The Significance of Respiratory Chain Oxidations 



631 



pathway not coupled with phosphorylation and insensitive to the above 

 inliibitors. 



Reference to Fig. 1 shows that if TPNH oxidation occurs either by the 

 pyridine nucleotide transhydrogenase (E) or by the Martius pathway (F) its 

 oxidation might be expected to be accompanied by a P/2e ratio of 3, just as 

 for DPNH oxidation. As already mentioned, such ratios have not been 

 obtained experimentally with TPNH. Table 3 shows some relevant data. 



Table 3. Oxidative phosphorylation in rat mitochondrial 



preparations with tpnh and dpnh 



(a) vignais and vignais (1957): substrate isocitrate 



(Activity of TPNH-cytochrome c reductase and of DPNH-cytochrome c reductase was approximately 



equal in all 3 tissues. No exogenous coenzyme added. Note P/O ratio increases with increasing 



transhydrogenase activity.) 



(b) Kaplan, Swartz, Frech and Ciotti (1956): liver mitochondria, depleted of 



COENZYMES 



(c) Ball and Cooper (1957): heart muscle particles 



Oxidation of TPNH occurred only after addition of DPN also; presumed therefore entirely through 

 transhydrogenase here. 



In the mitochondrial membrane fragments used by Lehninger, added 

 TPNH is oxidized only if DPN is also added, and the presence of a pyridine 

 nucleotide transhydrogenase has been directly demonstrated (Devlin and 

 Lelininger, 1958; Devlin, 1959). Since a^/^^^f DPNH oxidation (unlike that 

 of the internal DPNH) is not accompanied by much phosphorylation (Cooper 

 and Lehninger, 1956) presumably 'external' DPNH and TPNH are both 

 oxidized by the alternative non-phosphorylating route, the oxidation of 

 TPNH proceeding via DPNH by means of the transhydrogenase reaction 

 in this case. 



The study of mitochondrial oxidation of isocitrate is important in this 

 connexion. Both TPNH and DPNH are believed to be capable of being 



