METABOLIC ASPECTS 139 



of biological systems makes it possible to reexamine these ideas in a 

 more sophisticated and meaningful way. 



Net reducing power in bacterial photosynthesis 



There is still no unambiguous supporting evidence for the proposal 

 (see ref. 13) that a photoreductant provides the hydrogen atoms, or 

 electrons, required for net formation of reduced pyridine nucleotide 

 or molecular hydrogen. To my mind, it has become increasingly dif- 

 ficult to account for the available biochemical and physiological facts 

 on the basis of this kind of postulated mechanism, which has come to 

 be called "noncyclic electron flow," In this connection, Bose and I (47) 

 have recently examined the experimental basis of the claim (48) that 

 an antimycin- resistant "noncyclic photophosphorylation" (i,e,, aphos- 

 phorylation presumably dependent on light-stimulated"noncyclic elec- 

 tron flow") system operates in the normal metabolism of purple bac- 

 teria. The evidence cited in favor of this conclusion consists essen- 

 tially of the demonstration, using pigmented particles from /2. ruhnim, 

 of photophosphorylation ostensibly dependent on the presence of both 

 an added reductant (ascorbate + DPIP) and oxidant (NAD). Our studies 

 indicate that the phosphorylation observed in such experiments is in 

 reality cyclic photophosphorylation, which is antimycin- resistant be- 

 cause of the ability of DPIP to effect a by-pass in electron transfer 

 around the antibiotic- sensitive region. The effects of added reductants 

 or oxidants, or both, can be readily explained by the fact that optimal 

 photophosphorylation by isolated particles requires maintenance of a 

 suitable redox potential (49). The particles show very little inherent 

 poise and, consequently, the rate of phosphorylation is quite sensitive 

 to changes in the redox potential caused by electron donors or acceptors 

 which interact with the electron transfer system. It is easy to show, 

 in fact, that in the presence of excess reductant, phosphorylation will 

 not proceed at a significant rate unless a suitable oxidant is added, even 

 though there is no net electron transport (Table 3). 



The gas phase in both experiments was 100% H2. At 30 °C. (Exp. I), 

 the hydrogenase in the R. rubnim particle preparation is active and 

 when ascorbate + DPIP are also present the particles become "over- 

 reduced," leading to an inhibition of photophosphorylation. It is ap- 

 parent that the over- reduction effect can be completely prevented, or 

 reversed, by addition of fumarate. At 20°C. (Exp, II), the hydrogenase 

 is practically inactive and inhibition of light-induced phosphorylation 

 by over- reduction is, accordingly, not observed. It is of importance 

 to note that under the conditions employed there was no detectable 

 consumption of H2 and as much as 8.5 /imoles of Pi could be este ri- 

 fled in the presence of only 0,2 //mole of ascorbate. The results in 

 Table 3 also indicate that DPIP can catalyze an antimycin- resistant 

 bypass in electron transfer associated with photophosphorylation 

 when the overall redox potential is suitably adjusted. 



