METABOLISM. 53 
duction of urea in the dog, all the nitrogen of the asparagin together 
with an excess over that previously found in the urine being ex- 
creted. The sulphur in the urine also increased. Hagemann * 
has more recently fully confirmed this result. Salkowski + found 
that glycocol, sarkosin, and alanin were oxidized to urea and caused 
no gain of proteids. Apparently, then, this class of bodies, like 
ammonia, furnish material out of which the organism can con- 
struct urea. 
Can Amipes REepLace Prorerps?—Since the amides yield the 
same end products of metabolism as the proteids, it is natural to 
inquire whether they can perform any of the functions of those 
substances. 
Amides not Synthesized to Proteids—We have already seen that 
the albumoses and peptones resulting from the cleavage of the 
proteids during digestion are built up again into proteids in the 
process of resorption. The amides commonly found in vegetable 
feeding-stuffs are likewise simpler cleavage products of the proteids, 
and some of them are also formed in digestion by the proteolytic 
action of trypsin. Can proteids be regenerated from these simpler 
cleavage products? 
If this is the case, then it should be possible, under suitable con- 
ditions, to cause a gain of proteids, or at least to maintain the 
stock of proteids in the tissues, on a food free from proteids but 
containing amides. Up to the present time, however, all attempts 
of this sort have failed. With the most abundant supply of non- 
nitrogenous nutrients and ash, the animals perished when supplied 
with amides (asparagin) but not with proteids.{| What has thus 
been found to be true of asparagin we may regard as probably true 
of other amides and say that there is no evidence that the animal 
body can build proteids from amides. 
Partial Replacement of Proteids—But even if the amides can- 
not serve as a source of proteids to the animal, it seems not impos- 
sible that they may by their oxidation perform a part of the func- 
tions of the proteids, thus protecting a portion of the latter from 
oxidation and rendering it available for tissue production. 
*Landw. Jahrb., 20, 264. 
+ Zeit. physiol. Chem., 4, 55. 
{Compare Politis, Zeit. f. Biol., 28, 492, and Gabriel, b., 29, 115. 
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