74 E. C. Slater and W. C. Hulsmann 



theory, but it remains to be determined whether either of the 

 naturally occurring uncouphng agents works so selectively 

 as to cause an increase in the rate of synthesis of ATP, in the 

 presence of excess ADP. Incidentally, the sarcosomes used in 

 the experiment described in Fig. 4 probably contained the 

 uncoupling agent which is isolated from mitochrome prepara- 

 tions. This is indicated by the marked increase in the P : O 

 ratio brought about by the addition of serum albumin (Lewis 

 and Slater, 1954), first found by Sacktor (1954) to be useful 

 in the isolation of insect sarcosomes. Albumin has no appreci- 

 able effect on oxidative phosphorylation or on the ^^P-ATP 

 exchange reaction in rat heart sarcosomes. 

 (3) The third possibility is that an uncoupling agent might be 

 concerned in the utilization by the cell of the energy of one 

 of the intermediates of oxidative phosphorylation (see p. 67). 



Possible nature of the I compounds 



The exact location of the phosphorylation steps in the 

 respiratory chain is not yet established. However, it seems 

 very likely on thermodynamic grounds (Slater, 1959) that 

 the reaction between DPNH and fp is coupled with the 

 synthesis of ATP. 



According to equation (1), the first energy-rich intermediate 

 formed in the reaction between DPNH and fp would be 

 DPN '^ Ij. However, all the experiments described to date 

 could equally well be explained by the supposition that I 

 combines with the reduced product (BHg) of reaction (1), as 

 preferred by Chance and Williams (1956). If this were the 

 case, the product of reaction between DPNH and fp would 

 be fpHg '^ Ii rather than DPN '^ I^. Neither equation (1) as 

 written nor this modification could account for the formation 

 by this reaction of DPNH /^ I, which has been postulated by 

 Chance and Williams (1956). 



Purvis (1958) has recently obtained evidence for the 

 presence in liver mitochondria of a form of DPN which has the 

 properties predicted for DPN -^ I^. He measured the amounts 



