HYDROGENASE 447 



Inhibitors. Carbon monoxide inhibits the action of hydrogenase 

 (2628,2629). Hoberman and Rittenberg (1297) observed a reversal 

 of this inhibition by Hght. This could not be confirmed by other 

 workers (2651,3096), but insufficiently strong intensities of light may 

 have been applied. 



At a partial pressure of 0.07 atmosphere, carbon monoxide inhibits 

 the butyric acid fermentation about 50%; this is also completely 

 inhibited by 10" M cyanide (Kempner and Kubowitz, 1507,1513, 

 1590). Strong light reversed the inhibition by carbon monoxide, and 

 a few points of the photochemical absorption spectrum were plotted. 

 Too few points were taken, however, to allow the construction of the 

 curve; a maximum of absorption at about 560 m/x and a strong band 

 in the ultraviolet are indicated. 



The inhibition by carbon monoxide does not interfere with the initial 

 stage of the reaction in which glucose is split into C3 compounds. In an 

 atmosphere containing carbon monoxide, lactic acid is the end product, while 

 the formation of butyric acid and gaseous hydrogen are completely inhibited. 

 The enzyme apparently splits pyruvic acid into acetic acid, carbon dioxide, 

 and molecular hydrogen according to the equation: 



H3CC(OH)2C02H -^ CH3CO2H + CO2 + H2 



Hydrogenase must be an essential part of tliis enzyme system, which is also 

 reversibly inhibited by molecular hydrogen. 



Hydrogenase is inactivated by oxygen, the "Knallgas" reaction 

 only occurring when hydrogen is in excess, although some bacteria 

 (Bacillus pycnoticus) can react in mixtures of two parts of hydrogen 

 with one part of oxygen. The inactivated enzyme is reactivated by 

 molecular hydrogen as well as by hydrogen donors, glucose, pyruvate, 

 formate, f umarate, and succinate (1297) ; the action of succinate could 

 not be confirmed by Lascelles and Still (2651). Cyanide blocks this 

 reactivation and 10"^ 31 cyanide therefore inhibits when added to 

 the aerobic, but not when added to the anaerobic, system (1297,2626). 



Still and co-workers illo,26o]) found that the pure enzyme was not 

 inhibited by azide, which even accelerated the "Knallgas" reaction. They 

 found that with bacteria, the azide inhibition of tlie reaction with methylene 

 blue was small, and was unaffected by the state of oxidation of the enzyme. 

 On the other hand, with hydroxylamine and hydrazine, there was a clear 

 difference between the inhibition of the oxidized and the reduced enzyme, 

 the reduction of the former to the active reduced state evidently being 

 inhibited. These two substances also inhibited the "Knallgas" reaction 

 much less than the methvlene blue reaction. 



