Identification and Properties of 8-Hydroxyflavin : Adenine Dinucleotide in Electron-Transferring Flavoprotein from Peptostreptococcus elsdenii

1. A new flavin prosthetic group has been isolated in pure form from the electron-transferring flavoprotein of Peptostreptococcus elsdenii. Its structure has been established as the FAD derivative of 7-methyl-8-hydroxyisoalloxazine.Proof of this structure has been obtained by chemical synthesis of 7-methyl-8-hydroxyisoalloxazine models, and by stepwise degradation of the native compound to 7-methyl-8-hydroxyalloxazine. The orange chromophore is characterized by a strong absorption band with a maximum at 472 nm (ε = 41000 M-1 cm-1) and a pK at 4.8 due to the ionisation of the C(8)-OH group.2. The properties of a series of functionally substituted, derivatives of 8-hydroxy flavins and lumichromes have been investigated to provide a basis for interpreting the effects of pH on the spectroscopic properties of the 8-hydroxy derivatives of FAD and FMN.3. The 8-hydroxy derivative of FAD is bound by apo-d-amino acid oxidase; the complex shows no catalytic activity. The 8-hydroxy derivative of FMN is bound by apoflavodoxin to give a complex which has catalytic activity similar to that of native flavodoxin. The complex is reversibly reduced by dithionite, first to a relatively stable semiquinone and further to the dihydroflavin form