I. New flavins have been isolated from purified preparations of an electron-transferring flavoprotein (ETF) from Peptostreptococcus elsdenii and glycolate oxidase from pig liver. The structures of these new species have been established as FAD and FMN derivatives of 6-hydroxy-7,8-dimethyl-isoalloxazine, the chemical synthesis of which is described in the accompanying paper by Schollnhammer and Hemmerich. The chromophores are yellow a t pH 5 and green at pH 9 due to an ionization a t pK 7.1. BH3cp4;,7--4H0 0 H3C OH 2.6-OH-FAD is bound by apo-ETF and its pK is decreased. The complex is reduced by NADH and it couples the oxidation of NADH to the reduction of dichlorophenolindophenol. Unlike the complex of FAD and apo-ETF, 6-OH-FAD * ETF does not couple the oxidation of NADH to the reduction of butyryl-CoA dehydrogenase.3.6-OH-FMN is bound by the FMN-specific protein apoflavodoxin from P , elsdenii and the pK is increased to %9. This complex is reduced by sodium dithionite and an intermediate, presumed to be the semi-quinone, is formed a t half reduction. Long-wavelength absorption bands (A, , ,500 t o 800 nm) have been observed in the spectra of several oxidised flavoproteins (e.g. [l -3] and they concluded that there is a second chromophore in the enzyme. They showed that when FMN is selectively removed from glycolate oxidase, the resulting preparation is green, and it retains a broad absorption band centered a t 600nm and also a sharp maximum at 425nm. The chromophore giving this absorption spectrum was not identified.More recently we have observed a green chromophore with a similar spectrum in preparations of an electron-transferring flavoprotein (ETF) isolated from the anaerobic bacterium Peptostreptococcus elsdenii [7]. This enzyme couples the oxidation of NADH to the reduction of butyryl-coenzyme-A dehydrogenase and resembles the ETF of Beinert [S] which functions in the B-oxidation of fatty acids in aerobic systems. The spectrum of purified ETF from P. elsdenii, like the spectrum of glycolate oxidase, Eur. J. Biochem. 44 (1974)

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