New Aspects of the Structure of Corrinoid Coenzymes

Recently the structure of the cobalamin-coenzyme has been elucidated by X-ray analysis1 and has been confirmed by partial s y n t h e ~i s .~-~~ With both methods, however, it was not possible to estimate the precise structure of the chromophore in the corrin nucleus.We have been able to show that the reactivity of the corrin nucleus in the corrinoid coenzymes is greatly different from that in the cyano-or hydroxo-forms.Investigations by Todd and coworkers* have shown that cyanocobalamin is converted to cyano-dehydrocobalamin by oxidation with air in alkaline solution (FIGURE 1).Under the same experimental conditions 60 per cent of the cobalamin-coenzyme is unchanged and 40 per cent is converted in the first step to hydroxo-cobalamin which is rapidly oxidized to hydroxo-dehydrocobalamin.The reaction mixture was separated by paper electrophoresis in the presence of 0.2 per cent NaHS03 in acetic acid solution of pH 2.5.The hydroxo-dehydrocobalamin was identified and the coenzyme fraction was decomposed with cyanide.This fraction showed the same activity as cyano-cobalamin by the E .coli mutant tube assay and is also identical with cyano-cobalamin by paper chromatography in four different solvents.With [ Co] -methyl-cobalamin or cobalamin-sulf~nate,~ however, no reaction takes place.In both cases, 95 per cent remain unchanged and only traces of deaminated products of [ Co] -methyl-cobalamin or cobalamin-sulfonate are produced.Both unchanged compounds showed after