58 PHYSICAL CHEMISTRY IN BIOLOGY. 



If those bodies which accelerate a reaction are to be considered as 

 catalysts, then certainly the solvents must belong to the catalytes. 

 Attention must be called to the enormous influence which the solvent 

 can exert upon the velocity of a reaction under otherwise equal condi- 

 tions. Thus MENSCHUTKIN l found for the reaction 



= (C 2 H 5 ) 4 .N.L, 

 the following velocity in different solvents: 



Hexane.. ....... 0.00018 



Heptane ....................... 0.000235 



Xylene ......................... 0.00287 



Benzene ....................... 0.00584 



Ethyl alcohol ................... 0.03660 



Benzyl alcohol .................. 0. 13300 



Recently BREDIG and FAJANS 2 have been able to show that an opti- 

 cally active solvent can help in the decomposition of optical antipodes 

 to a varying extent. Of the optical antipodes of campho-carboxylic 

 acid, the d-form is 17 per cent more quickly decomposed than the Z-form, 

 when they are dissolved in nicotine or when nicotine is present, dissolved 

 with the catalyte, while in an optically indifferent solvent and without 

 any nicotine the catalyte decomposes both forms with equal rapidity. 



The reaction proceeds differently with or without catalyst, and the catalyst 

 acts not only upon the velocity of the reaction. It is apparent that this does not 

 conform with OSTWALD'S definition of a catalyst (page 54). 



IV. ENZYMES. 



As the enzymes are active in very small amounts without under- 

 going any essential change, they have for a long time been included 

 among the catalytic substances. 



In regard to the terminology it must be remarked that thfee bodies upon 

 which an enzyme acts is designated as the substrate, and often an enzyme is 

 named after the substrate (amylase, protease, lipase) ; in other cases the kind 

 of action determines the name (oxidase, reductase), and finally the name is 

 founded upon the product produced by the action (alcoholase) . 



We have no general method for preparing the enzymes. In certain 

 cases they are contained in secretions (gastric and pancreatic enzymes) ; 

 in others they are prepared from the cells by crushing and pressing 

 out the cell juice (zymase, organ enzymes), and finally, most enzymes 

 can be extracted from the cells with water or glycerin, and as this last 

 gives permanent solutions it has found great use as an extraction 

 medium. 



1 Zeitschr. f. physik. Chem., 6, 41, 1890. 



2 Ber. d. d. chem. Gesellsch., 41, 752, 1908. 



