PROTEINS 



107 



obtained in a crystalline state. Crystallinity, however, is no sure sign 

 of homogeneity (purity). The best tests of homogeneity are electro- 

 phoresis, ultracentrifuge sedimentation, solubility measurements, and 

 maximum biological activity (if tiie protein has such a property). 



If a solution of protein is placed in an electrophoresis cell, and an 

 electric current is passed througii the solution, the protein moves toward 

 the cathode or anode, depending upon the polarity of the electric charge 

 carried by the protein. The position of the protein in the solution is 

 marked by a change in the refractive index of the solution and shows 

 up as a boundary which can be determined by suitable optical means. 

 The migration of the protein, during a given period of time, can be 

 followed and its electrophoretic mobility calculated. If the protein 

 consists of only one component, there will be only a single boundary in 

 the solution. The protein is then said to be pure or homogeneous. If 

 the solution contains a second component, this will usually migrate at 

 a different rate of speed than the first so that it will be revealed in the 

 optical pattern. Electrophoresis can be used to determine not only the 

 number of components but also the amount of each component in the 

 solution. An example of its usefulness is found in the extensive study 

 being made of the proteins of blood in health and disease. 



In the ultracentrifuge, the protein solution is subjected to a centrifugal 

 force about 250,000 times as great as the force of gravity. Heavier par- 

 ticles settle faster than lighter ones. If the solution contains two or 

 more proteins, there will be either two or more boundaries, or else a 

 diffuse boundary in the optical pattern. A single protein in solution 

 shows only one boundary when it is sedimented. 



The solubility test of homogeneity depends upon the fact that the 

 quantity of dissolved protein, until saturation is reached, is directly pro- 

 portional to the weight of sample taken. After saturation there is no 

 increase in dissolved protein, irrespective of the weight of sample. How- 

 ever, if the sample contains more than one protein, the solubility curve 

 does not show a sharp break but continues to rise until the saturation 

 point is reached for each component. 



Few of the proteins isolated have been subjected to all of these tests. 

 Most of the proteins, e.g., casein, gliadin, that have been prepared from 

 our common foodstuffs appear to be mixtures of several components. 



The names and amounts of the principal proteins in some common food- 

 stuffs are given in Table 5-2. Tlie percentages given are, in most cases, 

 the amounts actually isolated. The totals, in general, agree well with 

 the figures for crude protein (N X 6.25), and hence, it may be assumed 

 that the kinds and amounts of protein in many of our staple foods are 

 well established. 



From Table 5-3 it is apparent that proteins have great importance 

 other than as food constituents. Some of our most imjiortant articles 



