METABOLIC ASPECTS 141 



Considering the relative redox potentials of the succinate-fumarate 

 (Eq = +0.031V.) and NADH2 - NAD (Eq = -0.32v.) couples, it canbe 

 concluded that an input of energy is necessary to drive the reaction 

 from left to right. This can be provided either by ATP or energy- rich 

 intermediates (precursors of ATP) associated with oxidative phos- 

 phorylation. Recent experiments by Griffiths and Chaplain (52) suggest 

 the intriguing possibility that a labile phosphorylated form of NAD may 

 be involved in this and analogous reactions. 



An energy- dependent reduction of NAD by succinate has been ob- 

 served with R. rubrum particles, in the sense that a net reaction 

 occurs only when the system is illuminated. The reaction catalyzed 

 by the Rho do spirillum system was found by Frenkel (53) to be sig- 

 nificantly inhibited when optimal amounts of ADP, Pi, and Mg++ were 

 added. This observation suggests, among other possibilities, that there 

 is a competition between the oxidation- reduction reaction and the 

 phosphorylating system for a common intermediate. 



The oxidation of succinate to fumarate is a particularly interesting 

 reaction in the metabolism of both mitochondria and photosynthetic 

 bacteria. In both instances, there appears to be a close structural 

 association between succinic dehydrogenase and the phosphorylating 

 electron transfer system. In mitochondria, this physical integration 

 presumably is one of the factors responsible for the unusually high 

 "electron pressure" exerted by succinate on the electron transfer 

 chain. Another property worthy of special attention is the relatively 

 high redox potential of the succinate-fumarate couple. It seems un- 

 likely that the presence of one oxidation- reduction step in the citric 

 acid cycle with this redox character is an accident of nature, and I am 

 inclined to believe that thisparticular aspect of the succinate-fumarate 

 conversion may be of great importance in the regulation of electron 

 transfer in both aerobic and anaerobic systems. 



Purple bacteria such as R. rubrum can grow anaerobically in the 

 light with succinate as the primary carbon source and evidently must 

 be able to oxidize succinate to fumarate under these conditions. What, 

 then, is the electron acceptor? There is some evidence (54) from in- 

 tact cell experiments that the anaerobic oxidation of succinate can be 

 coupled with CO2 fixation, i.e., the "direct" acceptor in this case is 

 presumably NAD, as in Frenkel's in vitro system. There is also evi- 

 dence (13,24) that a coupling with CO2 reduction is not obligatory, in 

 that glutamate- grown cells of R. rubrum can oxidize succinate with 

 the liberation of H2; the gas yield (~7 moles H2/mole succinate) under 

 optimal conditions indicates that the two hydrogen atoms removed in 

 the initial oxidation step must be convertible to molecular hydrogen. 

 To my knowledge, photosynthetic bacteria are the only known organisms 

 which have the apparent potentiality of producing H2 from the conver- 

 sion of succinate to fumarate. 



