152 PROTEINS AND AMINO- ACIDS [ch. 



Dihydroxy phenylalanine. This amino-acid, which contains two 

 hydroxyl groups in the ortho position, has not been detected as a con- 

 stituent of proteins. It occurs in the free state in all parts of the plant 

 of the Broad Bean ( Vicia Faba) (Guggenheim, 8) and it has also been 

 found in the Velvet Bean (Stizolobium). It readily oxidizes in air and is 

 doubtless responsible for the intense black coloration which appears in 

 all parts of the Broad Bean plant after death of the tissues. 



Bxpt. 142. Extraction of dihy droxyphenylalanine fromthe Broad Bean (Vicia Faba). 

 Take one kilo, of green pods of the bean and put them through a mincing machine. 

 Put the minced mass immediately into boiling water, boil for a few minutes and 

 filter through linen, squeezing the residue thoroughly. Then add lead acetate solution 

 to the filtrate until no further precipitate (consisting of lead compounds of proteins, 

 amino-acids, flavones, etc.) is formed, avoiding an excess of acetate. Filter off and 

 discard this precipitate. Then add ammonia to the filtrate until it is distinctly 

 alkaline to litmus. A yellow precipitate of the lead compound of dihydroxy- 

 phenylalanine comes down. Filter, and suspend the precipitate in 500 c.c. of water 

 and pass in sulphuretted hydrogen until the precipitate is decomposed. Filter, and 

 evaporate the filtrate to a small bulk in vacuo preferably in a current of carbon 

 dioxide. Crystals of dihydroxyphenylalanine will separate out. Make a solution of 

 the crystals and perform the following test. Add 5 ^j^ ferric chloride solution. A 

 green colour is formed. Then add a little 1 % sodium carbonate solution ; the 

 green colour changes to violet. 



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. 20) the hydrolytic activity of many enzymes is un- 

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



