PROTEIN METABOLISM 485 



occur, some of Lhcni pathogenic. Esclicricliia coli is the most abundant 

 bacterial species in man. Since riuninants depend upon extensive 

 bacterial jjopulations in their rumen digestive processes, many of 

 these special organisms are subsequently excreted. It is believed that 

 practically all the nitrogen of feces occurs in the bacteria and amounts 

 to a total of approximately 0.7 to 1.0 g. daily. By comparison with 

 Table 20-3, this quantity is seen to be a rather small fraction of 

 that in urine. 



Other nitrogeneous materials occur to some extent, but most of 

 these are attacked by the intestinal bacteria and the nitrogen assim- 

 ilated. Bile pigments and porphyrins are at least partial exceptions. 

 Of the latter, hematin is probably the most abundant (especially in 

 the carnivores) since ingested hemoglobin gives rise to this indigestible 

 substance. The dark color of large quantities of hematin may make 

 feces nearly black. Bleeding of the upper portions of the gastro- 

 intestinal tract produces this same coloration, also due to hematin. 

 Peptic ulcers and cancers are the commonest causes. Bleeding in the 

 lower part of the tract leads to bright red oxyhemoglobin excreted 

 as such because it does not encounter the digestive processes. 



PROTEIN SYNTHESIS 



The mechanism of the biosynthesis of proteins is one of the great 

 unsolved problems of biochemistry. Since living forms all synthesize 

 proteins as one of their principal classes of components, the processes 

 become important for quantitative as well as cjualitative reasons. The 

 importance of protein synthesis is further emphasized by enumerating 

 some of the diverse roles played by these materials. In addition to 

 acting as food for the newborn, proteins serve as catalytic, skeletal, 

 genetic, hormonal, and transporting materials of diverse types. The 

 total protein is a large fraction of the dry matter of any cell or any 

 animal. 



Because of the great fundamental interest in it and the probable 

 value to practical medicine, the mechanism of protein formation has 

 been widely sought. In spite of this activity, little has been learned 

 beyond the general outlines and even these are still in doubt in many 

 cases. Much of the difficulty seems to arise from two factors; first, the 

 great molecular size and complexity of proteins, and, second, the 

 specificity of their action, which is dependent in turn upon specificity 

 in structure. 



