266 rX/XC/I'LES OF GENERAL r/fYSIOLOGY 



the femoral artery. This implies that the liver has not added any more urea 

 to the blood than was already in that arriving to it from the other parts of 

 the animal. Should this be so, all tissues must take part in the formation of 

 urea. In the words of these authors (1912, p. 161), "the food protein 

 reaches the tissues in the form of amino-acids, and those amino-acids which are 

 not needed for the rebuilding of broken-down body material are not rebuilt 

 either into protein or protoplasm, but are broken down and their nitrogen 

 converted into urea." 



The results of van Slyke and Meyer (1913, 2), contrary to those of Folin 

 and Denis, are in favour of the first view, that the liver is the chief, if not 

 the only, situation where de-amination occurs. Amino acids, as we s.-i\v, air 

 taken up rapidly by all tissues. Those taken up by the liver disappear 

 again in a comparatively short time, but, during the time required for this 

 disappearance from the livei 1 , no appreciable diminution has occurred in that 

 stored in the muscles. From other organs also they disappear less rapidly 

 than they do from the liver. This diminution of amino-acid content of the 

 liver is accompanied by increase of urea in the blood. The liver, therefore, 

 continually tends to decrease the amino-acid content of the blood, and, since 

 there is always an equilibrium between the amino-acid concentration in the 

 blood and that in the tissues, as the liver removes these acids more passes from 

 other tissues to restore equilibrium. We see then that it is not neress.n v that 

 de-amination should be performed by any tissue other than the liver. In a 

 further paper (1913, 3) the same investigators show that in starvation the 

 ami no-acid content of the tissues does not decrease. It is most probably 

 renewed by autolysis of protein, so that the amino-acid protein system appears 

 to be that of a reversible chemical reaction. This view is supported by the 

 fact that feeding with large quantities of protein does not increase the amino- 

 acids of the tissues, so that any nitrogen stored beyond the normal amount 

 of amino-acid must apparently be in the form of protein. 



In any case, we may conclude that nothing is left of the old discussion bet\v"<>n 

 Pfliiger and Voit as to the necessity of food protein becoming living protoplasm 

 before being utilised. 



If the view taken be correct, we see also that, so long as the "accessory 

 factors " are present, there is no necessity for the food proteins to be of similar 

 constitution to the tissues. In fact, experimental evidence confirms this deduc- 

 tion ; except for differences in degree of digestibility, and so on, there does not 

 seem to be any particular preference for one protein rather than another. 



A further consequence is that feeding with pure amino acids should be possible. 

 The evidence that this can be done has been referred to above (page 256). 



As to the chemical mechanism of de-amination we have little information. 

 Probably all the three reactions given above (page 264) are concerned. 



An old observation hy Streckcr referred to by Bach (1911, pp. Io7, 158) is interesting a- n 

 possibility of the formation of hydroxy-aeids and aldeh3'des from amino-acids. Alloxan n M< i- 

 in -Mijes with amino-acids thus : 



/NH-CCK /NH, 



OC< >CO + R -CH< +3H.OH = 



X NH CO/ VjOOH 



/NH OX /OH /OH 



CO< 

 X)H 



/ 



OC< >CH.OH + R.CH< +CO< +NH,= 



CO/ X)H 



/ 



OC< 

 X 



NH CO, 



NH CO/ 



The general facts of protein metabolism may be looked at from a slightly 

 different point of view, as in the original expression of his theory by Folin (I !><>.', 

 who carried out a large number of analyses of urine on two kinds of diets, rich and 

 poor in nitrogen, but both practically free from purines, creatine or creatinine (see 

 later, page 270). Comparing the two series, we note that there are some products of 

 metal>olism which maintain a nearly constant figure, while others are much greater 

 under rich nitrogen food than under food poor in nitrogen. The constant products 



