ENZYMES OR FERMENTS 69 



It was thought that only the living yeast organism could bring 

 about this characteristic fermentation; after many attempts, how- 

 ever, a juice was expressed from the yeast cell which fermented no 

 less well than the living organism; thus the distinction between the 

 two kinds of ferments disappeared, and the term " organized ferment " 

 became unnecessary. 



When the action of enzymes, like those of yeast, is normally effected 

 within the cell, the enzymes are grouped as " intracellular " enzymes, 

 or endoenzymes; those which act when discharged from the cell are 

 classed as " extracellular " enzymes, or exoenzymes. Under this 

 latter group are placed the enzymes concerned in the processes of 

 digestion. There are granules of a precursor, or zymogen, stored 

 within the cells of the secreting glands of the stomach, pancreas, etc., 

 and these are discharged in response to certain definite stimuli. The 

 precursors when discharged require to be " activated " i.e., turned 

 into the active enzyme by the presence of some other body. Prob- 

 ably all enzymes require the presence of a " co-enzyme " before 

 they manifest their full activity. Thus yeast juice can be squeezed 

 through a porcelain filter candle impregnated with gelatin by a 

 pressure of 300 atmospheres; the expressed juice is found to have 

 no enzymic action until mixed with phosphates, and some other 

 substance that is diffusible and not destroyed by boiling, which is 

 left behind in the cell residues on the filter. These act as co-enzyme 

 to the expressed juice. The " intracellular " enzymes are concerned 

 intimately with processes of metabolism. If a piece of liver be kept 

 under aseptic conditions, it will be found that the longer it is kept 

 the less nitrogen it contains in the form of protein, the more in the 

 form of products of protein disintegration. Thus, of the nitrogenous 

 substances in some fresh liver, 90-4 per cent, were found to be in- 

 soluble and 9-6 per cent, soluble in water. After keeping under 

 aseptic conditions for twenty days, 39-4 per cent, of the nitrogenous 

 compounds were found to be insoluble, and 60-6 soluble. Similar 

 results have been obtained on keeping other organs, such as the 

 spleen, thymus, kidney. The products of this self-digestion, or 

 " autolysis," appear to be the same as those of ordinary intestinal 

 digestion, but the different stages have not yet been worked out. 

 Autolysis takes place in any part of the living organism when the 

 blood -supply is shut off from it. Thus, if an artery be blocked by a 

 thrombus, and the blood-supply cut off fiom part of the brain, it is 

 found that the central part softens and undergoes autolysis. The 

 same occurs in a part of the liver if the circulation be cut off from it. 

 The chief circumstance favouring this change appears to be the in- 

 creased acidity of the cell juice produced by want of oxygen. The 

 peripheral parts do not undergo the same degree of autolysis, owing 

 to the diffusion into them of oxygen and alkaline fluid from the neigh- 

 bouring cells. Such changes take place anywhere in the body as the 

 result of thrombosis or infarction. Autolysis also occurs in the living 

 organism in acute yellow atrophy of the liver, in phosphorus-poisoning, 

 and in certain acute fevers. The products of this digestion can in 



