Holotranscobalamin, also known as active B12, is the active fraction of vitamin B12 bound to the protein transcobalamin, making it readily available for cellular uptake. This form is critical for assessing vitamin B12 status in the body, as it directly reflects the bioavailable portion of the vitamin, unlike total B12, which includes both active and inactive forms.
Holotranscobalamin measurement is precious in diagnosing vitamin B12 deficiency. It is more sensitive and specific than total serum B12 levels, as it decreases early in the course of deficiency, often before total B12 levels drop. This early detection is crucial for populations at risk of deficiency, such as the elderly, vegans, and individuals with gastrointestinal disorders affecting B12 absorption.
Serum vitamin B12 is bound to two protein carriers. About 70-90% of vitamin B12 is bound to Haptocorrin (HC, also called Transcobalamin 1), and this form is metabolically inactive. The remaining 10-30% of B12 is bound to Transcobalamin 2 (TC2), and this is the bioavailable, active fraction, as it can bind to receptors on cell membranes and then be received by these cells. The Transcobalamin 2-Vitamin B12 complex is called Holotranscobalamin or Active B12.
Diet, absorption issues, and genetic factors can affect holo transcobalamin levels. A diet low in animal products, conditions like pernicious anemia, gastrointestinal surgery, or chronic gastrointestinal diseases can lead to decreased levels. Genetic variations in the transcobalamin protein can also influence holotranscobalamin levels and B12 metabolism.
Holotranscobalamin is a crucial biomarker for assessing vitamin B12 status, providing a more accurate assessment than total serum B12. It enables early detection and better management of deficiency, essential for preventing associated complications, especially in at-risk populations. Regular monitoring and appropriate intervention can help maintain adequate B12 levels and overall health.
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