8 June, 1907.] Elements of Animal Physiology. 339 



Single-celled animals have but a small equipment of enzvmes to 

 liberate, as compared with higher animals where special sets of catalysors 

 are relegated to special organs and the total number thus made very 

 great. 



Some special characters of enzymes may now be described in detail. 



1. Enzymes are probahh- closely related to proteins in constitution. 

 Chemical reagents which precipitate proteins destroy enzymes. They are, 

 like albumens and globulins, altered by heat, in fact no enzyme in 

 solution can stand a temperature of 7o"C. and manv not even 6o°C. 

 Thus it happens that an enzyme has an optimum temperature, i.e., a 

 temperature at which it acts most rapidly. If the temperature be in- 

 creased from zero upwards the activity of the enzyme increases too, but 

 when the heat becomes sufficiently great to injure the enzyme its activity 

 begins to fall off and will eventually disappear. With the majority of 

 enzymes the optimum temperature is just a little above the temperature 

 of the mammalian body. Unfortunately no enzvme has ever been isolated 

 in a pure form, in fact we have no idea, when we have a solution of 

 an enzyme, how much of the solid matter present is impuritv and how 

 much is ferment. 



2. Enzymes act specifically. An acid, as we have seen, can split 

 up all disaccharides, all polysaccharides and all proteins, but such uni- 

 versal application in an enzyme would be disastrous to the bioplasm. 

 We find for instance that the enzyme which accelerates the splitting of 

 cane sugar into dextrose and levulose, fails utterly to act on any other 

 disaccharide, not to mention polysaccharides and proteins. It is highlv 

 probable that each enzyme unites at first with the body acted on and has 

 ii structure related in some way to the structure of the latter as the wards 

 of a key are related to the lock. We find many instances of an enzyme 

 acting not on one but on a small number of bodies, yet when these are 

 investigated it will be found that they all present some striking similarity 

 in chemical structure. Thus the alcohol-producing ferment of yeast or 

 zymase, as it is called, can act not only on dextrose but on levulose and 

 another sugar called mannose. 



3. Enzymes determine the direction of change. One enzyme will 

 transform dextrose into alcohol and carbon dioxide, another will change 

 it into butyric acid and hydrogen, a third will transform it into lactic 

 acid. Here we have a quality which seems at variance with the law 

 concerning the accelerating action of enzymes. This point has not vet 

 been properly investigated, but the probable solution of the difficulty seems 

 to be that dextrose of itself tends to decompose into carbon dioxide and 

 water, whilst alcohol, butyric acid and lactic acid are all steps towards 

 this end. This brings us to the next property. 



4. Enzymes, much more than inorganic catalysors, tend to halt at some 

 stage of change. An acid will carry starch through dextrins and maltose 

 into dextrose, but the enzvme which acts on starch (diastase) will carry 

 the change as far as maltose and then stop. 



5. Enzymes like catalysors generally may be poisoned by minute 

 quantities of other substances. Prussic acid even in minute doses stops 

 the action of all enzymes and to this its great poisonous action in the 

 body is due. 



6. Enzymes, like other cataly.sors, may be the cause of extensive 

 changes though present in very minute amounts. Owing to the reason 

 already given the amount of an enzvme in solution cannot be determined 



