Hydroxocobalamin (B12)

Hydroxocobalamin is a naturally occurring form of vitamin B12 (cobalamin) with the chemical formula C62H89CoN13O15P and a molecular weight of approximately 1346.37 g/mol. It features a cobalt ion coordinated with a hydroxyl (-OH) group as its upper axial ligand, distinguishing it from cyanocobalamin (which carries a cyanide group) and methylcobalamin (which carries a methyl group). Key characteristics: **Bioactive compound:** Hydroxocobalamin itself is the primary bioactive agent, serving as a precursor that is enzymatically converted in vivo to the two metabolically active coenzyme forms — methylcobalamin (cytoplasmic; cofactor for methionine synthase) and adenosylcobalamin (mitochondrial; cofactor for methylmalonyl-CoA mutase). **Typical supplemental doses:** Oral supplements range from 1,000–5,000 µg per dose; intramuscular injections are commonly administered at 1,000 µg (1 mg) per dose, with therapeutic protocols for deficiency using 1,000 µg every other day for several weeks. High-dose IV formulations (e.g., Cyanokit®) deliver 5 g for acute cyanide poisoning. **Cobalt content:** Each molecule contains one atom of cobalt (~4.35% by weight), approximately 58.93 µg cobalt per 1,000 µg hydroxocobalamin. **Phosphorus content:** Contains one phosphate group per molecule, contributing trace phosphorus (~2.3% by weight). **Bioavailability notes:** Oral bioavailability of B12 is generally low and saturable — intrinsic factor-mediated absorption in the ileum absorbs approximately 1.5–2 µg per meal, with an additional ~1% absorbed passively at high oral doses. Intramuscular injection bypasses gastrointestinal absorption, achieving near-complete bioavailability and producing sustained elevated serum B12 levels due to hydroxocobalamin's strong binding affinity to transcobalamin II and serum proteins, resulting in a longer half-life (~6–8 days IM) compared to cyanocobalamin. Hydroxocobalamin has superior tissue retention and slower renal clearance relative to cyanocobalamin. **No macronutrient contribution:** Hydroxocobalamin provides negligible calories, protein, fat, carbohydrates, or fiber. It contains no other vitamins or significant minerals beyond the trace cobalt and phosphorus inherent to its molecular structure. **RDA reference:** The Recommended Dietary Allowance for vitamin B12 is 2.4 µg/day for adults, though therapeutic and supplemental doses far exceed this to compensate for absorption limitations and address clinical deficiency states.