158 PHYSIOLOGY OF NUTRITION 



i. Albumins. — The albumins are soluble in pure water and may be precipi- 

 tated by saturation of the solution with ammonium sulphate. They are coagu- 

 lated by boiling or by treatment with alcohol. 



2. Globulins. — The globulins are insoluble in pure water but are soluble in 

 solutions of neutral salts (sodium chloride, ammonium chloride, magnesium 

 sulphate, etc.). They are completely precipitated by a half-saturated 

 solution of ammonium sulphate and are coagulated by boiling or by addition 

 of alcohol. 



3. Albuminates. — These are formed when albumins and globulins are treated 

 with weak alkalies (alkali albuminates), or with weak acids (acid albuminates, 

 or syntonins). They are insoluble in water and in solutions of neutral salts 

 but are soluble in weak acids and alkalies; they are not precipitated by boiling 

 but are salted out by saturation with ammonium sulphate. They coagulate 

 with excess of alcohol. 



4. Albumoses and Peptones. — These compounds are the first products of the 

 hydrolytic cleavage of proteins by enzymes. The albumoses are precipitated 

 by ammonium sulphate, while the peptones remain in solution. 



The proteins occurring in plants have been only partially worked out, 1 even 

 for seeds. The simple ones may be illustrated by the phy toalbumins, the phy to- 

 globulins, and the peptones. d 



The phytoalbumins are not of common occurrence 2 and in most cases have 

 not been identified with absolute certainty. The proteins present in the cell 

 sap are usually globulins, since they are soluble only in the presence of salts and 

 are obtained as a precipitate by dialysis. 



The phytoglobulins are better understood. 3 They appear to compose the 

 principal reserve proteins of some seeds. Seeds of Lupinus luteus may be men- 

 tioned as one of the best materials for the demonstration of phytoglobulin. 



1 Abderhalden, Emil, Biochemisches Handlexikon. Berlin, 1911. Vol. 4. 



2 Martin, Sidney H. C, The nature of papain and its action on vegetable proteid. Jour. Physiol. 

 6: 336-360. 1884-1885.. Green, J. R., Proteid substances in latex. Proc. Roy. Soc. London 40: 28-39. 

 1886. Vines, S. H., and Green, J. R., The reserve proteid of the asparagus root. 76j'<2. 52 : 130-132. 1893. 



3 Weyl, Th., Beitrage zur Kenntniss thierischer und pnanziicher Eiweisskorper. Zeitsch. physiol. 

 Chem. 1 : 72-100. 1877-1878. Palladin, W., Beitrage zur Kenntniss der pflanzlichen Eiweissstoffe. 

 Zeitsch. Biol. 31: 191-202. 1895. Abderhalden, Lehrbuch, 1906. [See note 1. p, 155.] [Also see Os- 

 borne, 1909. [See note I, p. 155.] 



d The following additional information may help the reader to fom a more concrete picture. 



(1) Albumins are plentiful in animals but generally seem to occur only in small quantities in 

 plants. Examples of plant albumins are legumelin, from pea seeds, and leucosin, from the 

 seeds of wheat and other grains. (2) Globulins are the main reserve proteins in seeds, except- 

 ing those of the cereals. Examples are: excelsin, from the Brazil nut {Bertholetia excelsa); 

 legumin from pea seeds and seeds of other legumes; edestin, from hemp seeds (Canna- 

 bis saliva); conglutin, from lupine seeds. (3) Metaproteins is a better term than albuminates. 

 (4) Proteose is the general term, so that albumose results from splitting of albumin, and 

 globulose from splitting of globulin. It may be mentioned that the gluten of wheat, etc., is 

 largely composed of glutelins and gliadins, two other groups that might be inserted between 



(2) and (3) in the text. To the latter group, besides gliadin proper (which occurs in wheat 

 and rye), belong also hordein (in barley), and zein (in wheat and maize). It should be re- 

 membered that the classifications of proteins are based on temporary needs of description; 

 the chemical knowledge necessary for a really satisfactory classification is still lacking. — Ed. 



