Reduced flavin-adenine dinucleotide (FADH2), which was produced by flavin-adenine dinucleotide (FAD) -dependent sennidin reductase, reduced methyl orange non-enzymatically in a molar ratio of 2 : 1 under anaerobic conditions. Similarly, sennidins and sennosides were reduced nonenzymatically to rheinanthrone and 8-glucosylrheinanthrone, respectively, by the reduced forms of cofactors such as FAD, flavin mononucleotide (FMN), riboflavin and benzyl viologen. The reaction was inhibited by oxygen. FADH2 reduced sennidins equimolarly, but sennosides were reduced much less effectively than sennidins (about one-tenth).
The polarographic half-wave potentials of sennidins A and B, and sennosides A and B were-115, -133, -184and-201mV, respectively.
Therefore, it was confirmed that the reduction of sennidins and sennosides by FAD-dependent sennidin reductase consists of the following two reactions; (1) the reductase reduces cofactors such as FAD and (2) sennidins and sennosides are reduced non-enzymatically by the reduced forms of cofactors.
On the other hand, oxidation of rheinanthrone to sennidins in a neutral solution occurred with oxygen but not with FAD.