500 ENZYMES 



the potato oxygenase, peroxidase, and peroxide ; the peroxi- 

 dase transfers oxygen from the peroxide to the guaiacum, and 

 the oxygenase re-oxidizes the reduced peroxide. This may 

 be termed the direct action. On the other hand, in the 

 cucumber juice, only peroxidase is present, so that in order to 

 obtain the blue reaction with guaiacum, hydrogen peroxide, 

 or other peroxide, must be added. This is the indirect action. 

 According to Wheldale,* the direct oxidase reaction is 

 dependent upon the plant containing an orthodihydric phenol 

 grouping such as catechol (i.), protocatechuic acid (ii.), or caf- 



feic acid (in.) : — 



COOH CH^CH . COOH 



-OH 'v ;— OH 



—OH 



OH OH OH 



I. H. in. 



Such substances when exposed to the air undergo slow 

 autoxidation with the formation of brown oxidation products 

 and a simultaneous formation of peroxides ; according to 

 Wheldale it is the function of the enzyme oxygenase to catalyse 

 this oxidation and the resulting peroxide then sets free active 

 oxygen in contact with the peroxidase, which active oxygen 

 blues the guaiacum or, alternatively, is available for oxidation 

 of other substances in the plant. 



The following experiments due to Wheldale-Onslow illus- 

 trate the nature of the direct acting oxidase system. Slices of 

 peeled potato are pounded under 96 per cent alcohol avoiding 

 undue exposure to air. The peroxidase and oxygenase are 

 thereby precipitated upon the tissues while the catechol con- 

 taining complex remains in the solution which may be filtered 

 off on a filter pump. In order to remove it completely, the 

 extraction is repeated several times until a colourless powder 

 consisting of tissue residues and peroxidase and oxygenase 

 remains. An aqueous extract of this residue will not blue 

 guaiacum directly, but does so on addition of hydrogen per- 



* Wheldale : " Biochem. Journ.," 1920, 14, 353. 



