BRIXTON CHANCE 



yeast cells by the action of the cytoplasmic glyceraldehyde-3 

 phosphate dehydrogenase, cytochrome b shows a large and rapid 

 reduction (cytochrome c also shows a small reduction) (22 

 and unpublished data). If the yeast particulates can be 

 assumed to be similar to those of liver, this result indicates that 

 cytoplasmic DPNH is immediately available to the intramito- 

 chondrial cytochrome b. However, direct experiments on 

 intact liver cells are desirable to prove this point. 



A second variation is one described by Martius (53) to 

 explain the possibility that thyroxin could uncouple only one 

 site of phosphorylation. 



dicoumarol 

 antimycin-A thyroxin 



succinate 

 i 



O^ <^ Oi <r- c <r- factor ^ <— ^ <— vitamin K <— DPN <— substrate 



-fp^ 



In terms of the spectroscopic studies represented here, 

 Martius's hypothesis proposes that a cytochrome-c reductase — 

 flavoprotein — provides a nonphosphorylating shunt from DPNH 

 to cytochrome c, the shunt being sensitive to antimycin-A. Our 

 studies show to the contrary that flavoprotein is involved in the 

 pathway leading to cytochrome b. In addition, detailed 

 studies of the effect of triiodothyronine on intact mitochondria 

 failed to show a localization of the effect upon cytochrome b 

 (unpublished data in collaboration with Dr. H. A. Lardy). 

 On a kinetic basis, such a mechanism is unsatisfactory because 

 it requires a decrease of respiration when the P:0 value falls 

 according to the following reasoning: The system implies that 

 two electron donor systems compete for the factor, one phos- 

 phorylating and one nonphosphorylating. In order to give a 

 higher P:0 value when the vitamin K pathway is operative, 



334 



