17° NUTRITION AND METABOLISM 



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 

 Ijrotein 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 degradati9n, 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 Uttle known as 

 that of the protein bodies. We,,5peak-^of-^eptenisatio»-of-proteins, 

 e.g., in the clearing of milk orthe g elatin liquefac tion, meaning that the 

 insolubIe"proteiirEas''been made soluble. 



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

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

 substituted by a NH2 radicaL Some of them are simple compounds, 

 as the amino-acetic acid NH2CH2COOH and also the amino-capronic 

 acid usually called leucin (CH3)2CH CH2 CHCNHs) COOH. Others 

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

 aminopropionic acid, C6H4(OH) CH2CH (NH2) COOH, and the 

 tryptophan or indol-amino-propionic acid, CgHeN CH2CH(NH2) 

 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 (CsH/N) andmethyl-lndol or 

 skatol (CgHeNCHa). Indol gives a rose color with nitrites in acid 

 solution, and this convenient reagent is used in the identification of 



