CARBON MONOXIDE INHIBITORS 319 



These observations can be rationalized by assigning the site of CO 

 sensitivity to the enzymatic processes leading to ATP formation. 

 This site could well be a cytochrome oxidase, since its inhibition is re- 

 versed by yellow light which is shown here to be effective in raising 

 the ATP level without driving P''- labeling of hexose phosphates very 

 rapidly. Red light by it.self was also ineffective in promoting phos- 

 phate esterification even with 5% oxygen in the CO since the red 

 light cannot reverse the inhibition but can only generate the re- 

 ductant necessary to convert phosphoglycerate to triose and 

 hexose phosphates. The experiments reported here were long enough 

 so that the oxidase may be said to be participating in a steady-state 

 process rather than in an induction effect. 



It is possible that the lowered levels of sugar phosphates were a 

 result of a rapid fermentation under CO. This seems unlikely since, 

 in rapid fermentation, phosphorylation of glucose would be ex- 

 pected, giving higher levels of P^- in the HMP fraction. 



Thus we favor an explanation which differs from those advanced 

 by Gaffron in that we believe the effects observed to be neither a 

 direct inhibition of oxygen evolution nor an indirect effect by accumu- 

 lated products of anaerobiosis but the result of a light-reversible in- 

 hibition of a cytochrome oxidase involved in photosynthesis. Our 

 main reason for this is that yellow light of a wavelength known to be 

 very effective in dissociating the CO-cytochrome oxidase complex 

 raises only the ATP level and that red light of wavelength ineffective 

 in this process, but effective in photosynthesis, does not raise this 

 level but lowers it — a phenomenon most readily explained by saying 

 that photosynthetic reductant can be utilized by triose phosphate 

 dehydrogenase in forming triose and consumes all the available ATP 

 in the coupled phosphorylation of phosphoglycerate. This lack of 

 ATP then inhibits further triose formation, which inhibition can be 

 relieved indirectly'' by exposure to yellow light. 



SUMMARY 



Exposure to CO lowers the amount of P^- incorporated by illumi- 

 nated barley leaves almost to the level which obtains in darkness. 

 Illumination with red light lowers the ATP level in the presence of 

 CO below the level in the dark; illumination by yellow light raises 

 the ATP to the control (air-darkness) level. Only simultaneous il- 

 lumination with both red and yellow light in the presence of a CO 



