148 PHYSIOLOGY OF NUTRITION 



It is manifest that the proteins that are most important in the life-processes 

 are peculiarly constituted. The hydrolysis of these proteins does not primarily 

 result in mono-amino acids, but gives, to a much greater degree, heterocyclic 

 basic derivatives of purin, pyrimidin and imidazol. The structural formulas 

 of these three substances are given below. 



N=CH N=CH 



I ! II 



CH C— NH. CH CH HC— NH. 



CH 



N C N^ 



N— CH 



HC N 



CH 



Purin Pyrimidin Imidazol 



Mono-amino acids are especially scarce in fish sperms. These acids, which 

 are so predominant in reserve proteins (following the terminology of A. KosseP), 

 are practically without importance in the formative proteins. 



§3. Enzymes.2 — Most biochemical reactions occurring in plants and ani- 

 mals can now be interpreted in terms of enzymatic activity. To be sure, suc- 

 cess has not yet attended the effort to isolate enzymes in the free state, and the 

 presence of an enzyme is only inferred from its specific activity. The plants in 

 question must be killed in such a way that their enzymes are not destroyed. 

 One of the most useful methods involves the use of a water or glycerine extract 

 of the finely divided plant material. Brown and Morris' dried the plants at 

 from 40° to so°C. (higher temperatures are injurious to the enzymes) and showed 

 that the powder obtained by pulverizing the dried tissues exhibited enzymatic 

 activity. In the isolation of zymase, which accelerates alcohoUc fermentation, 

 E. Buchner found that the juice expressed from triturated yeast cells, by means 

 of a hydraulic press, possessed the properties of the enzyme. He also em- 

 ployed acetone in kilHng the yeast cells. Palladin* employed a method of kilUng 

 by low temperature, to demonstrate enzymes in seed-plants. The frozen plants 

 are dead when thawed out, but the efficiency of the various enzymes has not 

 been decreased. 



As to the mechanism of their action, enzymes are to be regarded as cataly- 

 zers. Catalysis may be defined as the acceleration or retardation of an other- 

 wise slow or Hmited chemical change, through the influence of a foreign sub- 

 stance. Many cases of the catalytic acceleration of various reactions are known 

 in general chemistry. For example, hydrogen is but slowly formed by the ac- 

 tion of pure sulphuric acid upon zinc. When a drop of platinic chloride solu- 



' Kossel, A., Einige Bemerkungen uber die Bildung der Protamine in Tierkfirper. Zeitsch. physio!. 

 Cliem. 44: 347-352. I90S- 



s Duclaux, E., Traits de microbiologic. Paris, 1898, 1899, 1900. Disastases, toxines et venins, in v. 2. 

 Oppenheimer, Carl, Die Fermente. 3te Aufl. Leipzig. 1909. Abderhalden's Handb, 1910. [See note i, 

 p. 140.) Green, J. Reynolds, The soluble ferments and fermentation. Cambridge. 1899. 2nd ed. Cam- 

 bridge. 1901. [Euler, 1908. (See note I. p. 139.) Idem, Allgemeine Chemie der Enzyme. Wiesbaden, 

 1910. Idem, General chemistry of the enzymes. Tr. from revised and enlarged Ger. ed. by Thomas H. 

 Pope. IX + 323 p. New York. 1912,] 



' Brown and Morris, 1893. [See note i; p. 28. 1 



* Palladin, W., Die Arbeit der Atmungsenzyme der Pflanzen unter verschiedenen Verhaltnissen. Zeitsch. 

 physiol. Chem. 47: 407-451. 1906. 



