Further Investigations about the Flavin in the L-Amino Acid Oxidase and a Possible Flavin in Photosystem II Complexes from the Cyanobacterium Anacystis nidulans

Abstract

The absorption spectrum of the previously purified ʟ-amino acid oxidase from the cyanobacterium Anacystis nidulans has shown considerable variation with each preparation and the spectrum in several preparations was quite different from the absorption spectrum of other simple flavoproteins (E. K. Pistorius and A. E. Gau, Biochim. Biophys. Acta 849, 203, 1986). Here we show that the spectral complexity and variability of the ʟ-amino acid oxidase can be largely explained by the presence of a modified flavin derivative of yet unknown structure besides oxidized FAD and FAD semiquinone. After removal from the enzyme this modified chromophore has absorption maxima at 260, 396 and in the 600 nm region. This derivative of FAD seems to be formed in variable amounts during the purification of the enzyme. On the other hand, extraction of Anacystis photosystem II complexes which contain the flavoprotein, almost exclusively yields modified flavin derivatives and practically no authentic oxidized FAD. The spectrum of the chromophores which have been extracted from photosystem II complexes at different purification stages, is either similar (although not identical) to the spectrum of the chromophore extracted from the isolated ʟ-amino acid oxidase or similar to the spectrum of reduced flavin. All extracted chromophores show a fluorescence emission in the 420 to 560 nm region when excited with light of 390 nm. These results indicate that the flavin present in the ʟ-amino acid oxidase protein as well as in photosystem II complexes from A. nidulans rapidly undergoes modification reactions of yet unknown nature to yield several closely related FAD derivatives. This might possibly be the reason why so far no flavin has been detected in photosystem II. The presence of such modified flavin derivatives in photosystem II complexes of A. nidulans as shown here is an additional support of our hypothesis that an unusual flavin is functional on the donor side of photosystem II.