SIDNEY F. VELICK 



137 



shift shown in Fiq,. 20 Avas obtained. It was found that TPN had 

 been formed and tliai one niolecide of TPN i)er niolecide of flavin 

 had l)econie liound to OYE to form a stable com]:)lex in whicli the 

 flavin also appears to be in the oxidi/.ed state. Upon incubation of 

 the reaction mixture anaerobically, an electron spin resonance signal 



400 



500 mp 600 



Fig. 20. The ab.sorption spettiuni of the old yellow enzyme ,.solid curve, and 

 of its complex with TPN, produced as described in the text. From Ehrenberg and 

 Ludwig (7). 



corresponding in magnitude to 15 per cent of flavin semiqiunone 

 was detected. The signal disappeared upon admission of oxygen 

 and returned when oxygen was removed, with no concomitant changes 

 in the absorption spectrum. The location of the odd electron in the 

 system is iniknown. In order to remove the TPN from the complex 

 by dialysis, it was necessary first to reduce both the pyridine and the 

 flavin with hydrosidfite. How such a complex would dissociate in 

 a catalytic situation, if it actually occurs as an intermediate, is not 

 clear. 



The acyl-coenzyme-A dehydrogenases. The reactions catalyzed by 

 these FAD enzymes are reversible a-/3 dehydrogenations of the fatty 

 acyl group in thio ester linkage with Co-A. Beinert (3) observed 

 the development and decline of a semiqiunone type of absorption 

 band at 570 ni/x during the reduction of the enzyme by substrate. 

 This enzyme, too, forms a stable complex with substrate or product, 

 but since the 570 m^ band can also be produced by non-specific 

 reducing agents it is assigned directly to the boiuid flavin and not 



