REVERSAL OF ELECTRON TRANSFER IX THE RESPIRATORY CHAIN 



129 



hydrosulphide or cvanide to block respiration causes no measurable 

 pyridine-nucleotide reduction, as observed fluorometrically with 365 m^Li 

 excitation and 450 m^t measurement [16]. Under such conditions reduction 

 of flavin, quinone and cvtochrome components can be observed spectro- 

 photometricallv. Thus the respiratory chain is in a condition where the 

 etTect of ATP on the sequence of reactions depicted by Fig. 8 could be 

 observed readily. Figure 13 shows that flavoprotein is oxidized at the 



Aerobic P.H. M 



w 



4mM succinate 



+ 



360 mM Na2S 



+ 



2y/mt. HO Q NO 



36/^M ATP 



Flavoprotein oxidation I 

 465-5l0mu, * 



_L 

 log Iq/I = 0005 



0-6mM 

 DPNH/sec 



50sec 



Pyridine nucleotide reduction I 

 365— 450myu -»- 



l-l/^M , DPNH 



Fig. 13. Flavoprotein oxidation and pyridine-nucleotide reduction caused by 

 ATP addition to pigeon-heart mitochondria inhibited with hydroxyquinoline-N- 

 oxide and sodium svilphide. Mitochondria suspended in mannitol-sucrose- 

 "tris" medium, pH 7-4, temperature 26 , protein concentration 1-2 mg./ml. 

 (Expt. 173). (Reproduced with permission of Xature.) 



same time that pvridine nucleotide is reduced upon addition of ATP to 

 the HOQXO- and XaoS-treated mitochondria. This result aflFords strong 

 support for the reaction of Figs. 8 and 11. It is also of interest that under 

 these conditions the reaction of ATP with the rfp-DPN couple is so 

 rapid that succinate cannot maintain flavoprotein reduced against the 

 oxidizing etfect of added ATP. However, when the DPN has been reduced, 

 the oxidized flavoprotein is reduced by succinate toward its initial level. 



An experiment of this tvpe suggests the possibility of generalized 

 reversal of electron transfer between all couples of the respiratory chain 

 involved in oxidative phosphorylation. It is apparent that if reduced 

 flavoprotein can be oxidized bv ATP a similar effect should be observed 

 at the level of the cytochromes under appropriate conditions. 



\\'e therefore repeated an experiment similar to that of Fig. 13 except 



VOL. II. K 



