lODOACETYL AND OTHER ORGANIC POISONS 319 



(complete inhibition required one or two hours). The free acid acted 

 particularly slowly in alkaline buffers and somewhat more rapidly in 

 acid solution (because of the greater ease with which neutral molecules, 

 which are present at the lower pH, can penetrate through cell mem- 

 branes). The amine, which does not dissociate, acts with the same 

 velocity in acid and in alkaline solutions. Its inhibiting effect becomes 

 manifest in concentrations from 10~* m./l. upwards; concentrations 

 below 10~^ m./l. may cause a slight stimulation. A concentration of 

 10~^ m./l. of the amine is sufficient for an almost complete suppression 

 of photosynthesis. Respiration is inhibited only above 10~^ m./l. 

 Inhibition is more complete in saturating than in " half -saturating " light. 

 This characterizes iodoacetyl as an "enzyme poison," similar to cyanide, 

 rather than as a narcotic. Kohn pointed out that iodoacetyl has no 

 tendency to form complexes with heavy metals, so that it should affect 

 other enzymes than those inhibited by cyanide. He suggested an effect 

 on sulfhj^dryl groups (with which iodoacetyl is known to react irre- 

 versibly). However, according to Nakamura (Table 11. 1), iodoacetyl is 

 also a strong inhibitor of catalase, which is a hemoproteid. Obviously, an 

 enzyme molecule may contain more than one grouping whose transfor- 

 mation can cause an inactivation of the molecule as a whole. 



Dinitrophenol affects strongly both photosynthesis and photoreduc- 

 tion; it has no specific effect on the adaptation reaction. Gaffron (1942) 

 found that dinitrophenol also inhibits hydrogen fermentation in the dark, 

 but does not affect hydrogen evolution in light (or even stimulates it), 

 which proves that the latter process is independent of an enzyme which 

 takes part in hydrogen production in the dark (c/. page 143). Dinitro- 

 phenol has no affinity for heavy metals and is therefore supposed to 

 act on enzymatically active proteins. Apparently, it affects photo- 

 synthesis by inhibiting the transfer of hydrogen from an intermediary 

 reduction product to carbon dioxide, since this stage is common to both 

 photosynthesis and photoreduction. (Catalytically active proteins may 

 be used to transfer hydrogen atoms, while heavy metal complexes transfer 

 electrons.) However, the influence of dinitrophenol on hydrogen fermen- 

 tation requires an independent explanation. 



Since dinitrophenol inhibits both photosynthesis and photoreduction, 

 it leaves the photochemical hydrogen liberation as the only light reaction 

 in adapted algae. The apparent stimulation of this reaction by dinitro- 

 phenol may be due to the prevention of losses usually caused by the 

 reaction of hydrogen with carbon dioxide (formed by fermentation, and 

 not absorbed rapidly enough by alkah). 



o-Phenanthroline and phthiocol were found by Gaffron (1944) to 

 produce in Scenedesmus effects very similar to those of hydroxylamine — 

 including inhibition of photosynthesis in small concentrations and of de- 



