82 PHYSICOCHEMIOAL BASIS OP PHYSIOLOGICAL PROCESSES 



6. Urease — accelerating hydrolysis of urea to ammonium carbonate 

 (in many microorganisms and in the soy bean). 



7. Glyoxylase — converting glyoxals into lactic acid (page 666). 

 Other enzymes accelerate oxidative processes and are called oxidases 



and peroxidases. Others bring about the displacement of an amino 

 group by hydroxyl (desamidases). Others cause coagulation (coagula- 

 tive ferments), e.g., thrombin, rennin. One of the enzymes present in 

 succus entericus acts by converting the zymogen (trypsinogen) into the 

 enzyme (trypsin). 



Enzyme Preparations 



So far it has been impossible to prepare enzymes in a pure state al- 

 though, being colloidal in nature, they are readily precipitated or ad- 

 sorbed along with, other colloids. 



Since most enzymes exist in cells, it is necessary to break up the cells 

 in order to isolate the enzyme. This is done in various ways. By one 

 method the cells are ground in a mortar with fine sand, then made into 

 a paste with infusorial earth (Kieselguhr), the paste enclosed in stout 

 canvas and placed under an hydraulic press at about 300 atmospheres 

 pressure ; a clear fluid separates and this contains the enzymes. An- 

 other way is to freeze the tissue with liquid air and grind it in a steel 

 mortar by means of a machine, ^till another and less expensive method, 

 and one which we have found most useful for organs and tissues, con- 

 sists in reducing the tissue to a pulp and, after sieving it to get rid of 

 connective tissue, etc., spreading the pulp on glass plates and drying 

 in a slightly warmed, dry air current. The scales of dried material are 

 then ground in a paint mill with toluene, and the resulting suspension 

 filtered ; the powder which remains on the filter, after thorough washing 

 with toluene, is dried and kept for future use. The toluene removes all 

 the fatty substances, so that when shaken with water, etc., the enzymes 

 dissolve. 



Conditions for Enzymic Activity 



Reactions brought about by intracellular enzymes are very readily 

 inhibited when there comes to be a certain accumulation of their prod- 

 ucts of action. Thus, yeast ceases to ferment sugar when the alcohol 

 has accumulated to a certain percentage. This action is partially due 

 to a toxic action of the alcohol on the cell, which paralyzes its power of 

 absorbing the substance to be acted on by the intracellular enzyme. If 

 these products be not in some way removed, they will ultimately kill 

 the cell and stop the fermentation. AVe have seen above how the ac- 

 cumulation of products may interfere with the activities of enzymes in 



