The Significance of Respiratory Chain Oxidations 637 



(b) at time 15 min after phosphate incubation, DPN = 3-1. 



The small discrepency (0-2 //mole/g) may be considered within the experimental 

 error of the method of analysis employed. These studies seem to question the relevance 

 of the additional pyridine nucleotide analysable with extracts from liver mitochondria 

 as DPN ~ I and the proposed correlation with the mechanism of oxidative phos- 

 phorylation. 

 Slater: I am glad to hear that Estabrook has confirmed Purvis's finding of additional DPN 

 on incubation of liver mitochondria with phosphate. Purvis did not investigate kidney 

 mitochondria in our laboratory. Estabrook's analysis raises two questions. One is, 

 as he points out, why do kidney mitochondria 'which show excellent control of respira- 

 tion and phosphorylation concomitant with respiration' contain little if any of the 

 additional compound, if the latter is indeed DPN -^ I as suggested by Purvis? The 

 second is why is there so much DPN in such mitochondria if the oxidation of DPNH 

 is inhibited? Further work is required to answer these questions. In any case, I 

 think that we should postpone further discussion of the possible function of Purvis's 

 compound until the completion of his attempts to isolate and characterize it. 



Possible Structure of Complexes of DPN or of DPNH Involved in Oxidative 

 Phosphorylation 



Chance: With reference to the paper by Dickens I cannot agree with the configuration 

 DPN -^ I; firstly, because of the crossover data which show that the inhibition must 

 lie in the reaction: 



DPNH + fp -^ DPN + rfp 



and secondly, because we do not observe inhibition of DPN reduction when substrate 

 is added to freshly prepared mitochondria suspended in a medium free of ADP 

 or phosphate. 



Would Slater indicate the state of reduction implied by DPN ~ I that is consistent 

 with the above two considerations ? 

 Slater: We have written DPN --^ I, because we believe that it is formed during the 

 oxidation of DPNH to DPN+, thus 



DPNH + fp + I . H ^ DPN '- 1 + fpHa (i) 



DPN ~ I represents a compound of oxidized DPN with I, without specifying the 

 nature of the compound. Indeed, we always had in mind the sort of addition compound 

 which DPN forms with HCN, thus 



Hv^CN 



■~ + HCN ^ (I Ip + H+ (ii) 



r I 



c.r 



which has structural and spectroscopic resemblances to DPNH 



H\/H 



a 



■N 

 I 



I have imderstood Chance's DPNH ~ I to refer to a compound of reduced DPN 

 with I, which if it were formed in a way comparable to that described in reaction (i), 

 must arise during the reduction of DPN+ by substrate, for example 



< /?-hydroxybutyrate + DPN+ + I ^ acetoacetate 4- DPNH -- I -|- H+ (iii) 



We believe that this reaction is rather unlikely. 



