Holotranscobalamin – An Early Marker for Laboratory Diagnosis of Vitamin B12 Deficiency

European Haematology, 2009;3(1):7-11 DOI: http://doi.org/10.17925/EOH.2009.03.1.7


Vitamin B12 deficiency is widespread. Among the population groups at risk are older people, vegetarians, pregnant women and patients with renal or intestinal diseases. The neurological symptoms of vitamin B12 deficiency are unspecific and can be irreversible. Early detection is therefore important. This article reviews the diagnostic performance of the different laboratory markers for vitamin B12 status. Total serum vitamin B12 is a relatively insensitive and unspecific biomarker of deficiency that does not reflect recent variations in cobalamin status. Holotranscobalamin (holoTC), the metabolically active portion of vitamin B12, is the earliest laboratory parameter that becomes decreased in case of a vitamin B12 negative balance. Concentration of methylmalonic acid (MMA) is a functional vitamin B12 marker that will increase when the vitamin B12 stores are depleted. Isolated lowering of holoTC shows vitamin B12 depletion (negative balance), while lowered holoTC plus elevated MMA (and homocysteine) indicates a metabolically manifested vitamin B12 deficiency, although there still may be no clinical symptoms. The diagnostic use of holoTC allows the initiation of therapeutic measures before irreversible neurological damage develops. Because the clinical manifestations of vitamin B12 deficiency are unspecific, people at risk should be identified and should regularly test their holoTC with or without MMA.
Keywords: Cobalamin, holotranscobalamin, diagnosis, deficiency
Disclosure: The authors have no conflicts of interest to declare.
Received: July 09, 2009 Accepted August 04, 2009
Correspondence: Wolfgang Herrmann, Klinische Chemie und Laboratoriumsmedzin, Universitätsklinikum des Saarlandes, Gebäude 57, D-66421 Homburg, Germany. E: kchwher@uniklinikum-saarland.de

Pernicious anaemia, which causes severe vitamin B12 (cobalamin) deficiency, used to be a fatal, non-curable disease. However, in 1926 George Minot and William Murphy discovered that pernicious anaemia can be treated by including large amounts of liver in patients’ food. Vitamin B12 is synthesised exclusively in microorganisms. Animals obtain vitamin B12 from foods contaminated with vitamin B12-synthesising bacteria, and thus foods of animal origin represent the only source of vitamin B12 in the human diet.
Together with 5-methyltetrahydrofolate, vitamin B12 is an essential co-factor in methyl group transfer, cell division and catabolism of homocysteine (Hcy). In addition, vitamin B12 is required by all cells for one-carbon metabolism and DNA synthesis and maintenance. Only two vitamin B12-dependent enzymes are known in humans: methionine synthase and L-methylmalonyl-CoA mutase. Methionine synthase mediates the formation of methionine from Hcy, and requires methylcobalamin as a co-factor. L-methylmalonyl-CoA mutase requires adenosylcobalamin, and catalyses the formation of succinyl-CoA from methylmalonyl-CoA. The latter compound can be converted into methylmalonic acid (MMA). Serum concentrations of MMA and Hcy are therefore considered metabolic indicators of vitamin B12 status.
Vitamin B12 is conserved in humans through the enterohepatic circulation and via active re-absorption in the proximal tubule. The estimated stores of vitamin B12 in the human liver are relatively high.1 Therefore, in the case of intact absorption, the depletion of the vitamin takes years to be expressed as a metabolic or clinical dysfunction. Acquired vitamin B12 deficiency can be related to prolonged insufficient intake, disturbed absorption, increased requirements or an accelerated loss of the vitamin.
Subtle Vitamin B12 Deficiency and Recommended Daily Amount
Subtle vitamin B12 deficiency is common in the general population and it is more widespread than has been assumed so far.2–4 Chronic insufficient intake or disrupted absorption of vitamin B12 are the most common causes of cobalamin deficiency.
According to the recommended daily amount (RDA) guidelines from the National Research Council of the US National Academy of Sciences, adults should ingest 2.4μg daily (pregnant women up to 6μg), which can be met by a typical western diet.5
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Keywords: Cobalamin, holotranscobalamin, diagnosis, deficiency