ENZYMES f>7 



(6) From such solutions they are precipitated by an excess 

 of alcohol. Enzymes also tend to adhere to any inert pre- 

 cipitate, and this is a recognized procedure in their isolation 

 and purification. 



(c) All have an optimum temperature lying somewhere 

 between 37 and 50 C. A rise of temperature to 80 C. kills 

 them. 



(d) The accumulation of the products of action hinders 

 further action and finally, apparently, stops it. This phe- 

 nomenon in some enzymes, at least, has been demonstrated 

 to be due to their reversibility of action. Lipase, for instance, 

 will not only break ethyl butyrate into ethyl alcohol and butyric 

 acid, but once having formed the latter, reunites them into 

 ethyl butyrate. So complete a reversal of action does not 

 always take place. Maltase acting on maltose will change 

 the latter to dextrose, but when maltase is allowed to act upon 

 a solution of pure dextrose some of the dextrose is converted 

 to isomaltose and dextrin-like bodies. 



(e) Enzymes often exist in active and inactive forms. The 

 latter, known as zymogen, is often visible in cells in the form 

 of granules. When a ferment is secreted in an inactive form 

 it often requires some second body to produce the conversion 

 into the active form. When this second body happens to be 

 an inorganic substance it is called an activator; when, on the 

 other hand, it is an organic substance it is called a kinase. 



The nature of the chemical change brought about by enzymes 

 is in very many cases, especially in digestive processes, one 

 of hydrolysis. In producing this reaction they are said to act 

 by catalysis, which means by their mere presence. It is con- 

 ceivable that a ferment may act through its physical properties 

 or through certain chemical changes that it undergoes, but 

 which leave it ultimately as it was at the beginning. Both 

 possibilities may be illustrated by phenomena from inorganic 

 nature. For instance, a trace of iodine added to amorphous 

 phosphorus will convert the entire mass into red phosphorus. 

 The iodine undergoes no chemical change, but acts through 

 its physical properties, possibly by inducing a more active 

 molecular vibration in the phosphorus, so that it assumes a 

 more staple structure. Chittenden has given an example illus- 

 trating the other possible method of action of enzymes. Carbon 



