The transcobalamin protein alone transports vitamin B12 from its site of absorption in the ileum to tissues and cells that express specific receptors. The vitamin is then internalised as the Active-B12 (vitamin B12 bound to transcobalamin) complex. This process delivers vitamin B12 into the cells of the body and provides the vitamin as an essential co-enzyme for essential cellular functions (eg DNA synthesis).

Less than 30% of the vitamin B12 in plasma circulates as Active-B12 (holoTC). The remaining ~70% is bound to haptocorrin, the function of which is unknown, but it is considered metabolically inert as cellular receptors for holohaptocorrin exist only on the liver.

Genetic absence of haptocorrin (rare) is not a serious condition and is usually discovered accidentally (1). On the other hand, genetic absence or abnormality of transcobalamin manifests as typical haematological and neurological pathologies of vitamin B12 deficiency (2). This is normally discovered shortly after birth (failure to thrive) and requires aggressive therapy with vitamin B12 if long term, irreversible, neurological damage is to be avoided. Frequently in such cases serum analysis shows misleading total vitamin B12 levels (within the normal range).

1. Carmel R and Herbert V, Blood, 1969;33: 1-12.

2. Hakami N et al., New Eng J Med., 1971; 285: 1163-70.

News

Coming Soon !  The full video recording of the Euromedlab 2007 Active B12 workshop will be available in the next few weeks, register for your copy.

Live CME Web Conference on vitamin B12 deficiency, Prof. Ralph Green, Dec 13th 2007, register now.

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Meetings

8-11 December, 2007
American Society of Hematology, Atlanta, booth 544
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