MECHANISM OF METABOLISM. 133 



their orginal state. They have, like organisms, a maximum, optimum 

 and minimum temperature of activity, and if heated above the maximum 

 they will be destroyed. In this respect they resemble albumin since the 

 maximum temperature for enzymes is very near the coagulating tempera- 

 ture of albumin. It is believed from this resemblance that enzymes 

 are of an albuminous nature. Another similarity is the fact that both 

 enzymes and albumins are precipitated by concentrated salt solutions. 

 Enzymes can further be inactivated by poisons. The same sub- 

 stances which kill living cells, like formaldehyde, hydrocyanic acid, 

 mercuric chloride, phenol, will also inactivate enzymes, though usually 

 stronger solutions are required for the destruction of the enzyme than 

 for killing the cell. It is the same with heat; a higher temperature is 

 generally required to destroy the enzyme than to kill the cell which se- 

 creted it. Light will also affect enzymes considerably. The great simi- 

 larity of enzymes and microorganisms in these respects, the similarity 

 of their reactions and the extreme minuteness of the bacteria render 

 it explicable why the chemists of eighty years ago could not deter- 

 mine the difference between microorganisms and enzymes. 



With the toxins, the enzymes have in common the great sensibility 

 to heat, light, and chemicals. Both of these groups are resistant to 

 drying to a limited extent. So far as body reactions are concerned these 

 two groups seem to belong to one physiological group of compounds. 

 When toxins are injected, the body responds by the production of anti- 

 toxins which inactivate the toxin. In the same way the body responds 

 to enzymes by the production of anti-enzymes which prevent the action 

 of the enzymes. It may be mentioned that against protein compounds, 

 precipitins are produced by the body which precipitate only that protein 

 which was injected. This "specific" action is also true with toxins and 

 enzymes. The anti-body will inactivate only the specific kind of toxin 

 or enzyme that was injected. 



What an enzyme really is cannot be defined. An enzyme is known only 

 by its reactions. Many chemists have tried to prepare pure enzymes 

 by continuously dissolving and precipitating, by dialyzing and other 

 means, but there are two great difficulties existing; there is no test for the 

 purity of enzymes, and they lose in activity if treated with chemicals. 

 The more they are freed from the protein bodies which always accompany 

 them, the more sensitive they are to injurious influences. The purest 

 enzymes that have been obtained do not give the reactions of protein 



