EFFECTS ON VARIOUS METABOLIC SYSTEMS 495 



Photosynthesis 



The photochemical release of oxygen in chloroplast suspensions (Hill 

 reaction) is classically demonstrated with p-benzoquinone as the hydrogen 

 acceptor, but a number of other quinones with values for Eq between 



7>Q + H.,0 -> v-qn^ + 1/2 O2 



— 0.1 and + 0.44 v can also function in this reaction, as can various non- 

 quinonoid substances such as NADP. A new type of quinone has recently 

 been found to occur in chloroplasts and is able to restore the Hill reaction 

 in extracted chloroplasts; this is the plastoquinone series (Bishop, 1960). 

 The plastoquinones differ from the ubiquinones only in lacking the ring 



of CH3 ~1 



HgC^Jl^^ CH— CH=C— CH£|-H 



H,cV 



o 



Plastoquinones 



methyl group, and having methyl groups instead of methoxy groups. 

 It has been postulated that the plastoquinones may be reduced directly 

 following the photoactivation of chlorophyll and function by transferring 

 hydrogen atoms to NADP, perhaps also being involved in photophosphoryl- 

 ation. Possibly the artificial quinones previously used in the Hill reaction 

 function as the natural plastoquinones. The discovery of these new qui- 

 nones enlarges the interest in the actions of exogenous quinones on photo- 

 synthesis and opens up interesting possibilities for inhibitory mechanisms. 

 Photosynthesis in Chlorella is quite potently inhibited by menadione and 

 phthiocol, while at the same time respiration may be stimulated or un- 

 affected (Gaffron, 1945). In general, photosynthesis is much more sensitive 

 than respiration to the quinones. The chelating or uncoupling mechanisms 

 suggested by Gaffron are unlikely explanations. When p-benzoquinone is 

 used as a Hill reagent, the ability of the cells to fix CO2 in photosynthesis 

 is entirely lost (Clendenning and Ehrmantraut, 1950), so that the inhibitory 

 action must be exerted on the later nonphotochemical steps. High concen- 

 trations of p-benzoquinone will actually inhibit photochemical oxygen 

 release or the Hill reaction, but the usual concentrations can block photo- 

 synthesis without affecting oxygen evolution or respiration. Despite the 

 fact that menadione and some other naphthoquinones can be used as Hill 

 reagents, they can occasionally inhibit oxygen evolution, as they do in 

 Elodea (Schopfer and Grob, 1949 a). p-Benzoquinone, duroquinone, and 



