VIII] THE PROTEINS AND PROTEASES 137 



Lysine has been isolated from seedlings of Lupinus, Vicia and 

 Pisum. Also from the inner leaves of the Cabbage and tubers of the 

 Potato. 



Phenylalanine has been isolated from seedlings of Lupinus luteus, 

 Vicia sativa and Phaseolus vulgaris. 



Tyrosine is very widely distributed. It is present in seedlings of 

 Vicia sativa, Cucurbita, Lupinus, Tropaeolum and tubers of Potato, 

 Turnip, Dahlia, Beet and Celery. Also in berries of Elder (Sambucus), 

 in Clover (Trifolium), Bamboo (Bambusa) shoots and other plants. 



Proline has been isolated in very small quantities from etiolated 

 seedlings of Lupinus albus. 



Histidine has been isolated from seedlings of Lupinus and tubers of 

 Potato. 



Tryptophane has been isolated from seedlings of iMpinus albus and 

 Vida sativa. 



The Proteases. 



We have seen in the previous pages that proteins can be hydrolyzed 

 artificially with the intermediate production of proteoses and peptones, 

 and the final production of a number of amino-acids. There is no doubt 

 that this process of hydrolysis takes place in the living plant, and it is 

 believed that the converse process, the synthesis of these proteins from 

 amino-acids, also takes place in the cell. 



There is evidence that this hydrolysis of proteins is catalyzed by 

 certain enzymes which have been termed proteases. On analogy with 

 other enzymes, we may suppose that these enzymes also catalyze the 

 synthesis of the proteins. 



It seems highly probable that the proteases are of two types : 



1. Pepsin-like enzymes, which catalyze the hydrolysis of proteins to 

 peptones, and, in all probability, the reverse process. 



2. Erepsin-like enzymes, which catalyze the hydrolysis of albumoses 

 and peptones to amino-acids, and, in all probability, the reverse process. 



We now turn to the evidence for the existence of proteases. In 

 autolysis (see p. 19) the hydrolytic activity of many enzymes is un- 

 controlled, and in the case of the proteins, the amino-acids are formed 

 as end-products. Amino-acids are rarely present in plants in sufiicient 

 quantity to be detected readily, at any rate in small quantities of 

 material, but if the tissues are put to autolyze at temperatures of 88— iO°C., 



