8 VETERINARY PHYSIOLOGY 



(2) Nature of Enzyme Action. — (i.) Enzymes act by 

 hastening reactions which go on slowly without their 

 presence, but they do not themselves take any direct part 

 in the reaction ; they modify its rate but not its extent. 

 Hence, a very small quantity may bring about an extensive 

 change in the substance acted upon, (ii.) The reaction 

 does not pass beyond the point of equilibrium. Thus, when 

 the enzyme maltase acts upon malt sugar, it converts it 

 only in part into dextrose, (iii.) With certain enzymes at 

 least, the action may actually be reversed ; the enzyme, 

 which splits esters into their component acid and alcohol, 

 may cause a linking of those components to form esters. 

 (iv.) The rapidity of the reaction is retarded as it proceeds 

 sometimes by the accumulation of H ions, sometimes by the 

 accumulation of the products of the change. When these 

 are removed the action may again be accelerated. 



The general action of enzymes is catalytic. It may be com- 

 pared to the action of an acid in the inversion of cane sugar — 



C.oH^Ai + H2O = 2(CeH,A)- 



Here an acid merely hastens a reaction which would go on 

 slowly in the presence of water alone. 



(3) Mode of Action of Enzymes. — {a) The precise way in 

 which such catalytic actions are brought about is still not 

 quite clear, but there is evidence that the catalyser acts as 

 a middleman between the reacting substances — in the case 

 of HgO., taking up the and then giving it off and in the case 

 of the decomposition of cane sugar taking up HoO and 

 handing it on. In the same way in the oxidation of 

 glucose which occurs when it is boiled with an alkali, a 

 metallic oxide, such as cuprous oxide, ma}^ take oxygen 

 from the air, becoming cupric oxide, and then hand the 

 oxygen on to the glucose, thus making the oxidation more 

 rapid. 



(6) While such catalysers as the inorganic acids act upon 

 many different substances, the enzymes have generally a 

 specific action upon one substance alone, the substrate 

 of the enzyme. It is as if each enzyme fitted one special 

 substrate as a key fits one special lock. They are generally 



