FEEDING EXPERIMENTS WITH ISOLATED FOOD SUBSTANCES. 3 



So ' mg as it was still assumed that the proteins are only slightly 

 modified within the alimentary tract, it was not easy to appreciate 

 how these widely differing amino-acid complexes could be converted 

 into a common protein or group of proteins. But with the introduc- 

 tion of the evidence that proteins experience a profound cleavage 

 prior to absorption that the organism is equipped with an outfit of 

 enzymes easily capable of effecting such intense hydrolyses, and that 

 the proteins in all probability normally disappear from the alimentary 

 tract as amino-acid fragments and relatively simple polypeptides 

 our conception of protein assimilation has changed notably. 



It is, perhaps, too early yet to determine to what extent the 

 theories of protein metabolism and reconstruction, which Loewi and 

 Abderhalden in particular have championed, accord with the facts 

 of experience. Whether the organism synthesizes blood and tissue 

 proteins from the amino-acid rests of digestion, and thus only; 

 whether there are no rearrangements whereby one amino-acid may 

 give rise to another; and whether under these circumstances the 

 synthetic power of the organism is limited to a choice by the mini- 

 mum of all essential protein building complexes the so-called " Bau- 

 steine" can not yet be profitably debated in detail. 



The problem of protein synthesis is further complicated by a 

 consideration of the activities of bacteria in relation to the food-prod- 

 ucts which enter the alimentary tract. It is now well appreciated 

 that microorganisms grow and die in large numbers throughout the 

 lower parts of the digestive tube, so that it is not uncommon to find 

 the faeces to be composed in a very considerable degree of the bodies 

 of dead bacteria. It is not unlikely that the organisms which thus 

 perish in the intestinal canal are subject to autolytic or other diges- 

 tive degradation by which their protoplasmic constituents may 

 become available as food sources to the organism of the host. Bear- 

 ing in mind the synthetic possibilities inherent in plant cells, such 

 as bacteria represent, it is by no means beyond the bounds of reason- 

 able interpretation to assume that the amino-acids first formed by 

 digestion of food proteins may experience a synthesis into new forms 

 of protein complexes prior to a subsequent digestion and utilization. 

 Viewed in this light, the immediate hydrolysis products of our food- 

 stuffs may become available only after they have been in greater or 

 less part reconstructed by these preeminently synthetic symbiotic 

 bacteria into products of more uniform character, possibly widely 

 different from the original intake. Nucleoprotein synthesis, for 

 example, may thus become referable to bacterial intervention. 



No one can say at the present time to what extent, if at all, 

 such synthetic possibilities enter into the problems here discussed. 

 The subject has recently again been referred to by Luthje.* The 



*Iuthje: Ergebnisse der Physiologie, 1908, vn, p. 826. 



