748 PHYSIOLOGY OF DIGESTION AND SECRETION. 



The blood-serum may not contain any proteolytic enzyme capable 

 of hydrolyzing egg-albumin, but if this protein is injected into the 

 animal the serum will then be found to contain an enzyme that 

 digests egg-albumin. Similarly, if cane sugar is injected the blood- 

 serum will show the presence of an inverting enzyme that splits 

 the cane-sugar to its constituent monosaccharids. It has been 

 claimed, moreover, that if any of the cellular material of the 

 animal's own body, not normally present in the blood, finds its way 

 into this liquid, enzymes are produced capable of digesting this 

 blood-foreign material. There is reason to believe that the pro- 

 tective enzymes thus developed show a considerable degree of 

 specificity to the particular foreign material responsible for their 

 formation, and efforts have been made, therefore, to utilize the fact 

 in the diagnosis of abnormal conditions, the method .being desig- 

 nated as the Abderhalden test, after the name of the physiologist 

 who first proposed its use, or more generally as the method of sero- 

 diagnosis. Thus, in pregnancy, cells from the chorionic villi of the 

 placenta get into the blood of the mother, and examination of her 

 blood-serum shows the presence of an enzyme that digests placental 

 proteins. The biological reaction, whereby these protective en- 

 zymes are produced, is similar to that already described in the case 

 of the formation of precipitins (p. 424) and antibodies in general. 

 There can be little doubt that it constitutes a part of the mechan- 

 ism by which the body acquires immunity to certain infectious dis- 

 eases, and possibly also it may serve to explain the development 

 of the condition known as hypersensitization or anaphylaxis. 



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, particularly for the endo-enzymes; 

 the enzyme is either insoluble or is destroyed in the process of ex- 

 traction, 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, Adsorption. The enzymes are precipitated from 

 their solutions in part at least by excess of alcohol. This precipi- 

 tation is frequently used in obtaining purified specimens of en- 

 zymes. The enzymes, moreover, show an interesting tendency to 

 be carried down mechanically by flocculent precipitates produced in 



