114 NUTRITION AND METABOLISM. 



molecules. This is generally believed to be a hydrolytic process similar 

 to the decomposition of starch to maltose, or of saccharose into dextrose 

 and levulose. 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 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 compounds, 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 per- 

 centage 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. 



The amino-acids are the first well-known compounds of protein de- 

 composition. They are organic acids, in which a hydrogen atom is 

 substituted by a NH 2 radical. Some of them are simple compounds, 

 as the amino acetic acid NH 2 CH 2 COOH and also the amino-capronic 

 acid, usually called leucin, (CH 3 ) 2 CH CH 2 CH(NH 2 ) COOH. Others 

 are of a more complex nature, such as the tyrosin or hydroxy-phenyl- 

 amino-propionic acid, C 6 H 4 (OH) CH 2 CH (NH 2 ) COOH, and the 

 tryptophan or indol-amino-propionic acid, C 8 H 6 N CH 2 CH(NH 2 ) COOH. 



Of other nitrogenous products which are not amino-acids, a few 

 are of striking significance. The very disagreeable odor of putrefying 

 proteins and of excreta is due to indol (C 8 H 7 N) and methyl-indol or skatol 

 (C 8 H 6 N CH 3 ). Indol gives a rose color with nitrites in acid solution, 

 and this convenient reagent is used in the identification of bacteria. 

 Another group are the amins, hydrocarbons in which one or several 

 hydrogen atoms are replaced by an NH 2 radical. The simplest amins 

 are the methyl-amins, of which the tri-methylamin (CH 3 ) 3 N is produced 

 by several bacteria. The fishy odor of the brine of salted herring is 

 largely due to this compound. In this group belong also a large number 

 of the so-called ptomains. 



