VARIOUS METABOLIC PATHWAYS 891 



1-786). The increase of the inhibition with rise of temperature was inter- 

 preted by Houck in terms of an equilibrium between active and denatured 

 forms of the attacked enzyme, especially luciferase. Actually from this 

 work one cannot locate the site of the inhibition, or even be certain it is 

 on the bioluminescent reactions themselves, since interference with glucose 

 uptake or oxidation, or the supply of ATP, could be responsible. However, 

 it has been found that Achromohacter luciferase is markedly inhibited in the 

 range of Hg++ concentrations found to inhibit luminescence (Table 7-13), 

 so it may well be that luciferase is the major site of action. This is somewhat 

 substantiated by the fact that luminescence in extracts of Renilla reniforniis, 

 the sea pansy, is strongly inhibited by p-MB (Cormier, 1960). 



Photosynthesis and Photophosphorylation 



The marked inhibition of certain phases of photosynthesis by iodoacetate 

 and iodoacetamide (III-1-156) indicates the necessity of SH groups, so that 

 one would expect the mercurials to be effective inhibitors, and this is borne 

 out. The photoreduction of various dyes in isolated chloroplasts or grana 

 (Hill reaction) is very sensitive. In spinach chloroplasts it is inhibited 90% 

 by 0.005 mi/ Hg++ (Macdowall, 1949). The dye reduction may be me- 

 diated through NADPH, which is the initial acceptor. The photosjmthetic 

 NADP reductase from spinach is inhibited 50% by 0.012 mM and 90%by 

 0.016 mM p-MB (San Pietro and Lang, 1958) and the photoreduction of 

 NADP in chloroplasts is similarly inhibited, although slightly less potently 

 (J. S. C. Wessels, 1959). The photoreduction of cytochrome c and NADP 

 by the chloroplast enzyme is 50% reduced by 0.004 mM p-MPS and the 

 enzjTne is bleached by the mercurial (Keister and San Pietro, 1963). In 

 Chromatium, illumination causes a blue fluorescence presumably due to 

 bound NADH, indicating that here there is a photoreduction of NAD. 

 This fluorescence change is completely abolished by 0.02 mM PM (Olson 

 et al, 1959). Finally, a NADPH diaphorase from chloroplasts, possibly in- 

 volved in the reduction of the Hill dyes by NADPH, is inhibited 53% by 

 0.023 mM Hg++ and 50% by 0.13 mM p-MB (Avron and Jagendorf, 1956). 

 The initial photoreductive changes upon illumination are thus quite po- 

 tently inhibited by the mercurials, and this must certainly be one site of 

 action on over-aU photosynthesis. Other evidence for a primary interference 

 with the photolysis of water was obtained by Damaschke and Liibke (1958), 

 who showed that Chlorella under anaerobic conditions produces a sudden 

 burst of Hg upon illumination and that this is completely inhibited by 0.2 

 mM p-MB (lower concentrations not tested), and by Whittingham (1956), 

 who found that althoagh 0.12 mM p-MB does not inhibit the initial evo- 

 lution of Oo by illuminated Chlorella, the steady-state formation of Og is 

 strongly depressed. It may be mentioned that even high concentrations of 

 Hg++ do not react with chlorophyll (Macdowall, 1949). 



