20 THE PHYSIOLOGY OF PROTEIN METABOLISM 



amount which passed along this vein was 594 c.c. per kilo animal per 

 hour, which works out about a third slower than the flow as estimated 

 by Cybulski. Now Pfliiger (330) showed in a series of experiments in 

 which cats were fed with lean meat that the maximal rate of absorption 

 of the protein for an animal of one kilo in weight was ri4 grm. pro- 

 tein per hour. Pfliiger (331) maintains that this is a good maximum 

 for human beings as they cannot digest protein at the same rate as 

 animals. In support of this statement he cites the case of a dog of 30 

 kilos which digested 2500 grms. of meat in twenty- four hours, whereas 

 a man of twice the weight could hardly manage half this amount with 

 comfort. If then the rate of absorption betaken at 1.14 grm. protein 

 per kilo per hour, and using Burton Opitz's figures, the 1-14 grm. is con- 

 tained in 594 c.c. of blood, thus the percentage concentration of protein 

 digestion products in the blood is '19. If on the other other hand we 

 use the figures obtained by Cybulski the result is still lower, for here 

 we have the ri4 grm. dissolved in some 950 c.c. of blood, i.e. a con- 

 centration of *I2 per cent. Other workers, Bergmann and Langstein 

 (63), for example, put the amount to be looked for as low as '005 per 

 cent. But the difficulty does not end here as in the first place we 

 know only a fraction of the substances obtainable by the digestion of 

 proteins, even leucine, the most abundant amino acid, common to most 

 proteins, is present on the average only to the extent of 20 per cent., 

 and in the second place this search for digestion products is being 

 made in a fluid which already contains some 3 per cent, of nitrogen in 

 the form of coagulable protein, and about '03 per cent, nitrogen in the 

 form of nitrogen non-precipitable by tannic acid. Unless the search 

 for products of digestion be made in the portal blood the chances of 

 detecting them must be small, as, in addition to the liver acting as 

 an efficient filter and deaminizing organ, the tissues probably fix a large 

 percentage of the circulating nitrogen at a very rapid rate. 



In spite of all these difficulties, a certain amount of evidence does 

 exist in support of the contention that the simple products of diges- 

 tion are to be found in the blood more particularly in the portal stream 

 at the height of digestion. Bergmann and Langstein (63) examined 

 the portal and systemic blood of well-fed dogs for total nitrogen and 

 non-coagulable nitrogen, and found that there was always a slight gain 

 in the non-protein nitrogen " residual " nitrogen after a meat meal. 

 The percentage amount of non-protein nitrogen of the total nitrogen 

 of the blood varied between 77 and 147 with an average of 107. 

 This coagulation method, however, cannot be regarded as satisfactory 

 as the non-coagulable nitrogen cannot with certainty be pronounced 



