66 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



It appears also that some enzymes are oxygen carriers and pro- 

 duce oxidation : they are termed oxidases. 



A remarkable fact concerning the ferments is, that the substances 

 they produce, in time put a stop to their activity ; thus in the case 

 of the organised ferments the alcohol produced by yeast, the phenol, 

 cresol, &c., produced by putrefactive organisms from proteins, first 

 stop the growth of, and ultimately kill, the organisms which produce 

 them. In the case of the unorganised ferments the products of their 

 activity hinder and finally stop their action, but on the removal of 

 these products the ferments resume work. 



This fact suggested to Croft Hill the question whether ferments 

 will act in the reverse manner to their usual action ; and in the 

 case of one ferment, at any rate, he found this to be the case. 

 Inverting ferments, as we have just seen, usually convert a 

 disaccharide into monosaccharides. One of these inverting ferments, 

 called maltase, converts maltose into dextrose. If, however, the 

 ferment is allowed to act on strong solutions of dextrose, it converts 

 a small proportion of that sugar back into maltose. ' Reversible 

 action ' has since this been shown to occur in the case of other 

 enzymes. 



Ferments act best at a temperature of about 40 C. Their 

 activity is stopped, but the ferments are not destroyed, by cold ; it is 

 stopped and the ferments killed by great heat. A certain amount 

 of moisture and oxygen is also necessary ; there are, however, certain 

 micro-organisms that act without free oxygen : these are called 

 anaerobic, in contradistinction to those that require oxygen, and 

 which are therefore called aerobic. 



The chemical nature of the enzymes is very difficult to investigate ; 

 they are substances that to a great extent elude the grasp of the 

 chemist. So far research has taught us that they are either protein 

 in nature or are substances closely allied to the proteins. 



The distinction between organised ferments and enzymes is, how- 

 ever, more apparent than real ; for the micro-organisms exert their 

 action by enzymes that they secrete. This has long been known 

 in connection with the invertin of yeast and with the enzyme 

 secreted by the micrococcus urese which converts urea into ammonium 

 carbonate. In recent years Buchner by crushing yeast cells succeeded 

 in obtaining from them an enzyme which produces the alcoholic 

 fermentation, and there is now no doubt that what is true for yeast 

 is true for all the organised ferments ; in several cases this has 

 already been proved experimentally. 



The view at present current regarding ferment action is that they 



