DIFFERENT OXIDANTS 1585 



In evaluating the possible significance of these results for the mecha- 

 nism of photosynthesis, Anion referred to the tracer experiments of Calvin 

 et al. (to be described in chapter 30), indicating that malate is not normally 

 an intermediate in the main reaction sequence of photosynthesis. 



Earlier in this section we mentioned the recently discovered "coenzyme 

 A." Attempts to use this compound as ultimate Hill oxidant have not yet 

 given positive results; whether it can serve as intermediate in the photo- 

 chemical reduction of metabolites whose formation in respiration is coupled 

 with the reduction of coenzyme A (such as the formation of acetate by oxi- 

 dative decarboxylation of pyruvate) remains to be ascertained. An im- 

 portant role was ascribed to this compound in photosynthesis by Calvin; 

 but since this hypothesis was not based on photochemical experiments 

 with chloroplasts, we will consider it in chapter 36 when dealing with vari- 

 ous suggested chemical mechanisms of photosynthesis. 



More than halfway on the redox potential scale between the systems 

 H2O/O2 (^0' = -0.81 volt at pH 7) and H,TPN/TPN (^0' = +0.282 volt 

 at pH 7) lies the system ferrocytochrome c/ferricytochrome c (£'0' = —0.26 

 volt at pH 7). Despite this favorable position, oxidized cytochrome c was 

 found by Holt and French (1948) not to produce oxygen in light in the 

 presence of chloroplast suspensions. Holt (1950) reinvestigated this sys- 

 tem, using a photometric method to observe the reduction of cytochrome c. 

 He found that oxidized cytochrome c was reduced by chloroplasts (from 

 Spinacia or Phytolacca amcricana) in light, but that the reduction was 

 rapidly reversed in the dark. Experiments with added reduced cyto- 

 chrome c in the presence of oxygen showed that the chloroplasts contained 

 an enzyme ("cytochrome oxidase") capable of transferring electrons from 

 reduced cytochrome c to oxygen. The oxidation was inhibited by cyanide, 

 azide and carbon monoxide. The inhibition by the latter, however, could 

 not be reversed by light as is the case with cytochrome oxidase in respira- 

 tion. 



The presence of a cytochrome oxidase can explain the lack of oxygen 

 production by illuminated chloroplast suspensions in the presence of oxi- 

 dized cytochrome c; but it can not explain why practically complete reduc- 

 tion of cytochrome c could be observed spectroscopically — unless this reduc- 

 tion occurred not by hydrogen transfer from water, but by reaction with 

 some cellular reductant. 



Mehler (1951-) also found rapid autoxidation of cytochrome c to follow 

 chloroplast-sensitized photochemical reduction (revealed by spectroscopic 

 observation). He attributed the failure of cj-anide to stimulate oxygen 

 evolution from the cytochrome-chloroplast system (by poisoning the cyto- 

 chrome oxidase) to a destructive effect on cyanide of concentrated spinach 

 chloroplast suspensions (such as are used in manometric, in contrast to 



