SIDNEY F. VELICK 



135 



intermediate in the oxidation and reduction ot (lavins had pre- 

 viously been indicated by the potentionietric titrations of Michaelis 

 (16), who postulated long ago that a seniiquinone intermediate was 

 obligate in the reduction of a univalent cytochrome by the divalent 

 flavin in the pathways of biochemical electron transport. 



Flavin Nucleotide Enzyme Complexes 



Old yellow enzyme (OYE). Although its catalytic activity has not 

 yet been thoroughly investigated, this flavoprotein was the first enzyme- 

 coenzyme complex to be purified, resolved, and reconstituted (30). In 



0.800 



230 



250 



300 



400 500 



m/i. 



Fig. 18. The absorption spectrum of 0.25 M FMX in a 0.05 cm cell at various stages 

 of reduction, pH 5.2. Curve 1, reduced; curve 10, oxidized. From Beinert (2.) 



recent years Theorell and coworkers have completed the purification 

 of the protein and have extended the early characterization of the 

 complex (31, 32). The protein, of molecular weight 102,000, carries 

 two molecules of FMN. In the complex the fluorescence of FAIN 

 is almost completely quenched. This effect has been utilized in the 



