450 IX. HEMATIN ENZYMES, II 



by the removal of hydroxylamine by oxime formation with oxaloacetic acid. 

 Hydroxylamine is known to form hem/globin and choleglobin from hemo- 

 globin. The role of oxaloacetic acid is to prevent this back reaction which 

 destroys the effective hemoglobin catalyst, but it is interesting to note that 

 this is evidently not done very efficiently. The nitrogen fixation according 

 to Virtanen's theory is thus due to a reversal of the action of hydroxylamine 

 on hemoglobin. This reaction is normally not reversible and must certainly 

 still require the presence of another factor activating the nitrogen.* 



Virtanen has assumed a relation between hemoglobin and hydrogenase in 

 the root nodules. At present it is difficult to see a connection between the 

 hydrogenase in root nodule bacteria and the role of hemoglobin in the 

 nitrogen fixation, or between these two phenomena in root nodules and the 

 same phenomena in Azotobader. No hemoglobin has been found in Azoto- 

 bacter. These experiments are of importance with regard: first, to the 

 appearance of hemoglobin at an early stage of evolution for a purpose not 

 connected with its oxygen-carrier role; and second, to the mode of break- 

 down of hemoglobin to bile pigment, in a way which evolution has not 

 essentially changed. These aspects will receive further discussion in later 

 chapters. 



5.3. Possible Hematin Nature of Catalysts 

 in Photosynthetic Processes 



A full treatment of this subject would require a detailed discussion of 

 many aspects of photosynthesis which cannot be given in this book. The 

 reader is referred to the excellent monograph of Rabinowitch {2198), as well 

 as to reviews by Franck, Gaffron, van Niel, and French {9J^0,951,97J^.,2050y 

 2051). 



Photocatalase {oxygen-liberating enzyme, deoxidase) . It is well known that 

 the carbon dioxide assimilation of green plants consists of photochemical and 

 "dark" reactions. This was revealed by the studies of Blackman {286) on 

 the factors limiting the rate of photosynthesis, and by Warburg's investi- 

 gations with intermittent light {2923). Willstatter and Stoll {3092) assumed 

 that the dark reaction consisted in the liberation of oxygen from a peroxide 

 formed in the photochemical reaction, and was catalyzed by an enzyme which, 

 because of this catalase-like action, was called photocatalase. The obser- 

 vations of Warburg and others that the dark reaction was inhibited by 

 cyanide, sulfide, azide, and hydroxylamine (Shibata and Yakushiji, 254-5), 

 indicated that the enzyme was possibly a hematin compound. 



Later investigations have shown that several different reactions and cata- 

 lysts are involved in the dark reaction, and that some of the above-mentioned 

 inhibitors, notably cyanide, interfere primarily with reactions other than the 

 liberation of oxygen. Hill and Scarisbrick {1285) have even constituted a 

 system with isolated chloroplasts and ferr'c oxalate which developed oxygen 

 without being inhibited by cyanide, azide, or hydroxylamine. Nevertheless, 



* Keilin and Smith {1502a) have been unable to confirm the results of Virtanen and 

 Laine. 



