372 ENZYMES 



The action of this enzyme is promoted by the presence of 

 acids, e.g. hydrochloric or citric, but if too much acid be added, 

 the action is inhibited. 



To study the action of diastase on starch a mixture of 

 these two substances may be tested from time to time with 

 iodine solution. 



Appleman * gives the following experiment. A number 

 of test tubes, say ten, each containing I c.c. of a one per cent 

 solution of starch paste, are placed in ice. The extract of the 

 material to be examined for its diastatic activity is added to 

 the mixture in increasing amounts. Thus to the first tube is 

 added I c.c. of extract, to the second I 'I c.c., to the third I '2 

 c.c., and so on. A drop or two of toluol are also added as an 

 antiseptic. The tubes are then removed from the ice and 

 placed in an incubator, kept at a temperature of 40 C., for 

 forty-eight hours. An equal amount of water, roughly enough 

 to fill the test tubes, is added to each test tube and, after shaking 

 up, three drops of iodine solution. The first tube in the descend- 

 ing series which showed a blue or violet colour was taken as 

 the index for comparison. 



QUANTITATIVE DETERMINATION OF THE ACTIVITY OF 

 DIASTASE. 



The amount of optically active or reducing sugars produced 

 may be followed polari metrically or by means of Fehling's 

 solution. Due allowance should be made for any sugar 

 contained in the enzyme. 



PROTEASES. 



For many years it has been known that the fluids ex- 

 creted by many insectivorous plants are capable of digesting 

 proteins ; proteolytic ferments are now known to occur in the 

 juice of a good many plants. Some indeed, e.g., erepsin, are 

 almost universal. Amongst the better known ones may be 

 mentioned papai'n which occurs in the fruit of Carica papaya 

 (papaw), bromelin in the fruit of the Ananas sativa (pine- 

 apple), and cradei'n in the latex and fruit of Ficus (fig). 



* Appleman : " Bot. Gaz.," 1911, 52, 306. 



