104 



PROTEINS 



beans, peas, etc.) as well as important clothing and furnishing materials 

 (e.g., woolens, felts, furs, silks, leather, etc.). Table 5-1 gives data 

 for some industries utilizing protein materials. 



Comparison wdth Table 3-1 (p. 103) shows that the protein industries 

 approach the machinery industry in monetary value of products, though 

 they do not give employment to as many workers. Since Table 5-1 is 

 not exhaustive, it does not include any nonindustrial business based on 

 protein materials; for example, eggs, which had a value in 1950 of 

 $1,811,387,667 at the farms where they were produced. 



Several pure proteins are prepared and sold — gelatin for food and film 

 industries; insulin for the treatment of diabetes; pepsin, trypsin, and 

 other enzymes; and the peptide antibiotics, bacitracin and tyrothricin. 

 Other purified protein materials are marketed as vaccines, toxins, and 

 antitoxins. 



Occurrence and preparation . 



Every kind of cell contains its own special proteins, and, therefore, the 

 number of individual proteins must be enormous. About 700 have been 

 isolated and examined. Perhaps 200, contained in the most important 

 foodstuffs and biological materials, have been studied in some detail. 

 Since there are about 40 known amino acids, each one of which may be 

 used many times, it is evident that an enormous number of proteins is 

 possible. Perhaps comparison with the number of words in the English 

 language will make the possibilities more evident. We have 26 letters, 

 each of which may be used several times in a given word. They comprise 

 about 600,000 words listed in an unabridged dictionary. Many new 

 words are added yearly, just as many new proteins are discovered each 

 year. There are probably thousands of new proteins in the many species 

 of plants, animals, and microorganisms that have not been investigated. 



The general method of preparing a protein is to dissolve it in its 

 particular solvent, water, salt solution, or alcohol, and then alternately 

 precipitate impurities or protein by changing one or more factors such as 

 pH, salt concentration, or temperature. Solution and reprecipitation are 

 repeated many times until the protein is obtained as pure as possible, 

 and preferably in crystalline form. Some proteins, for example, egg 

 albumin, can be obtained crystalline after only one or two operations, 

 but others, such as muscle phosphorylase, require about ten different 

 treatments before they will crystallize. A twentyfold purification is 

 usual, but in the case of botulinum toxin A, purification increases the 

 potency more than 200 times that of the crude material. 



Isolation of proteins challenges the skill and resourcefulness of the 

 most experienced investigators. Great progress, however, has been made 

 in the last twenty years. To date, more than 150 proteins have been 



