PROTEINS. 65 



there are a diverse assortment of monamino and diamino acids 

 which constitute the final products of the protein decomposition. 

 These acids are devoid of any protein characteristics and are there- 

 fore decidedly different from the original substance from which 

 they were derived. From a protein of huge molecular weight, a 

 typical colloid, perhaps but slightly soluble, and entirely non-dif- 

 fusible, we have passed by way of proteoses, peptones, and pep- 

 tides to a class of simpler crystalline substances which are, for the 

 most part, readily soluble and diffusible. 



These amino acids after their production in the process of diges- 

 tion, as just indicated, are synthesized within the organism to form 

 protein material which goes to build up the tissues of the body. 

 It is thus seen that the amino acids are of prime importance in the 

 animal economy. Moreover, it is important to remember that these 

 essential factors in metabolism and nutrition cannot be produced 

 within the animal organism from their elements, but are only yielded 

 upon the hydrolysis of ingested protein of animal or vegetable origin. 



When we examine the formulas of the principal members of the 

 crystalline end-products of protein decomposition we note that they 

 are invariably acids, as has already been mentioned, and contain an 

 NH 2 group in the a position. This relation of the NH 2 group to 

 the acid radical is constant, no matter what other groups or radicals 

 are present. We may have straight chains as in alanine and glu- 

 tamic acid, the benzene ring as in phenylalanine or we may have 

 sulphurized bodies as in cystine and still the formula is always of 

 the same type, i. e., 



NH 2 

 E - CH - COOH 



It is seen that this characteristic grouping in the amino acid pro- 

 vides each one of these ultimate fragments of the protein molecule 

 with both a strong acid and a strong basic group. For this reason 

 it is theoretically possible for a large number of these amino acids 

 to combine and the resulting combinations may be very great in 

 number, since there is such a varied assortment of the acids. The 

 protein molecule, which is of such mammoth proportions, is prob- 

 ably constructed on a foundation of this sort. Of late much valu- 

 able data have been collected regarding the synthetic production of 

 protein substances, the leaders in this line of investigation being 

 Fischer and Abderhalden. After having gathered a mass of data 

 6 



