METABOLISM OF PROTEINS 589 



The significance of these autolytic enzymes in the normal metabo* 

 lism of proteins has been already discussed (p. 569) ; indeed, so many 

 of the chemical reactions of the body have been found to depend 

 upon enzymes that modern physiology may at first thought seem 

 almost to have reverted to the position of van Helmont and his 

 school in the seventeenth century, who resolved all difficulties by 

 murmuring the magic word ' ferment.' No fewer than eleven fer- 

 ments have been stated to be present and active in the liver alone 

 viz., a proteolytic and a nuclein-splitting ferment, a ferment 

 which splits off ammonia from amino-acids, a milk-curdling ferment, 

 a fibrin ferment, a bactericidal ferment, an oxydase, a lipase, a 

 maltase, a ferment called glycogenase, which changes glycogen into 

 dextrose, and an autolytic ferment. In the presence of such an 

 array of enzymes the organs might seem to be little more than 

 incubators in which the ferments do their work. It must not be 

 supposed, however, that the intracellular ferments, whether they 

 cause decomposition or synthesis, oxidation or reduction, work in- 

 dependently of what, for want of a better name, we must call the 

 organization of the cell. We may be sure they are the servants 

 and not the masters of the protoplasm, and that a drop of an extract 

 containing intracellular ferments has very different powers from a 

 living cell. ' It is not in the existence of the ferments, but in their 

 combined action at the proper time and in the proper intensity, that 

 the riddle of metabolism lies ' (Hober). 



Summary. At this point let us sum up what we have learnt as 

 to the relation between the proximate principles of the tissues and 

 the proximate principles of the food. Inside the body we recognize 

 representatives of the three groups of organic food-substances in a 

 typical diet proteins, carbo-hydrates , and fats. But we should 

 greatly err if we were to imagine that the three streams of food- 

 materials have flowed from the intestines into the tissues each in its 

 separate channel, neither giving to nor taking from the others. 

 The fats of the body may, indeed, in part be composed of molecules 

 which were present as fat in the food ; but they may also be formed 

 from carbo-hydrates, and probably from proteins. The carbo-hydrates 

 of the body the glycogen of the liver and muscles, the sugar of the blood 

 may undoubtedly be derived from carbo-hydrates in the food, but they 

 may also be derived from proteins and from fats (certainly from their 

 glycerin constituent, perhaps from the fatty acids as well). The pro- 

 teins of the body come mainly, if not solely, from the proteins of the food. 

 Although, of course, neither fats nor carbo-hydrates can by themselves 

 form protein, being devoid of nitrogen, it is possible that products 

 arising in the intermediary metabolism of either may, by combining 

 with nitrogenous groups, be transformed into amino-bodies, which can 

 then take part in the synthesis of proteins. In any case there is no 

 doubt that both carbo-hydrates and jals can economize proteins and 

 shield them from an overhasty metabolism. 



