496 5. QUINONES 



chloranil can quench the fluorescence of chlorophyll and other porphyrins, 

 but the concentrations required (3.4-11,6 mM) make them unlikely to 

 be of significance in photosynthetic inhibition (Livingston and Ke, 1950; 

 Livingston et al., 1952). 



One might postulate that exogenous quinones reoxidize reduced plasto- 

 quinones and thus divert the electron flow to NADP: 



Q 

 / 

 H^O -> PQ -> FAD(?) -> NADP 



the reduction of which is necessary for the operation of the photosynthetic 

 carbon cycle, and perhaps also prevent the phosphorylation occurring 

 during the normal electron transfer. Miyachi et al. (1955) suggested a 

 similar scheme but assumed a substance, R, formed in the photochemical 

 reaction and necessary for CO2 fixation; inhibiting quinones oxidize R 

 so that it cannot function. The plastoquinones could be visualized as R. 

 Higher concentrations of quinones would irreversibly inactivate enzyme 

 systems in addition to this oxidation of reduced plastoquinones. 



Bioluminescence 



The effects of the quinones on bioluminescence have been studied more 

 thoroughly than most other metabolic systems because for many years it 

 was thought a naphthoquinone might be involved in the normal production 

 of light. Harvey (1929) showed that bacterial luminescence is inhibited by 

 p-benzoquinone, p-xyloquinone, 1,2-naphthoquinone, 1,4-naphthoquinone, 

 and 2-Cl- 1,4-naphthoquinone, and attributed this to a direct oxidation 

 of luciferin, since anthraquinone-2-sulfonate and anthraquinone-2,6-disul- 

 fonate do not inhibit and have very low redox potentials. However, these 

 latter two substances are not ideal substances for comparison, since anthra- 

 quinones are often abnormally reactive and the sulfonate groups might 

 well interfere with their penetration. p-Benzoquinone is not a very potent 

 inhibitor and yet has a high redox potential, as shown in the study of 

 Spruit and Schuiling (1945), who thought that luciferin might be a 1,4- 

 naphthoquinone and hence examined several quinones in the hope of com- 

 petitively inhibiting the reaction (see accompanying tabulation). Lumi- 

 nescence is always more sensitive than respiration, and this is evident in 

 the curves of Fig. 5-4. Bacterial luciferin, which is now considered to be 

 a complex of FMNHg and a long-chain aldehyde, was assigned an E^ 

 of —50 mv, so it could be readily oxidized by most of the quinones studied. 

 Since cyanide can counteract the inhibition, it was postulated that the hy- 

 droquinones must be reoxidized through the cytochrome system in order 

 that the luciferin be kept oxidized. The specificity of quinones for the in- 

 hibition of bacterial luminescence was stressed by Rake et al. (1943), who 



