304 BIOLOGICAL EFFECTS OF RADIATION 



differ from that of crystalloids, for it has been shown that proteins com- 

 bine stoichiometrically {i.e., by the purely chemical forces of primary 

 valency) with acids and alkalies, forming protein salts which dissociate 

 electrolytically. The colloidal nature of the proteins is attributed to the 

 large size of their ions which enables them to fulfill the conditions neces- 

 sary for the establishment of a Donnan equilibrium, namely, that the 

 protein ion is prevented from diffusing through membranes which are 

 permeable to the smaller crystalloidal ions. Proteins are formed by the 

 condensation of a number of molecules of various amino acids, and amino 

 acids, by virtue of their amino and carboxyl groups, react with either 

 acids or bases to form salts {i.e., they are amphoteric electrolytes). The 

 salts formed with a base (for instance NaOH) dissociate to give a posi- 

 tively charged sodium ion and a negatively charged protein ion. The 

 salts formed with an acid, such as HCl, dissociate to form a negatively 

 charged chlorine ion and a positively charged protein ion. Therefore at 

 a certain hydrogen ion concentration, called the isoelectric point, protein 

 molecules exist uncharged, but on the alkaline side of the isoelectric point 

 they carry a negative charge and on the acid side a positive charge. 

 The isoelectric points of some proteins are given in Table 1. 



Table 1 



pH, Isoelectric 



Protein • point 



Glutenin 4 . 45 



Serum albumin 4.7 



Casein 4.7 



Gelatin 4.7 



Egg albumin 4.8 



Fibrinogen 5.0 



Serum globulin 5.4 



Oxyhemoglobin 6 . 74 



Gliadin 9.0 



The osmotic pressure, viscosity, and swelling of proteins are lowest 

 at the isoelectric point and their stability in water solution is least at this 

 hydrogen ion concentration. 



Some proteins (globulins) lose their water of hydration and precipitate 

 spontaneously at the isoelectric point. Other proteins (albumins) retain 

 their water of hydration and stay in solution even at the isoelectric point 

 where they are uncharged. The difference between an albumin and a 

 globulin is therefore fundamentally a difference in affinity for water. 

 Globulins are hydrated only when charged, and even when charged, they 

 have a lower affinity for water than albumins, for they are more easily 

 precipitated by salts. 



The following definitions for the various changes produced in the 

 state of a protein are given by Lewis (36) . Crystallization is the precipita- 

 tion in solid crystalline form of undenatured protein. Denaturation is a 



