The main cobalamins with vitamin B12 activity in human nutrition are adenosylcobalamin, methylcobalamin, aquacobalamin and the conjugate base of aquacobalamin, hydroxocobalamin. Cyanocobalamin is a synthetic vitamin B12 analogue that can be converted in animals to active vitamin B12. It has been speculated based on in vitro studies that vitamin C may interact with vitamin B12 in the gut. In particular, it has been suggested that vitamin C could destroy vitamin B12 in the gut before it has been absorbed. For example, aquacobalamin is quickly degraded by vitamin C as demonstrated using in vitro models. However, aquacobalamin is readily reduced to cob(II)alamin by many reducing reagents and so there is nothing unique about vitamin C in this regard. Nonetheless aquacobalamin may interact with vitamin C in the gut and this may prevent its absorption. However the degradation of vitamin B12 by vitamin C is controversial because high intakes of vitamin C in animals do not lower vitamin B12 plasma levels.
Cyanocobalamin, methylcobalamin and adenosylcobalamin are all stable at high temperatures (94 °C for 5 hours) and at a wide range of pHs. Although both adenosylcobalamin and methylcobalamin are highly photosensitive and degrade quickly when exposed to light. The first step in the degradation of the cobalamin corrin ring structure by vitamin C is likely the reduction of the cobalt atom to cob(II)alamin. In this regard addition of vitamin C to solutions of red aquacobalamin changes the colour to brown. The subsequent slower degradations reactions yield yellow products that signify further destruction of the corrin ring. However, only aquacobalamin is readily reduced by vitamin C in this way. As adenosylcobalamin and methylcobalamin are the main biological forms of vitamin B12, and cyanocobalamin is the main synthetic form of the vitamin, all of which are not reduced by vitamin C, it is unlikely that even high concentrations of vitamin C can cause degradation of vitamin B12 in the gut.
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