388 LIFE : OUTLINES OF GENERAL BIOLOGY 



obtain these substances in a pure state ; they are probably allied to 

 proteins, but their chemical composition is unknown; they are 

 destroyed by heat; they form colloidal solutions in water. We may 

 be pardoned for repeating that the colloidal state is one in which 

 the dissolved substance exists in the form of very minute particles 

 or droplets, and has therefore a very large surface, which is no doubt 

 of importance in the activity of the enzyme. As examples of 

 enzymes may be mentioned those of the digestive juices, which are 

 formed in the cells lining the alimentary canal and its glands, but 

 act outside the cell on the nutritive substances that have passed 

 into the stomach or the intestine, or, in Invertebrates, into out- 

 growths of the food-canal such as the pyloric caeca of starfishes. 

 As we have noticed in connection with "Digestion" {q.v.), the 

 digestive juices include enzymes which split proteins into amino- 

 acids. others which attack starch and complex sugars, others which 

 work on fats. The function of one and all is to simplify the foodstuffs 

 to relatively simpler forms which can be absorbed by the cells 

 lining the food-canal, whence, in higher animals, there is a diffusion 

 into the general blood-stream. But it must be understood (i) that 

 there are similar enzymes that remain within cells, and do their 

 work there; and (2) that some of the ferments serve to hasten 

 chemical reactions very different from simplifications, for example, 

 oxidations. 



Asking again for a little patience in order to reach an appreciation 

 of a great discovery, we must linger for a little over oxidising 

 enzymes. There are two well-known and widely distributed oxidising 

 enzymes which have the power of attacking hydrogen peroxide 

 (H2O2), and converting it into water and oxygen; but whereas the 

 one, catalase, sets free the oxygen in inactive, molecular form, the 

 other, peroxidase, sets free active oxygen, which is of great chemical 

 potency. The relation of these enzymes to each other, and to the 

 enzymes which oxidise other substances directly, is very difficult 

 and not well understood; but it may be possible to give an intelli- 

 gible picture of how they work, though it must, unfortunately, deal 

 with the enzymes in milk, not with those in the living cell. 



If there is introduced into milk the nitrogenous base called 

 xanthine — near uric acid — it is immediately attacked by an 

 enzyme which splits off from the xanthin molecule hydrogen in the 

 active state. In other words, the xanthin is oxidised by the removal 

 of hydrogen. The active hydrogen does one of two things: (a) it may 

 combine with some other organic substance which it reduces; or, 

 (6) if air be present, it may unite with molecular, inactive oxygen 

 to form hydrogen peroxide. But milk also contains peroxidase, 

 which attacks the hydrogen peroxide and sets free oxygen in the 

 active state, and this oxygen combines either with the xanthine or 

 with some other organic substances, which it oxidises. The function 



