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R. A. Dilley, M. D. Henninger and F. L. Crane 



Table III shows the effect of adding back the various quinones to acetone 

 extracted spinach chloroplasts. Following extraction there is a decrease in 

 the rates of reduction of ferricyanide, cytochrome c and NADP, but on a 

 chlorophyll basis there is an increase in the photoreduction of indophenol. 

 On a dry weight basis there is a decrease in all the reductase activities. 



It is apparent that there is considerable variation in the extent of stimu- 

 lation of a particular photoreduction by the various quinones. 



The photoreactions are inactivated by heating the chloroplast suspension 

 to 100° C for two minutes. 



Two general statements can be made about the restoration of photo- 

 reductase activities by plastoquinones and tocopherylquinones as shown in 

 Table III. First, none of the quinones stimulate all activities. Secondly, the 

 tocopherylquinones tend to be effective in restoration of certain activities at 

 much lower levels than the amounts of plastoquinones required for restora- 

 tion of activity. It may also be noted that the tocopherylquinones occur at 

 much lower concentrations in vivo. Ferricyanide reduction is the least 

 selective process and is stimulated to some extent by all quinones except 

 «-tocopherylquinone. ^ and y- tocopherylquinones as well as a mixture of 

 PQ-C +PQ-D are also effective for restoration of cytochrome c reduction. 

 Purified PQ-C is also slightly active in this system. The ascorbate dependent 

 NADP reduction is stimulated by Jj'-tocopherylquinone, PQ-B and PQ-C. In 

 contrast to this the reduction of NADP from water requires the addition of 

 PQ-A + PQ-C + PQ-D on the one hand or PQ-B alone on the other (n). The 

 combined effects of more than one quinone are consistent with observations 

 of Trebst of more than one quinone site '^^^ in the electron transport system 

 of chloroplasts. 



After acetone extraction there is the appearance of a new type of activity, 

 namely a photooxidation of reduced cytochrome c or NADP in the presence of 

 certain added quinones. This type of activity islllustrated by negative values 

 shown in Table III. In the experiments illustrated in the table where negative 

 values are shown, addition of quinone in the dark leads to a partial reduction 

 of cytochrome c or NADP. When the system is exposed to light, the reduced 

 acceptor is oxidized. This type of activity is best seen when PQ-A or PQ-D 

 is added to the acetone-extracted chloroplasts in the NADP system. Similar 

 effects can be shown when reduced NADP is added at the start with subse- 

 quent oxidation occurring in the light. These dark reduction and photo- 

 oxidation processes are readily reversible. After photooxidation in the light 

 the NADP can be shown to return partially to the reduced form when the 

 system is returned to the dark. It would appear that the addition of certain 

 quinones makes a pool of reducing power available for reduction of NADP in 

 the dark, and that the electron flow is reversed in light to regenerate the 

 pool of reducing power with consequent oxidation of the NADPH2. 



The photoreduction of both NADP and cytochrome c has been shown by 

 Keister et al ^^' to require the presence of an enzyme, photosynthetic 



