7-8-dimethylalloxazine and lipoamide

The uptake of reducing equivalents of X CO-2 by lumichrome in spectrophotometric titrations has been re-examined in the light of a recently reported extinction coefficient of 10 500 M-1 cm-1 at pH 6, which is in agreement with 10 270 +/- 100 M-1 cm-1 determined here. The average uptake was 1.8 +/- 0.1, independent of pH in the range 6.3-9.0. The major product appears to be a dihydro-alloxazine, which can be reoxidized quantitatively to lumichrome by X Br-2 radicals or by O2. As in the case of dihydroflavins, oxidation by O2 is biphasic. As in the case of flavins, a two electron reduction of lumichrome was also observed with the disulphide monoanion of lipoamide (LS X 2-), but that reduction does not go to 100 per cent yield. Contrary to our earlier conclusions, which were based on an erroneous extinction coefficient, the combination of lumichrome radicals (2 X LcH----HLc--LcH) was of relatively little (less than or equal to 20 per cent) importance, and the behaviour of lumichrome on treatment with reducing species was rather similar to that of flavins.

Topics: Anions; Carbon Dioxide; Flavins; Free Radicals; Oxidation-Reduction; Thioctic Acid