The kinetics of photostabilization of cyanocobalamin in liposomal preparations

Cyanocobalamin (B₁₂) is a photosensitive vitamin, and its photodegradation to hydroxocobalamin (B_12b) in liposomes has been investigated. The values of apparent first-order rate constants (k_obs) for the photodegradation of B₁₂ in liposomes (nine preparations) are in the range of (0.52-2.24) × 10^–3 min^–1, compared to 3.21 × 10^–3 min^–1 for B₁₂ in aqueous solution (pH 5.0). The entrapment efficiency of B₁₂ in liposomes is 26.4-38.8%. The values of k_obs show a linear relation with phosphatidylcholine (PC) content in liposomes, indicating the influence of PC in inhibiting the rate of photolysis of B₁₂. The value of the bimolecular rate constant for photochemical interaction of B₁₂ and PC is 0.32 M^–1 min^–1, indicating the stabilizing effect of PC on the photolysis of B₁₂. The ratio of B₁₂ stabilization in liposomal preparations is in the range 2-6 compared to that of the unentrapped vitamin The stabilization of B₁₂ is mediated by a photoinduced charge-transfer B₁₂-PC complex that leads to the reduction of B₁₂ to B_12r, which is then oxidized to B_12b that has low susceptibility to photolysis. The extent of stabilization of B₁₂ probably depends on the degree of interaction between the two compounds under the reaction conditions, indicated by the loss of B₁₂ fluorescence.