Bakers' Yeast Lactate Dehydrogenase 557 



ferricyanide, while in the crude state the enzyme can reduce all of them. On 

 the other hand, cytochrome b has been purified in an apparently water- 

 soluble state with the aid of detergents and proteinase digestion (Sekuzu and 

 Okunuki, 1956). Attempts to link cytochrome b with the solubilized suc- 

 cinate dehydrogenase have not yet succeeded. Chance (1954) has assumed 

 that the electron transport between the succinate dehydrogenase and cyto- 

 chrome b may be mediated by another flavoprotein, but all attempts at 

 separation of such a flavoprotein have been unsuccessful. Keilin and King 

 (1958) have demonstrated that the water-soluble succinate dehydrogenase 

 purified by the method of Wang, Tsou and Wang (1956) can transfer electrons 



Fig. 2. Models of bakers' yeast lactate dehydrogenase. Schematic representations 



of the relationship between bakers' yeast lactate dehydrogenase activity and the 



reduction of the haem of bakers' yeast lactate dehydrogenase. 



liberated from succinate to the cytochrome system contained in a mito- 

 chondrial particle preparation which has no succinate dehydrogenase. The 

 similarity in the modification of the enzymic activities between Y-LDH 

 and succinate dehydrogenase might indicate similarities in their electron- 

 transferring systems (compare, however, Morton, 1955). 



At present, the two models of Y-LDH may be considered to be as shown 

 in Fig. 2, where {A) indicates that Y-LDH is a protein complex which is 

 composed of a cytochrome plus a dehydrogenase containing a bound flavin. 

 In this model the dehydrogenase moiety may be equivalent to the modified 

 Y-LDH or to solubilized succinate dehydrogenase and the cytochrome 

 moiety may be equivalent to crystalhzed cytochrome b^ or to highly purified 

 cytochrome b. Other concepts of an enzymic protein complex may be deduced 

 from studies of phosphorylase a and b (Green and Cori, 1943) and L-amino 

 acid oxidase (Blanchard, Green, Nocito and Ratner, 1944, 1945, 1946). At 

 present, however, there is no evidence to deny the scheme (5), where the 

 dehydrogenase protein itself holds both the 6-type haem and a flavin, except 

 the observation that: Y-LDH and cytochrome b^, show identical a-, /?-, y- 

 and <5-absorption maxima (in wavelength and in ratio). This is in contrast 

 to the behaviour of cytochrome c, for native cytochrome c and cytochrome 

 c digested by proteolytic enzymes are spectrophotometrically different, 

 though the native cytochrome c is not spectrophotometrically distinguished 

 from the cytochrome c modified only in its secondary structure (Nozaki, 



