HENRY R. MAHLER 



The metalloflavoprotein model may constitute the simplest 

 examiple of a system capable of linking the areas of H-transport 

 enzymes (pyridinoproteins and flavoproteins) and true electron 

 transport enzymes (the cytochrome system). Where true 

 hydrogen transport (or a 2 e~ transfer) is possible from flavin 

 to oxidizing agent, i.e., in the interaction of flavoproteins with 

 dyes, quinones, or directly with oxygen, the presence of the 

 metal is not necessary and its absence does not affect the mecha- 

 nism of the catalysis observed. Metalloflavoproteins are of 

 interest not only because of their inherent importance as electron 

 transport enzymes but also because of the fact that certain 

 extensions of principles first observed and easily tested with this 

 class of enzymes may be possible to more complex enzymatic 

 arrays as well. 



Hemo-Flavoproteins 



In the case of the metalloflavoprotein enzymes just described 

 the interaction between reduced flavin, and the preceding 

 H-transport system, with the cytochrome system is mediated by 

 a metal, and this linkage between the flavoprotein and the 

 isolated cytochrome (cytochrome c) is produced in a transient 

 manner only during the existence of the enzyme-acceptor 

 complex. As an extension of this principle, possibly depicting 

 a status more closely akin to that to be found in the mitochon- 

 drion, it has been possible in at least three instances to isolate 

 enzymes containing flavin and an iron-porphyrin linked in a 

 more permanent manner. One such example is the crystalline 

 lactic dehydrogenase of yeast, containing FMN and cytochrome 

 ^2, isolated by Appleby and Morton (1). 



Another example is the aldehyde oxidase of mammalian 

 liver (30), which has the following four prosthetic groups: thiol 

 groups, FAD, molybdenum and iron-protoporphyrin (with a 

 spectrum very similar to that of cytochrome b). Finally solu- 

 bilized enzymes have been obtained from DPNH oxidase, which 

 will be described below (25), and from complex succinic dehydro- 



260 



