THE CHEMISTRY OF THE DIGESTIVE JUICES 331 



by their mere growth, the distinction between formed and unformed 

 ferments has lost its significance, and has to a great extent been 

 superseded by the distinction between intra- and extra-cellular 

 enzymes (also called endo- and exo-enzymes) i.e., between 

 ferments which normally act in the interior of the cells where they 

 are produced and ferments which act outside of the cells that secrete 

 them. From yeast cultures, for instance, by crushing the cells,*, 

 a ferment (zymase*) can be obtained which in the complete absence 

 of living yeast-cells, and, indeed, of any living micro-organism, forms 

 alcohol and carbon dioxide from sugar, just as living yeast does. 

 There is every reason to believe that it is by the intracellular action 

 of this endoenzyme that the yeast-cell normally causes alcoholic 

 fermentation. The digestive ferments are typical extracellular 

 enzymes. Their chemical nature has not been exactly made out; 

 some of them at least do not appear to be proteins, or to contain 

 a protein group. Many of them apparently exist in the colloidal 

 condition, although this has not been shown for all. In certain 

 cases the more or less stable union of a definite inorganic substance 

 with the ferment, or its actual inclusion in the ferment molecule, 

 seems to be a condition of its action. Thus there is reason to believe 

 that in gastric digestion hydrochloric acid is loosely combined with 

 the pepsin. In the plant oxydase, laccase (p. 268), manganese is 

 present. And the fact that manganese salts oxidize certain substances 

 as laccase does suggests that it is the manganese in combination 

 with some protein or other organic compound in the ferment 

 molecule which confers upon laccase its oxidizing power. A similar 

 relation between iron and some animal oxydases is possible, though 

 not definitely proved. But none of the ferments of the digestive 

 juices has as yet been satisfactorily isolated, and at present it is 

 only by their effects that we recognize them. The difficulty of 

 isolating them is increased by the fact that, like other colloids, 

 they readily adhere to surfaces, and are carried down by the most 

 diverse precipitates of substances to which they are chemically 

 indifferent. On the other hand, this very property is taken advan- 

 tage of to procure more concentrated, although still impure, solutions 

 of them than exist in the natural secretions. Thus in the prepara- 

 tion of many ferments the first step is to produce an inert pre- 

 cipitate, such as calcium phosphate, in the juice or extract. Some 

 of the ferments act best in an alkaline, some in an acid medium. 

 They all agree in having an ' optimum ' temperature, which is more 

 favourable to their action than any other; a low temperature sus- 

 pends their activity, and boiling abolishes it for ever. The 

 optimum temperatures of the majority of enzymes lie between 



* Ferments are usually designated by names with the termination ' ase,' 

 and indicating the kind of substances on which they act, or sometimes their 

 source. Thus proteases are ferments acting on proteins, amylases ferments 

 acting on starch, etc. 



