83 



by heat, and are precipitated by alcohol, concentrated mineral acids and 

 the salts of heavy metals. They are quantitatively precipitated by satura- 

 tion with ammonium sulphate in neutral solution. Most of them may be 

 obtained in pure crystalline form. 



The globulins are also present extensively in the animal and plant 

 kingdoms. They are found in the blood as serum globulin, fibrinogen 

 and its derivative fibrin ; in the muscles as myosinogen and myosin ; in the 

 egg as ovoglobulin ; in milk as lactoglobulin ; in the crystalline lens of the 

 eye as lentoglobulin ; in the thyroid gland in combination with iodin as 

 thyreoglobulin or iodothyreoglobulin ; in the urine as Bence Jones' pro- 

 tein. 



As a class they are characterized by their insolubility in pure distilled 

 water and dilute acid solutions. They are, however, soluble in dilute neu- 

 tral salt solutions and in dilute alkaline solutions. They are coagulated 

 by heat and precipitated by alcohol. They are completely precipitated 

 by saturation with magnesium sulphate and by half saturation with am- 

 monium sulphate. They are strongly acid in reaction and possess the 

 power of turning blue litmus red. 



The glutelins are a group of proteins which are present in the plant 

 kingdom only. We know the glutelin of wheat and the oxyzenin of rice. 

 They are soluble in dilute alkali, forming salts. 



The prolamins or alcohol soluble proteins are a group of proteins found 

 in cereals. They are gliadin of wheat, hordenin of barley and zein of maize. 

 They are characterized by their solubility in 70 to 80 per cent alcohol, 

 and by their insolubility in water, neutral solvents and absolute alcohol. 



The albuminoids or sclero proteins are a group of proteins found in the 

 framework of all connective tissues. In this group belong elastin, gelatin 

 and collagen, keratin from hair, bones, hoofs, nails, turtle shell, also silk 

 gelatin, reticulin, etc. They are characterized by their marked insolubility 

 in any of the neutral solvents and their resistance to chemical decom- 

 position. 



The histones are a sharply defined group of proteins strongly alkaline 

 in reaction, and not found free in nature but in combination with acids or 

 other proteins. They contain a large amount of the dibasic amino acids 

 (see page 87), lysin, arginin and histidin. They are found in com- 

 bination with nucleic acid in the nucleoproteins and with hematin in hemo- 

 globin. They are soluble in water and precipitated by alkali. They are 

 coagulated by heat in the presence of small amounts of salts, and are pre- 

 cipitated by other proteins. 



The protamins are the simplest of all the proteins. Similar to the 

 histones, they are strongly alkaline in reaction. They contain 25 to 30 

 per cent of nitrogen and are made up almost entirely of the dibasic amino 

 acids (ninety per cent). They are found in combination with nucleic 

 acid in the nuclei of the spennatozoa of numerous fish. They are soluble 



