KINETICS 1613 



of the Hill reaction by a variety of agents; his results are summarized in 

 Table 35.XI. 



Wessels (1954) found partial inhibition of the photoreduction of quinone 

 and DCPI by 6 X IQ-" mole/1., and full inhibition by 6 X IQ-' mole/1. 

 2,4-dinitrophenol, in agreement with table 35. XI. In agreement with 

 Aronoff (1948) and Clendenning and Gorham (1950), but contrary to 

 French et al. (1946), he found no marked effect of sodium fluoride on the 

 Hill reaction. Thymol inhibited quinone reduction by 50% in a concentra- 

 tion of 3 X 10~^ mole/1. In contrast to Macdowall's data, iodoacetamide 

 (3 X 10"- mole/1.), p-chloromercurihenzoate (6 X 10^* mole/1.) and p- 

 aminophenyldichloroarsine (6 X 10"* mole/1.) (all three strong sulfhydryl 

 inhibitors), were found without effect. Only slight inhibition was ob- 

 served with 3 X 10-2 mole/1, nitrite. Phthiocol (6 X 10"^ mole/1.) in- 

 hibited the reduction of DCPI, but not that of quinone; 0.001 M penicillin 

 or Chloromycetin had no effect. Sulfanilamide and nicotinic acid were with- 

 out effect even at 10 "^ mole/1. Of the cations studied by Wessels (all 

 10-* M), Zn++ was found to stimulate the reaction with quinone, but not 

 that with DCPI; Mn++ stimulated both of them. Mg++ ions had no ef- 

 fect, while Ca++ ions accelerated only the reaction with quinone. 



Cu++ ions inhibited strongly at 10-" mole/L, Hg++ ions even at 10"^ 

 or 10"® mole/1, in agreement with table 35. XI. 



No effect was caused by oxidized glutathione, DPN (cf. section 4(/) 

 above) or ATP (6 X 10 -" mole/L). 



Figures in Table 35. XI and the inhibition experiments discussed before, 

 support the view that the cyanide- and azide-sensitive component of the 

 photosynthetic mechanism is not involved in the Hill reaction, and is prob- 

 ably concerned with the transformation of carbon dioxide. Hydroxyla- 

 mine, o-phenanthrolin, and dinitrophenol are powerful inhibitors of the Hill 

 reaction, affecting it either equally strongly in weak and strong light, or 

 preferentially in weak light. Of the two heavy metals, cupric ions inhibit 

 much stronger in weak light {cf. Vol. I, page 340 for a contrary observation 

 by Greenfield on photosynthesis in Chlorella), mercuric ions in strong light. 

 The narcotics act, as usual, at all light intensities; but phenylurethan and 

 chloroform show a somewhat enhanced action in weak light. 



Strychnine (5 X 10"* M) strongly stimulated the Hill reaction in 

 strong light (46 klux), but inhibited it slightly in weak light. 



In addition to the specific inhibitors listed in Table 35. XI, Holt, Smith 

 and French (1950) and Macdowall (1949) observed also the influence of 

 various organic reagents: sucrose, formaldehyde, acetone, ethanol, 

 methanol, carbitol, isopropanol, n-propyl glycol, propyleneglycol, glycerol, 

 toluene, dioxane; and of salts, such as MgS04, Na2S04(NH4)2S04 and 

 CaCla. All had a more or less pronounced inhibiting effect. Complete 



