GENERAL CHARACTERISTICS OF THE PROTEINS 121 



conditions, be obtained in crystalline condition. This is particularly 

 true of hemoglobin, of egg-albumin, the serum-albumin of the horse, 

 and a variety of vegetable proteins. The solutions of the proteins 

 are always optically active and with the exception of the solutions of 

 hemoglobin and the nucleo-proteins, are levorotatory. 



The great majority of the proteins are soluble in water or in very 

 dilute acids or alkalies. Some exceptional proteins, however, such as 

 Elastin from elastic fibers of connective tissues, Keratin from horn, 

 Fibroin from silk and Spongin from sponges, are insoluble in water or 

 in dilute acids and alkalies and require strong acids or alkalies to bring 

 them into solution. The action of strong soda upon a sponge may 

 be cited in illustration. The proteins are usually insoluble in organic 

 reagents, although some of the vegetable proteins, particularly those 

 obtained from a variety of grains, are soluble in 80 per cent, alcohol. 

 Many of the proteins not commonly regarded as alcohol-soluble are, 

 however, soluble in faintly alkaline alcohol, if they are first dissolved 

 in alkaline water, and alcohol added up to 80 or 90 per cent. Casein 

 is soluble in warm anhydrous formic acid, but the protein undergoes 

 decomposition if the solution is allowed to stand. 



The proteins combine with both acids and bases, neutralizing them 

 wholly or in part, and causing a diminution of hydrogen ions in the case 

 of combination with acids, or of hydroxyl ions in the case of combi- 

 nation with bases. They therefore belong to the class of substances 

 designated Amphoteric Acids, or acids which are simultaneously 

 capable of acting as bases. Under certain conditions the proteins 

 are also capable of combining with neutral salts. 



When dissolved in water, especially in faintly acid solutions, the 

 proteins are usually modified by heat in such a way as to render them 

 less soluble. This generally leads to flocculation or Coagulation of the 

 protein, or if the solution be very concentrated, to the formation of a 

 firm jelly, such as, for example, the white of a hard-boiled egg. 



The true characterization of the proteins depends upon the presence 

 among their hydrolytic cleavage products of a preponderating pro- 

 portion of Amino-acids. No other single "test" can be relied upon 

 to demonstrate the presence of a protein in a solution containing 

 unknown substances, nor can the individual proteins be accurately 

 characterized, as a general rule, in any other terms than the propor- 

 tions of various cleavage-products which they yield on hydrolysis. 

 By employing a multiplicity of tests, however, the presence of protein 

 in a solution may be established by the fact that the unknown sub- 

 stance yields several positive reactions. For the identification of any 

 particular protein we depend upon slight peculiarities of solubility 

 in various salt solutions, dilute acids and alkalies, etc., and upon 

 physical peculiarities and the nature of the tissue or fluid in which the 

 protein occurs. 



The various reactions which the majority of the proteins yield may 

 be subdivided into Coagulation-reactions which involve or depend 



