HYDROGEN ADAPTATION AND DE-ADAPTATION 135 



majority of the primary oxidation products HZ is disposed of by reaction 

 (6.6c). If this is true, then the absence of oxygen evolution during 

 photoreduction indicates that the oxygen-liberating enzyme, Eo, is in 

 an inactive state. 



Another argument in favor of the absence of an active enzyme, Eo, 

 in the adapted state is, according to Gaffron, the fact that the adaptation 

 process shares with the oxygen evolution in photosynthesis a sensitivity 

 to very small quantities of hydroxylamine and phenantroline. To ex- 

 plain this similarity, one can assume that the complex formation with 

 hydroxylamine "freezes" enzyme Eo in the oxidized state, thus inhibiting 

 its function in photosynthesis, but at the same time preventing its de- 

 activation by reduction during anaerobic incubation. 



Gaffron (1943) suggested that the de-activation of Eo does not 

 eliminate this enzyme altogether as a catalytic agent, but converts it 

 into an "oxidase" Eo' (whose presence is revealed by the oxyhydrogen 

 reaction). However, it is thermodynamically impossible for Eo' to 

 catalyze the formation of the same complex {O2}, whose decomposition 

 was catalyzed by Eo. (A catalyst has the same effect on the velocity 

 of reaction in both directions, because it cannot shift a thermodynamic 

 equilibrium.) Therefore, we must assume that the transformation of 

 Eo into Eo' brings about a change in specificity— in other words, that 

 enzyme Eo catalyzes the formation of a complex {02}' which is different 

 from the complex {O2}, decomposed by enzyme Eo. 



As a result of this discussion, we attribute the de-adaptation by 

 excess oxygen to the reactions: 



(6.9) O2 >!02!' 



(6.10) {021' + 2Eh >2EhO 



which is analogous to, but not identical with, reaction (6.8), and occurs 

 whenever the removal of {02}' by the hydrogenase system (i. e., the 

 oxyhydrogen reaction) lags behind the formation of this intermediate by 

 reaction (6.9). 



It will be noted that, according to (6.9), the formation of the oxidant 

 {O2}' in the oxyhydrogen reaction cannot be avoided; but its rapid 

 consumption (by reaction 6.11) can prevent the deactivating reaction 

 (6.10) from destroying the hydrogenase more rapidly than it is restored 

 by reaction (6.5). As mentioned before, Gaffron assumed that, in photo- 

 reduction too, a certain quantity of the oxidant {O2} is formed despite 

 the removal of the preponderant part of the oxidation product, HZ, by 

 reaction with the hydrogenase system. 



A summary of the reactions associated with the adaptation and 

 de-adaptation phenomena is given in scheme 6.1. 



