Catalysts of Organic and Inorganic Origin. 311 



The Potassium Iodide and Starch Test for Oxidase in Living Tissues. 



Moore and Whitley consider that where a plant extract gives a blue with 

 guaiacum without the addition of hydrogen peroxide being necessary, this is 

 due to the production of peroxides by the dying protoplasm during extraction 

 or to their presence in the guaiacum solution. If H2O2 is added to a solution 

 ■of potassium iodide, iodine is liberated and gives the usual blue with starch. 

 On applying potassium iodide to the freshly cut surface of a potato a blue is 

 also slowly formed, as well as with the cut surface of an apple or carrot 

 smeared with starch. Bach and Chodat* consider this to prove that the living 

 cells develop peroxides. If, however, the material is pounded to a fine pulp 

 and potassium iodide applied to the surface no liberation of iodine takes place, 

 and yet in freshly pounded pulp any peroxides produced by drying cells should 

 be more abundant than in a freshly cut surface. Further, the pounded pulp 

 gives a strong blue with guaiacum without the addition of hydrogen peroxide 

 being necessary. With strong potassium iodide, pounded pulp browns, and 

 the starch grains swell but remain uncoloured, although staining readily when 

 free iodine or when hydrogen peroxide is added. As a matter of fact the 

 liberation of iodine from potassium iodide appears to be due to the oxidase 

 present in the tissues used. If a slice is boiled and a fresh surface cut no 

 liberation of iodine is shown. Actual tests showed that slices soaked in 

 hydrogen peroxide contained some of the latter undecomposed after again 

 boiling. 



Certain metallic oxidases such as ferric chloride, black oxide of manganese 

 and potassium ferricyanide will also liberate iodine in a solution of potassium 

 iodide. Hydriodic acid is a substance which readily undergoes oxidation with 

 a production of free iodine, and dilute hydrochloric acid liberates free iodine 

 at the surface of a solution of potassium iodide, giving a blue colour in the 

 presence of starch. Bach and Chodatf have shown that the oxidases in the 

 sap of plants can decompose hydriodic acid, although Asot considers this action 

 to "be due to the presence of nitrates or nitric acid. The solution of potassium 

 iodide we may suppose to contain in addition to ions and undissociated 

 molecules of KI also KHO and HI. The latter would be liable to oxidation 

 by organic oxidases when applied on one side of a semipermeable membrane. 

 The action is naturally favoured by the presence of free acid, and is only shown 

 by tissues rich in oxidase. The apple, potato and carrot, which are all acid, 

 give the change readily and the iodine is liberated first over the parts rich in 



* ' Ber. d. D. Chem. Gesell.,' vol. 35, p. 2464 and p. 3943 (1902). 

 t ' Ber. d. I). Chem. Gesell.,' vol. 37, p. 36 (1904). 

 X ' Bull. Coll. Agric., 1 Tokio, vol. 5, p. 481 (1903). 



