114 AROMATIC COMPOUNDS AND [CH. 



tissue residue, as does the peroxidase [Expt. 118 (A)], but the tyrosine is 

 almost entirely washed away. 



Expt, 120. Demonstration of the presence of tyrosinase in the Potato. Take about 

 half a potato and proceed as in the preparation of peroxidase [see Expt. 118 (A)]. 

 Koughly dry the powder left on the filter and then add about 100 c.c. of water and 

 allow to stand for 15 mins. Filter, and divide the filtrate into six portions a, 6, c, rf, 

 e and /. Make a suspension of a little tyrosine in water (tyrosine is only slightly 

 soluble in cold water). 



To a add 5 c.c. of tyrosine suspension. 



To b add 5 c.c. of tyrosine suspension and a few drops of hydrogen peroxide. 



To c add 5 c.c. of tyrosine suspension and boil. 



To d add a few drops of hydrogen peroxide only. 



To e add some p-cresol. 



To /nothing is added. 



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

 Note that tubes a and 6 fairly rapidly turn red, then brown and finally black. 

 Tubes d and / may darken a little owing to the action of tyrosinase and peroxidase 

 on the traces of plant aromatics left in the tissue. Tube c remains unaltered. Tube 

 e gives an orange-red colour. 



Another point which may be touched upon is the significance of the 

 oxidizing enzymes in plant metabolism. The facts as they present 

 themselves may be stated thus. On death or injury we discover a 

 system (oxidase) in some plants which can oxidize certain acceptors, 

 either artificially introduced, such as guaiacum, or other aromatic sub- 

 stances present in the plant itself. 



The question is whether the oxidase system as described above 

 functions in metabolism for the purpose of oxidation and respiration in 

 the living plant. If it does, there are various suggestions as to how it 

 may act. For instance, it is possible that aromatic substances are oxi- 

 dized to pigments by the oxidase system, and the pigment is then 

 deoxidized by products of anaerobic respiration, hence constituting a 

 mechanism for this function. Such pigments have been termed " respi- 

 ration pigments" (Palladin, 12-16). It is suggested that they are not 

 present in any quantity in the living plant and their production is 

 controlled by oxidizing and reducing systems : 



autoxidizable substance + molecular oxygen = peroxide 



peroxide + peroxidase = active oxygen 



aromatic substance (acceptor or chromogen) -f active oxygen = 



pigment 

 pigment 4- reducing substance = chromogen. 



The above hypothesis is only applicable to plants containing oxi- 



