VIII] AROMATIC COMPOUNDS 129 



Plug all the tubes with cotton-wool, put in an incubator at 38° C. for 2-3 hrs. 

 Note that tube a fairly rapidly turns red, then brown and finally black. Tube d may 

 darken a little owing to the action of tyrosinase and oxygenase on the traces of 

 plant aromatics left in the tissue. Tube h remains unaltered. Tube c gives an 

 orange-red colour. 



It is probable that tyrosinase is a mixture of enzymes, of which an 

 oxidase is one component. It appears to be a fact that the plants which 

 give the tyrosinase reactions are always oxidase, and not peroxidase, 

 plants. 



Reductases. (Oxido-reductases.) These enzymes (Bach, 4) catalyze 

 the decomposition of water into hydrogen and oxygen, provided sub- 

 stances are present which will accept the hydrogen and oxygen re- 

 spectively. Such an enzyme has been shown to be present in the tuber 

 of the Potato. It will reduce nitrates to nitrites, provided acetaldehyde 

 is present, the latter being oxidized to acetic acid. 



Expt. 126. Demonstration of the presence of a reditctase in the Potato. Prepare a 

 crude enzyme extract of the tuber as in [Expt. 123 (A)]. Take 10 c.c. of a 4 % 

 solution of sodium nitrate in a test-tube, heat it in a beaker of water to 60° C. and 

 then add 10 c.c. of the enzyme extract, followed by 3 drops of IOo/q acetaldehyde 

 solution. Prepare a control tube with boiled enzyme extract. Keep the tubes at 

 60° C. for 2-3 minutes. Test for nitrite with a few drops of an alcoholic solution of 

 indole and a few drops of strong hydrochloric acid. The unboiled tube should give 

 a red colour. 



Catalases. These enzymes are probably present in all plants. They 

 decompose hydrogen peroxide with the formation of molecular oxygen 

 (see Expt. 15). 



The function of the peroxidases, reductases, catalases and tyrosinase 

 in the living cell is not known. It would appear that the oxidase reaction 

 (as detected by guaiacum, etc.) is the outcome of post-mortem changes 

 after the death of the cell. It is probable, however, that the processes 

 giving rise to it may take place to some extent, though under control, 

 in the living cell and it has been suggested, in fact, that oxidases play 

 a part in respiration (Palladin, 10). There is certainly reason to believe 

 that the first stages of respiration in plants involve a fermentation of a 

 hexose similar to that taking place in yeast. The enzymes, zymase and 

 carboxylase have been shown to be present in the tuber of the Potato 

 and the root of Beet (Bodnar, 5). Hexosephosphatase has also been 

 demonstrated in the bran of Wheat and seeds of the Castor-oil Plant 

 (Ricinus communis) (Plimmer, 20). Whether oxidases act upon the 

 products formed by the preliminary action of zymase remains an open 

 question. The fact that they are not universally present in plants 

 presents a difficulty. 



o. 9 



