240 NUTRITION AND METABOLISM 



casein and serum albumin are more readily decomposed, though their 

 solubility is quite limited. Peptones which are readily soluble are 

 used by the vast majority of microorganisms. Of interest in this con- 

 nection is the fact that the fresh white of egg is poisonous to most bac- 

 teria, and fresh blood and animal tissues as well as freshly drawn milk 

 have also germicidal properties which are lost by heating or upon 

 standing. 



PROTEIN BODIES are as numerous as plants and animals. Each 

 species of organism seems to have its particular protein which differs 

 from that of other species. With the more highly developed organisms, 

 there are several distinctly different proteins found in the same individ- 

 ual in different parts of the body. The constituents, carbon, oxygen, 

 hydrogen, nitrogen, and sometimes sulphur and phosphorus can be 

 determined in their relative amounts without, however, furnishing any 

 knowledge of the structure of the molecule. The molecular weight of 

 proteins is estimated to be at least 10,000, while the weight of the very 

 large molecule of saccharose is only 342. The protein molecule can be 

 broken up into smaller molecules. This cleavage is generally believed 

 to be a hydrolytic process similar to the decomposition of starch to 

 maltose. The first products of protein decomposition do not differ 

 essentially from the original protein, but they can be hydrolyzed again 

 and again, until finally products of a crystalline nature are found which 

 are well-defined chemical bodies. Among the very first products of 

 protein degradation it is usually impossible to determine single com- 

 pounds, but several groups of compounds may be separated by certain 

 precipitants, as acetic acid, ammonium sulphate, zinc sulphate, copper 

 sulphate, tannic acid and others. In order to determine the degree of 

 protein degradation, e.g., in the analysis of cheese, it is customary to 

 determine the nitrogen of compounds precipitated by these various 

 reagents, and state it in percentage of the total nitrogen. Thus the 

 terms "water-soluble nitrogen," "acid-soluble nitrogen" and others 

 originated, meaning the nitrogen of the compounds soluble in water or 

 in acid respectively. Some of these groups of degradation products 

 have been named and defined more accurately, of which the albumoses 

 and peptones are the most common and best described compounds. 

 Their chemical nature and structure is, however, just as little known as 

 that of the protein bodies. We speak of peptonisation of proteins, 



