COMPOSITION OF FOOD AND ACTION OF ENZYMES. 663 



C U H B 11 + H 2 = C 6 H 12 6 + C 6 H 12 6 . 



Maltose. Dextrose. Dextrose. 



And the hydrolysis of the neutral fats by lipase may be expressed 

 so: 



/-i TT /r\ TT /~v \ _t_ OU" /"i f IT ff\~fT\ _i_ 1(C* TT C\ \ 



^3-"-5\^18-'^*-35^2/3 ' *Ji*-yJ ^a-'-lsv^'-tl./S r "V^18-*"*-36^2/ 



Tristearin. Glycerin. Stearic acid. 



General Properties of Enzymes. The specific reactions of the 

 various enzymes of the body are referred to under separate heads. 

 The following general characteristics may be noted briefly : 



Solubility. Most of the enzymes are soluble in water or salt 

 solutions, or in glycerin. By these means they may be extracted 

 conveniently from the various tissues. In some cases, however, such 

 simple methods do not suffice; the enzyme is destroyed in the process 

 of extraction, and to prove its presence pieces of the tissue or the 

 juice pressed from the tissue must be employed. 



Temperature. The body enzymes are characterized by the fact 

 that they are destroyed by high temperatures (60 C. to 80 C.) and 

 that their effect is retarded in part or entirely by low temperatures. 

 Most of them show an optimum activity at temperatures approxi- 

 mating that of the body. 



Precipitation. The enzymes are precipitated from their solutions 

 in part at least by excess of alcohol. This precipitation is frequently 

 used in obtaining purified specimens of enzymes. The enzymes, 

 moreover, show an interesting tendency to be carried down mechani- 

 cally by flocculent precipitates produced in their solutions. If 

 proteid present in the solution is precipitated, for instance, the 

 enzymes may be carried down with it in part. 



Incompleteness of their Action. In any given mixture of a 

 substance and its enzyme the action of the latter is not complete, 

 that is, all of the substance does not disappear. An explanation for 

 this fact has been found in the reversibility of the action of the 

 enzyme. If the reaction proceeds in both directions, then evidently 

 under fixed conditions a final equilibrium will be reached in which 

 no further apparent change takes place, although in reality the 

 condition is not one of rest, but of balance between opposing proc- 

 esses proceeding at a definite rate. Within the body itself, on 

 the contrary, the action of an enzyme may be complete, since the 

 products are removed by absorption and the possibility of a re- 

 versed reaction is removed. In some cases for instance, the co- 

 agulating enzymes the action is apparently always complete. 



Active and Inactive Form. In many cases it can be shown that 

 the enzyme exists within the cell producing it in an inactive form 

 or even when secreted it may still be inactive. This antecedent or 

 inactive stage is usually designated as zymogen. The zymogen may 

 be stored in the cell in the form of granules which are converted into 



