Reduction of lumichrome by the radical anions of CO2 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.