632 F. Dickens 



produced at the isocitrate dehydrogenase stage of the Krebs cycle oxidations, 

 by two dehydrogenases each of which is specific for one of the pyridine 

 nucleotides and both of which occur in mitochondria, while only the TPN- 

 linked dehydrogenase is present in the soluble fraction (Ernster and Navazio, 

 1957). These authors found very low activities of transhydrogenase in liver 

 mitochondria and believed, therefore, that the greater part of isocitrate 

 oxidation proceeded via the DPN-linked isocitrate dehydrogenase. More 

 recently Purvis (1958b) has reinvestigated this and finds a much higher 

 transhydrogenase activity (cf. Stein, Kaplan and Ciotti, 1959), so that both 

 in liver mitochondria and in heart sarcosomes he believes that isocitrate is 

 oxidized almost entirely via the TPN-enzyme coupled with transhydrogenase. 

 After depletion of pyridine nucleotides in the mitochondria by prehminary 

 incubation at 30°C, added DPN was not reduced in presence of isocitrate until 

 TPN was also added. The reason for the discrepancy with Ernster's findings 

 is not clear, but may possibly find its explanation if a substance is formed 

 from TPN (possibly TPN ^l, as already discussed) which blocks the 

 reaction, presumably because it cannot liberate its high energy component 

 unless DPN is also present (Purvis, 1958c). It is probable that the trans- 

 hydrogenase reaction in mitochondria is much more complex than the simple 

 reaction : 



TPNH + DPN ^ DPNH + TPN 



indicated in Fig. 1 would suggest. 



A number of other possible mechanisms for interconversion of oxidized 

 and reduced DPN and TPN have already been discussed in an earlier review 

 (Dickens, Clock and McLean, 1959). To these should be added the system 

 of 3-a-hydroxysteroid dehydrogenase, which acts as a transhydrogenase by 

 reacting reversibly with both DPN and TPN, and thus resembles the previously 

 described placental oestrone-oestradiol system, but is apparently much more 

 widely distributed in animal tissues (Hurlock and Talalay, 1958). In the 

 present state of ignorance of what actually constitutes the physiologically 

 active transhydrogenase system, it is not hard to understand how such 

 widely different results on the mechanism of TPNH oxidation could have 

 been obtained in different laboratories, and this is hkely to be an active 

 field of future study. 



REGULATION OF METABOLIC PATHWAYS THROUGH 

 THE OXIDATION LEVEL OF TPN 

 One can readily see the outstanding advantage to the regulation of cellular 

 metabolism due to the presence of the pools of the two pyridine nucleotides, 

 DPN and TPN, of which only DPN is maintained largely in the oxidized 

 state, is coupled directly with oxidative phosphorylation, and is oxidized by 

 an efliicient oxidation chain localized in the mitochondrial membrane. On 



