346 Mr. R. H. A. Plimmer [April 8, 



biscuits entirely. The separate parts circulate to the tissues ; the 

 tissues select the ones they require and form another arrangement of 

 the units, or simply replace those which have been used in their 

 metabolism. Digestion and metabolism are a sort of re-shuffling of 

 the units. In the absence of any particular unit the tissue can no 

 longer rebuild its substance and consequently suffers. The old 

 example of the inadequacy of gelatin is now explained. The tissues 

 require tryptophan, tyrosine and cystine ; gelatin cannot provide 

 them. 



The protein molecule can be represented better by a series of 

 coloured blocks— the body colour representing the particular group 

 to which the unit belongs, and the ends being white and black to 

 represent the amino or carboxyl groupings. White is always joined 

 to black, and a very long chain can be imagined. This can be 

 made into a compact model by supposing that there is a tautomeric 

 grouping at certain points ; it would cause a folding back of the 

 chain at these points or a connection to another row of units. The 

 separation of blocks would be like the formation of proteoses and 

 peptones ; the separation of the individual pieces would be like 

 complete hydrolysis. 



In nutrition there are essentially two problems to study — the 

 formation of new tissues as in the growth of young animals, and the 

 maintenance of tissue, which undergoes so-called wear and tear, in 

 adult animals. In the latter case we have ultimately to ascertain if 

 every unit of the molecule breaks down, or certain selected units only. 

 If these are in the middle of a chain, it would follow that the whole 

 molecule would undergo metabolism and not units at the ends alone. 

 The problem resolves itself into ascertaining the function of each 

 amino acid. 



Since the practical difficulties of feeding animals with a mixture 

 of pure amino acids are far too great, advantage may be taken of 

 feeding incomplete proteins and adding to them the missing unit 

 or units. 



Wilcock and Hopkins made the first experiment of this kind in 

 1906. They selected zein as protein and fed it to mice, in one set 

 alone, in another set with the addition of 2 per cent of its amount 

 of tryptophan. Young mice on zein alone immediately began to 

 lose weight and generally died in 16 days ; decline in weight also 

 occurred in the other set, but with the added tryptophan death did 

 not occur till the 30th day. Adult mice lived 27 days without 

 tryptophan, 49 days with tryptophan. Tryptophan had thus an 

 appreciable effect ou the survival peiiod of the animals. Zein is 

 incomplete in respect of other units, and death was probably on this 

 account. The experiment was repeated in HUG by Ackroyd and 

 Hopkins under different but better conditions. The animals were 

 first given a mixture of amino acids from casein (i.e. without 

 tryptophan, which is destroyed in hydrolysis by acid) to which 



