THE UTILISATION OF PROTEIDS IN 



THE ANIMAL 



BY F. G. HOPKINS, D.Sc, F.R.S. 

 Reader in Chemical Physiology at the University of Cambridge 



As an essential part of the all-important results of photosyn- 

 thesis in the green plant, there arise from inorganic precursors 

 those complex nitrogenous substances, the proteids or proteins, 

 which form the chemical basis of every living cell, animal or 

 vegetable. 



For its supply of these the animal is ultimately dependent 

 upon the plant, and cannot maintain its tissue equilibrium 

 without consuming pre-formed proteid as part of its dietary. 

 How precisely the ingested proteid is utilised ; in what form 

 the dead proteid of the food reaches the living tissues ; whether 

 it all becomes part of the living tissues before it can be broken 

 down and energy extracted from it ; what, in any case, are the 

 precise chemical changes associated with the liberation of its 

 energy — all these are questions which it has proved very difficult 

 to study experimentally, and they still await final solution. 

 They are related, however, to problems which concern a branch 

 of physiology now at a very interesting stage of its development, 

 and they yield a case in which opinion is at the moment 

 emerging from the influence of older teachings. It is justifiable 

 therefore to consider here the present attitude of physiologists 

 towards them. 



The study of animal metabolism reached its scientific phase 

 under the influence of Liebig, and the questions which concern 

 us were perhaps first clearly stated, as they were certainly first 

 clearly answered, rightly or wrongly, by him. To Liebig, 

 indeed, the fundamental aspects of proteid metabolism in the 

 animal appeared to suggest no great difficulties, and his answers 

 to the questions partake of the lusty self-confidence which was 

 characteristic of biological science in his day. Deceived by the 

 fact that the percentage analysis of any one proteid (its content, 

 that is, of carbon, hydrogen, nitrogen, and oxygen) differed 

 so little from that of any other, Liebig conceived of a practical 



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