Maria Wolak (a,b), Achim Zahl (b) Thorsten Schneppensieper (b) Grazyna Stochel (a) Rudi van Eldik (b).
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(a) Faculty of Chemistry, Jagiellonian University, 30060 Krakow, Poland
(b) Institute for Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstr.1,
91058 Erlangen, Germany
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Vitamin B12 derivatives participate in a variety of biological pathways involving enzyme-catalysed molecular rearrangements and methyl group transfer reactions. One of the recent interests in the biological functions of vitamin B12 concerns the use of aquacobalamin and its reduced form to modify physiological actions of nitric oxide.
The reduced form of vitamin B12 (vitamin B12r, Cbl(II)) at physiological pH efficiently binds NO to yield the nitrosyl adduct, Cbl(II)-NO. From the biological point of view formation of a stable Cbl(II)-NO complex may influence the physiological activity of nitric oxide and cobalamin-dependent enzymes. In particular, inhibition of biological actions of NO in the presence of cobalamin, and NO-induced inactivation of cobalamin-dependent methionine synthase has been observed.
In this context, we studied the kinetics of binding and dissociation of NO from the reduced form of cobalamin in aqueous medium. The combination of ambient and high pressure kinetic techniques (laser flash photolysis, stopped-flow and 17O NMR) enabled a quantitative kinetic and mechanistic description of the reaction system on the basis of rate constants and activation parameters ((H(, (S( and (V(). In particular, the nature of the reactive aqua intermediate generated in the laser flash was resolved. In addition, the reaction product formed at physiological pH has been characterised with the use of several spectroscopic techniques (UV-Vis, 1H, 15N and 31P NMR).

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