SECRETION 55 



important for an understanding of chemical equilibrium in the 

 organism is the complete reversibility of their action, first 

 shown by Croft Hill (1898). The same is true of inorganic 

 catalysts ; thus in the preparation of ethyl acetate, to take an 

 illustration from the historic researches of Berthelot and Pean 

 de Saint Gilles, if molar equivalents of acid and alcohol are 

 mixed, equilibrium is reached when two- thirds of the acid and 

 alcohol are converted into ester, and precisely the same 

 equilibrium point is reached when a molar solution of ethyl 

 acetate is subjected to hydrolysis. It is usual to regard both 

 reactions as contemporaneous and equiUbrium as the equalisa- 

 tion of their respective velocities. The catalyst, in this case 

 usually a mineral acid, acts by accelerating both reactions ; 

 the actual point of equilibrium is unchanged and the total 

 energy of the system unaffected. This is also the case with 

 enzyme catalysis. Thus maltase accelerates both reactions 

 symbolised by maltose $ ^/-glucose. As with inorganic 

 catalysts {e.g. finely divided platinum) an enzyme is often 

 reclaimable at the end of a reaction ; this is well illustrated in 

 the case to be discussed immediately. Another aspect of 

 enzyme reaction illustrated by the phenomenon of biolumi- 

 nescence is specificity. Any enzyme is able to catalyse only 

 a restricted range of reactions, usually all of a well-defined type. 

 The recognition of separate enzymes in a tissue-extract depends 

 on the possibility of differential destruction of one or other of 

 the catalytic activities of the extract. When different tissue- 

 extracts have different optima of temperature or hydrogen- 

 ion concentration, etc., for catalysing precisely the same reaction, 

 there is good reason to believe that their enzymes are distinct 

 chemical entities. 



The term " enzyme " was introduced by Kiihne (1878) to 

 obviate the confusion resulting from the use of the older term 

 " ferment," a term originally used to include the activities of 

 micro-organisms like yeast. Pasteur's researches led to a 

 distinction being drawn between the fermenting action of 

 gastric juice ('* unorganised ferment ") and that of micro- 

 organisms (" organised ferments "). This confusion was 

 finally dissipated when Buchner (1903) extracted from crushed 



