PROTEINS 123 



cedures are based on differences in the degree of adsorption of amino acids 

 by solids, in the reaction of amino acids with ion exchange materials, and 

 in the solubility of amino acids in organic solvents. For details regard- 

 ing the operation of these methods, the original papers of Moore and Stein 

 and the book by Block and Boiling should be consulted. 



Table 5-4 gives the percentages of amino acids found in some typical 

 proteins. The table includes proteins representative of foods, enzymes, 

 hormones, viruses, antibiotics, and fibers. In a given protein the figures 

 vary greatly. In egg albumin, for example, there is about 14 times as 

 much glutamic acid as there is tryptophan. In another common food 

 protein, gliadin, the glutamic acid exceeds the tryptophan more than one 

 hundredfold. 



If all the proteins are considered, glutamic and aspartic acids and 

 leucine are seen to be the most abundant amino acids, while tryptophan, 

 histidine, and methionine are least abundant. The amino acids present 

 in largest amounts are those most closely related to the intermediary 

 compounds of carbohydrate metabolism, e.g., glutamic acid and a-keto- 

 glutaric acid form a pair, and aspartic acid and oxalacetic acid make up 

 a second pair (p. 331). The least abundant amino acids such as tryp- 

 tophan and methionine are the most complex in structure, probably in- 

 volving also the largest number of steps in synthesis. 



Proteins that contain considerable amounts of all of the amino acids, 

 e.g., egg albumin and casein, are called "complete"; those that are lacking 

 or very high in certain amino acids, e.g., gelatin and zein, are said to be 

 "incomplete." A better term to denote the uneven composition of pro- 

 teins is disproportionate. 



Certain highly specialized proteins such as fibroin and salmine are 

 conspicuously disproportionate in make-up. However, other proteins 

 such as pepsin, insulin, and botulinum toxin A having marked biological 

 properties show no unusual features in amino acid content. Their bio- 

 logical properties must be related to the structure of the molecule as a 

 whole and not to the kind or amount of amino acids that are found in 

 the molecule. 



In most cases the sum of the figures for the amino acids amounts to 

 more than 100 per cent. Because of the water taken up in hydrolysis, 

 the total should be about 115 per cent of the starting material. With 

 the improved methods now available, the total nitrogen of the protein 

 can usually be accounted for in the individual amino acids. Likewise, 

 the extent of carbon and sulfur recoveries are useful criteria in judging 

 the validity of the analytical data. 



From a practical viewpoint, data on the amino acid composition of 

 foods are more useful than those on the amino acid content of individual 

 proteins. Such data are gradually becoming available, and figures for 

 some of our staple foods are given in Table 5-5. However, we need many 



