DIGESTION OF THE PROTEINS 239 



To some extent, but a very limited extent therefore, parenterally 

 introduced protein, that is, protein injected directly into the circu- 

 lation, may be utilized by the tissues, since a proportion of the protein 

 is evidently converted into a normal product of protein catabolism, 

 namely, urea. But it is also evident that the utilization of protein 

 thus introduced is imperfect, that it is abnormal because the urine 

 contains protein which is not the case when proteins are absorbed 

 from the digestive tract, and that the utilization of the protein which 

 does occur is preceded by hydrolytic cleavage. Moreover it is not 

 even certain that the additional urea-output which results from the 

 injection of foreign proteins is due to utilization of the protein itself, 

 since it has been found by Mendel and Rockwood that the intro- 

 duction of a foreign protein, such as Edestin or Casein into the cir- 

 culation leads to a considerably more than proportionate increase of 

 the nitrogenous secretion, in other words to actual destruction of 

 tissue-proteins. 



In the second place, the intravenous or subcutaneous injection of 

 proteins which are foreign to the tissues of the animal receiving them, 

 results in the production of a variety of specific Antibodies or substances 

 appearing in the circulation which have the property of precipitating 

 or otherwise modifying the protein employed for injection. If the 

 injections are repeated, and successive injections are separated by 

 only a few days from one another, the result after some weeks is the 

 production of a specific Precipitin which circulates in the blood of the 

 immunized animal, so that if the blood-serum of the animal be now 

 mixed with a solution of the protein which was employed for injection, 

 that protein, but no other, is precipitated. If a single injection be 

 made and then a second only after a considerable interval, e. g., three 

 or four weeks, the effect of the second injection is to induce Anaphy- 

 lactic Shock, a condition which so strikingly resembles peptone-poison- 

 ing that many investigators are of the opinion that it is due to the 

 development in the tissues of the sensitized animal of an enzyme having 

 the specific ability to rapidly break down the particular protein 

 employed and to convert it into proteoses or peptones. 



Now it has been shown that even after the introduction of excessive 

 amounts of native protein into an isolated loop of intestine, although 

 the protein disappears and would seem to have been absorbed, yet no 

 evidence is obtainable of the development of antibodies in the circula- 

 tion of the animal so treated, nor is there any sensitisation, so that a 

 second dose of the protein, after a considerable interval, does not give 

 rise to symptoms of anaphylactic shock. 



There can be little doubt therefore that proteins are not absorbed 

 without previous hydrolysis, and there is much ground for supposing 

 that even that proportion of parenterally introduced protein which is 

 utilized by the tissues, is utilized simply because it has been excreted 

 into the intestine and reabsorbed therefrom after digestion. 



The case against the direct absorption 1 of peptones from the intestine 



