Active-B12 (holotranscobalamin) ELISA

Early and accurate measurement of Vitamin B₁₂ deficiency

is essential for preventing the occurrence of irreversible neurological dysfunction. IBL International offers the Active-B₁₂ (holotranscobalamin) ELISA for measuring Vitamin B₁₂ which is bound to transcobalamin (TC), the only biologically active fraction that can be delivered to cells. Less than 30% of the total Vitamin B₁₂ in plasma circulates as Active-B₁₂ (HoloTC). The remaining 70% are bound to haptocorrin (HoloHC). Because changes in HoloTC levels occur earlier than Total-B₁₂ in Vitamin B₁₂ depletion, it constitutes a faster way to prove Vitamin B₁₂ deficiency. Independent clinical studies show the improved clinical diagnostic value obtained with measuring Active-B₁₂ compared to assessing the Total Plasma Vitamin B₁₂ concentration. The Active-B₁₂ (holotranscobalamin) ELISA does not require pre-analytical steps, which is the main source of laboratory variation.

Active B12 (holotranscobalamin) ELISA

The Active-B12 (Holotranscobalamin) assay is a manual enzyme-immunoassay (EIA) 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, in apparently healthy individuals. For professional use only.

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-B showed that Active-B (HoloTC) is a more sensitive and more specific biomarker for assessing Vitamin B deficiency than Total-B and Methylmalonic Acid (MMA). Table 1: Sensitivity and specification comparison between Active-B , Total-B and Methylmalonic Acid (MMA) for assessing Vitamin B deficiency

Figure: Measurement of B-12-status with the Active-B12 (HoloTC) ELISA. [2]

Excerpt from the Instructions for Use

Enzyme immunoassay for the quantitative determination of holotranscobalamin (HoloTC) in human serum.

Vitamin B12 deficiency is a major public health issue; it is widespread, but the elderly are the group most at risk. It is estimated that 10-15% of over-65’s will have B12 deficiency. Neurological symptoms may be the only sign of deficiency, and can be irreversible if treatment is delayed. The only biologically-active fraction that can be delivered to cells is Vitamin B12 attached to transcobalamin (TC). Less than 30% of the Vitamin B12 in plasma circulates as Active-B12 (HoloTC). The remaining 70% or so is bound to haptocorrin. Accurate measurement of Vitamin B12 status is needed because of the high prevalence and serious complications of deficiency. Total plasma Vitamin B12 concentration is the current standard clinical test for Vitamin B12 deficiency, but it has various limitations: in particular, most of the Vitamin B12 measured is the haptocorrin-bound fraction. This suggests a prima facie clinical benefit from measuring Active-B12, in terms of remedying the poor sensitivity and specificity of Total-B12 measurement.

For concrete data please consult the Instruction for Use in the download box on the top right side.