68 THE ACTION OF ENZYMES 



tioned later, can act upon several different kinds of 

 compounds of the same class, many examples are 

 known of enzymes which are very specially concerned 

 with the decomposition of one definite kind of chemical 

 substance only and are inactive upon any others even of 

 very similar or allied composition. They are generally 

 soluble in water, and although their solutions may be 

 obtained in a concentrated form by extracting the 

 tissues of plants and animals containing them, they 

 have not yet been isolated in a pure state. One of 

 their most striking properties is the power which very 

 small quantities have of decomposing or changing large 

 amounts of the particular chemical compounds upon 

 which they work, and that without suffering much 

 diminution or loss during the process. Enzymes are 

 active only within a limited range of temperature. 

 Those obtained from the tissues of warm-blooded animals 

 carry on their work best at about 37 C, while those of 

 vegetable origin are generally most effective about 2 5 C. 

 At low temperatures their action is checked but not per- 

 manently destroyed ; they are, however, destroyed when 

 their solutions are heated to 70 or 80 C. for a short 

 time. In a dry state they resist a temperature of 

 120 C. or more without damage. The majority act 

 only in neutral or faintly acid solutions, but a few are 

 effective in an alkaline medium. 



As already indicated enzymes are very sensitive com- 

 pounds and much influenced by external conditions. 

 They are readily destroyed by high temperatures and 

 strong acids or alkalis, but neutral salts are less injurious 

 and often precipitate the enzymes along with proteins 

 and other substances with which they are associated in 

 living tissues of plants and animals. 



