sensitive and more specific biomarker for assessing Vitamin B₁₂ deficiency than Total-B₁₂ and Methylmalonic Acid (MMA).

Table 1: Sensitivity and specification comparsion between Active-B₁₂, Total-B₁₂ and Methylmalonic Acid (MMA) for assessing Vitamin B₁₂ deficiency

IBL International Active-B₁₂ (HoloTC) ELISA outcome algorithm

Subjects at risk of B₁₂ deficiency

*Please note that leading experts in the Vitamin B12 field recommend a cut-off value of 35pmol/L for most populations (Herrmann W., Obeid R., Schorr H., Geisel J. The usefulness of holotranscobalamin in predicting Vitamin B12 status in different clinical settings. J Curr Drug Metab. 2005; 6(1):47-53.)

External comparison study shows excellent correlation between IBL International’s Active-B₁₂ (HoloTC) ELISA and the Abbott AxSYM^® Active-B₁₂-Assay