446 NITROGEN METABOLISAI 



1. Albumins. The albumins are water soluble and are coagulated by heat. 

 They occur in both plants and animals and are commonly present in 

 seeds. Leucosin from cereals, legumelin found in legume seeds, and 

 ricin from the castor bean seed are examples. 



2. Globulitis. These proteins are insoluble in water but are soluble in 

 dilute solutions of neutral salts. Globulins are found in many seeds. 

 Edestin from hemp seeds and tubcrin from potato tubers are examples. 



3. Glutelins. Proteins of this group are insoluble in water and in dilute 

 salt solutions but are readily soluble in dilute acids and alkalies. Glu- 

 tenin in wheat, oryzenin in rice and glutelin from corn are examples. 



4. Prolamines. These proteins are insoluble in water, dilute salt solu- 

 tions, dilute acids and alkalies, and absolute alcohol but are soluble in 

 70-go per cent alcohol. Gliadin from wheat, zein from corn, and hordein 

 from barley are examples. Aside from one member of the group which 

 has been extracted from milk all the prolamines have been found in 

 the seeds of cereals. 



5. Albuminoids. Proteins of this group are insoluble in dilute acids and 

 alkalies, and in solutions of all neutral solvents. Keratin from hair and 

 fibroin from silk are examples. Gelatin is also classified in this group 

 although it docs not conform strictly to the definition given. No 

 members of this group are known to occur in plant tissues. 



6. Histones. These are soluble in water and dilute acids but are insoluble 

 in very dilute ammonia. They are not coagulable by heat. The globin 

 of hemoglobin is the most familiar example. No members of this 

 group are known to occur in plant tissues. The known histones do 

 not contain sulfur. 



7. Protamines. Proteins of this group are soluble in water and ammonia, 

 and form salts with mineral acids. They are not coagulable by heat. 

 Like the two preceding groups the protamines have not been found in 

 plant tissues. These are the simplest known natural proteins in terms 

 of both number and kinds of constituent amino acids. They have been 

 found principally in fish sperm. The known protamines do not contain 

 sulfur. 



II. The Conjugate Proteins. — The members of the group are proteins 

 with which is combined some non-protein group. They are classified upon the 

 basis of the non-protein group present. 



I. Nucleoproteins are complex compounds composed of one or more pro- 

 tein molecules combined with a nucleic acid. They are found in cell 

 nuclei. 



