224 TEXTBOOK OF PLANT PHYSIOLOGY 



globulins, legumin and vicilin are found in the seeds of peas and 

 some other leguminous plants; phaseolin, in seeds of the kidney- 

 bean; conglutin, in those of lupine; and edestin, in hemp seeds. 

 Of prolamins the most important is gliadin, which enters into the 

 composition of wheat gluten; hordein, in the grain of barley; and 

 zein, in the grain of maize. Of the glutelins we should note glu- 

 tenin, the other part of the gluten of wheat. Reserve proteins of 

 plants have been studied in great detail by Osborne and his col- 

 leagues and by Prianishnikov. 



71. Changes in Nitrogenous Substances at Germination. 

 Enzymatic Destruction of Proteins and Synthesis of Asparagine. — 

 The protein reserves of seeds, like the polysaccharides, are subject 

 to hydrolysis at germination. This is brought about by special, 

 so-called proteolytic enzymes, or proteases. 



The first of these to be studied were the ones present in the 

 digestive tract of animals. Soon two types of them were estab- 

 lished: pepsin, secreted by the walls of the stomach and causing 

 a rather weak hydrolysis of proteins into the so-called peptones; 

 and trypsin, secreted by the pancreas and splitting proteins into 

 amino acids. Peptones are quite complex bodies of a colloidal 

 nature, but the degree of their dispersion is considerably greater 

 than in the true proteins. It is usually thought that peptones are 

 the first products of the decomposition of the protein molecule, 

 but recently evidence has accumulated which shows that in the 

 formation of peptones the protein molecule itself is not decomposed, 

 but only the larger colloidal aggregates, composed of several mole- 

 cules, are broken down. Peptones are not fully determined chem- 

 ical compounds. 



Proteolytic enzymes, or proteases, also can be found in plants. 

 The most active and best-known plant protease is papain, 

 obtained from the juice of the fruit of the papaw tree (Carica 

 papaya). The isolation of proteases from germinating seeds is more 

 difficult than the isolation of diastase. Their behavior is usually 

 studied by employing autolysis, that is " autodigestion " of the pro- 

 tein substances, first by pulverizing the seeds, and then by soaking 

 them in water with the addition of chloroform or some other neu- 

 tral antiseptic, which prevents the development of microorganisms, 

 and kills the remaining uninjured living cells of the seed embryo. 

 An extremely active protease is found in yeast. During the 

 autolysis of yeast the protein substances are split very rapidly. 



