PROTEIN METABOLISM 81 
obtained by hydrolysis with acid were of no use. In a recent 
paper, it is true, Henriques and Hansen! have shown that the 
acid products can act to some extent as sparers of protein. 
How, then, do the acid hydrolysis products differ so much in 
this vital action from the products obtained by digestion with 
ferments? Is it that by the more active boiling acid products 
which are of absolute necessity for the maintenance of life 
are too far broken down, or is it perhaps due, in part at 
least, to the racemisation which takes place? One series of 
bodies which are known to be formed in tryptic digestion, 
and not during acid hydrolysis, the polypeptides—bodies 
formed by the linking together of two or more amino acids— 
cannot play any part, as it has been shown (Henriques and 
Hansen) that when they were removed by precipitation 
the monamino acids which were left sufficed. Whatever 
the cause, the main fact still stands that animals fed with 
acid hydrolysis products perish, whereas animals fed with 
ferment digest products not only live, but may even thrive 
on them. 
As regards the statement that the products of protein 
digestion were absorbed in the form of albumoses and peptones, 
it rested mainly on the work of Salvioli,? carried out many 
years ago. This worker stated that if he isolated a loop of 
intestine, and into it put the products in question (albumoses 
and peptones), on perfusion with blood he found that they 
disappeared. Further, Salvioli was unable to detect them in 
the blood used for perfusion, therefore he concluded that the 
absorbed products were regenerated to more complex bodies 
before they reached the blood. Recently the writer, working 
in conjunction with Dr. Leathes,? repeated the work of Salvioli, 
and found that albumoses and peptones introduced into the 
loop of intestine did indeed disappear, but the interpretation, 
supported by analytical data, was quite other. We found that 
the peptone, etc., introduced into the loop was merely split 
up into simpler bodies which do not give the biuret reaction, 
in all probability due to the action of the ferment erepsin. 
If it be due to the erepsin, then this ferment could not have 
acted intracellularly as Cohnheim holds, as so far as we could 
1 Henriques and Hansen, Zezt. f. Ghysiol. Chem. 49, 1906, 113. 
? Salvioli, Du Bots Reymond’s Archiv, 1880, Suppl. p. 95. 
* Cathcart and Leathes, Journ. of Physiology, 33, 1906, 462. 
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