NUTRITION 361 



with hydrogen as a definite radical, NH 2 , known as the amide radical. 

 It may replace an H or OH group in the various carbohydrates and their 

 allied acids, converting them by this slight change into quite different 

 substances. 



Thus, either acetic acid, CH<r COOH, or glycolic acid, CH 2 OH COOH, be- 

 comes amido-acetic acid (glycin), CH 2 (NH 2 ) COOH, by the substitution of the 

 amide radical NH2 for hydrogen (H) or hydroxyl (OH), respectively. Glucose, 

 COH CHOH CHOH CHOH CHOH CH 2 OH, becomes glucosamin COH 

 CH(NH 2 ) CHOH CHOH CHOH CH 2 OH, by a like substitution. On the 

 other hand, some of the constant decomposition products of the more complex 



CH 3 v 

 proteins are glycin (ante)', leucin, V;H CH 2 CH(NH 2 ) COOH ; tyrosin, 



HC CH 

 OH C^ %C CH 2 CH(NH 2 ) COOH; asparagin, CO(NH 2 ) CH(NH 2 ) 



HC=CH 



CH 2 COOH; in all of which the amide radical has replaced H or OH of an allied 

 substance. 



Proteins. Proteins are the substances which compose the larger part 

 of the cytoplasm; protein foods, therefore, .are those which can be most 

 directly used for nourishment, and so represent the end of food making. 

 To define proteins is quite impossible; they are so numerous and so 

 varied that scarcely any characteristic is universal. Within this huge 

 group are included some substances which are relatively simple, and 

 others whose complexity defies all analysis. Even the simplest are 

 scarcely known chemically, the actual knowledge permitting only theo- 

 ries of their construction. It has been possible in most cases to deter- 

 mine only the percentage composition, which with a study of the decom- 

 position products sometimes permits the establishment of an empirical 

 formula. The more complex proteins contain sulfur, and some have 

 also phosphorus in addition to the carbon, hydrogen, oxygen, and 

 nitrogen of amides, with traces of ash, which may or may not be struc- 

 turally a part of the protein. One nearly pure protein is familiarly 

 known, the albumin or " white " of eggs; perhaps the best known plant 

 protein is the one longest known, the gluten of wheat grains. 



To illustrate the complexity of these substances and, as well, the uncertainty re- 

 garding their composition, the following formulas, though hardly more than guesses, 

 are quoted. A crystalline vitelfin from squash: C 2 92H48iNgoO83S 2 . An albumin: 

 C7 2 oHn34N 2 i 8 O 2 48S6. Hemoglobin (of the blood): C7i2Hu3oN 2 uO245FeS 2 ; the 

 same, another guess, 



