192 NUTRITION AND METABOLISM 



can, like coagulated albumin, by no means be brought back to their 

 original state. This temperature 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 

 secreted it. Light will also affect enzymes considerably. The great 

 similarity of enzymes and microorganisms in these respects^ the simi- 

 larity of their reactions and the extreme minuteness of the bacteria 

 render it exphcable why the chemists of eighty years ago could not 

 determine the difference between microorganisms and enzymes, and 

 called them both "ferments." 



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

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

 drying to a hmited 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 wliich 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 en- 

 zymes 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 in- 

 fluences. Mineral salts seem essential for their action, because con- 



