HYDROGEN SULFIDE AND OTHER INORGANIC POISONS 315 



Here, {O2}' is the primary oxygen-acceptor compound of the oxidase 

 system, and H2AH the primary hydrogen-acceptor compound of the 

 hydrogenase system. 

 ~ If we replace in the first of the two coupled reactions, 4 HzAh by 

 8 HX, and 2 { O2 } ' by 8 Z, and assume that two quanta are required (as 

 usual) for the production of the pair HX and Z (whether by a two-step 

 photochemical transfer, or by a one-step transfer followed by "energy 

 dismutation," is irrelevant), we obtain the following reaction system: 



(12.4a) 8 X + 8 HZ > 8 HX + 8 Z 



n2 4M r8HX + 8Z >8Z + 8HX 



{ ZAb) I 2 HsAh + {CO2} > {CH2O} + H2O + 2 Ah 



This is the suggested mechanism of photoreduction of phthiocol- (or 

 phenanthroline-) poisoned algae. It requires 16 quanta, instead of the 

 8 which are sufficient for the normal mechanism of photoreduction: 



8hy 



(12.5a) 4 X + 4 HZ > 4 HX + 4 Z 



(12.5b) 4 HX + {CO2I > {CH2O} + H2O + 4 X 



(12.5c) 4 Z + 2 H2AH * 4 HZ + 2 Ah 



This explanation implies that the poisons under consideration in- 

 hibit reaction (12.5c). It was suggested before that hydroxylamine 



affects, in the first place, the reaction { O2 } > O2 (thus preventing 



the liberation of oxygen), and, in larger quantities, also the reaction 



2 Z + { H2O } > 2 HZ + ^ { O2 } , thus preventing de-adaptation by 



intense light in adapted algae. We have now to postulate a third effect 

 of the same poison, an inhibition of reaction (12.5c). However, this 

 third effect may be related to the second one, since both can be caused 

 by an association of the poison with the substrate Z, or with a catalyst 

 which brings about a preliminary transformation of this substrate, and 

 without which the latter undergoes immediate recombination (as in 

 11.4b). We may recall in this connection a previous discussion of the 

 probable necessity of stabilizing, in photosynthesis, both the primary 

 reduction product (by means of catalyst Eb) and the primary oxidation 

 product (either by the same or by another catalyst). 



4. Hydrogen Sulfide and Other Inorganic Poisons 



(a) Hydrogen Sulfide 



This compound is a poison for most, if not all, enzymes containing a 

 heavy metal. Its effect on photosynthesis, discovered by Negelein 

 (1925), is even stronger than that of cyanide. A concentration of 10~' 

 m./l. of hydrogen sulfide reduces the photosynthesis of Chlorella (in 



