28 IMMUNO-CATALYSIS 



But under completely dry conditions a noticeable combination will not 

 be observed. They will, however, combine on ignition forming oxide of 

 iron. The applied heat accelerates the combination till sufficient heat 

 of reaction is produced to make it self-sustaining. On the other hand, 

 when we bring oxygen and iron together in the presence of moisture 

 at ordinary temperature the reaction starts automatically, the moisture 

 acting as catalyst. The role of moisture here was to reduce the energy 

 of activation necessary for the attainment of the equilibrium of the re- 

 action between oxygen and iron. 



As previously mentioned, there is no stable union between a cata- 

 lyst and the reaction products or reactants although it may form in- 

 termediate, reversible and labile complexes with these substances; 

 but the lives of such complexes are very short; the combination is 

 fugitive. It is stated that the mean life of an active H202-catalase com- 

 plex (catalase contains trivalent iron) is 1.2X10"'^ second. This mean 

 life is unique in being derived from purely chemical data, and does 

 not involve any hypothesis as to collision. Likewise, the mean life of 

 the excited oxyhaemoglobin complex (contains bivalent iron) is 

 4.0XlO-« second (Haldane, 1931). 



The fact that a minimum amount of a catalyst is capable of trans- 

 forming a large quantity of substrate is understandable for the above 

 mentioned reason that after the breakdown of the catalyst-substrate 

 complex the catalyst can complete another cycle of acceleration, and 

 continuously thereon. It is true that the unchangeability of an ideal 

 catalyst during a reaction is one of its essential characteristics and 

 responsible for its continued activity, but it does not need to be so, 

 for when a catalyst takes part in a reaction and establishes the equilib- 

 rium it is possible that a certain amount of catalyst is tied by the 

 reaction products as specific inhibitors, or used up by the system so 

 that its redelivery from the theoretical equation: (A and B reactants, 



A+C^AC; AC+B^AB+C 



C catalyst, AB reaction products, AC catalyst-reactant complex) is not 

 complete. Also the catalyst might chemically combine by a side-reaction 

 and thus be removed from the field of action. The catalyst might be 

 destroyed by one of the reaction products, such as the destruction of 

 the oxidative enzyme system of xanthine oxidase or pneumococcus by 

 the hydrogen peroxide formed as one of the reaction products. Addi- 



