UTILISATION OF PROTEIDS IN THE ANIMAL 169 



of human urine have shown that the nitrogen elimination which 

 varies so intimately with the amount of proteid ingested concerns, 

 alone that which is in the form of urea. Other nitrogenous 

 substances which we excrete in much smaller quantities — such 

 as creatinine, uric acid, and ammonia — are much more stable in 

 their output. The urea, according to Folin, is, in the main, the 

 result of that preliminary treatment of the diet proteid which 

 we have discussed — it is of " exogenous " origin ; the other 

 substances arise from the living tissues, and their amount is a 

 measure of tissue breakdown — their origin is " endogenous." I 

 am at one with Dr. Noel Paton, and differ from its author in 

 feeling that the chief support of Folin's wholly acceptable theory- 

 is not derived from these considerations, but rather from its 

 satisfactory dissociation of the phenomenon of nitrogen elimi- 

 nation from what is, in a stricter sense, the true period of the 

 utilisation of the proteid. 



It must of course not be supposed that all the amino-acids 

 derived from a meal of proteid are thus further disintegrated, 

 with loss of their nitrogen, before entering the more intimate 

 domain of tissue metabolism, though the proportion under 

 average conditions may be large. However great may be 

 the stability which we ascribe to healthy tissues, we must 

 recognise that their repair in some degree is always necessary, 

 and the materials for the re-formation of proteid must always 

 be reaching them. It is still unsettled whether or not fresh 

 synthesis of proteid occurs in the intestinal wall. If it does, 

 some part of the blood proteids must represent transport 

 material. More likely free amino-acids are transported, as such, 

 and it is only the analytical difficulty which prevents their 

 identification in the blood. 



It is interesting to remember that the proteids of one tissue 

 differ from those of another, and the individual tissue has, like 

 the whole animal, a need for preserving its specific stamp. If 

 material for repair reaches it in the form of proteid in the blood, 

 then each tissue must deal in its own way with this common 

 supply, and a second breakdown, preceding synthesis, would 

 seem to be necessary. For the determination of this we should 

 look to the activity of those intracellular ferments which are to 

 be discussed immediately. If the free amino-acids circulate, 

 material for selective synthesis is directly presented to the 

 cell. 



