476 DIGESTION 



effect of some substance or factor which inhibits the operation of the enzyme. 

 Specific enzyme accelerators are generally called co-enzymes. The enzyme 

 zymase (Chap. XXX), for example, will not operate unless two co-enzymes 

 are present in the same medium with it. 



8. Inhibitors. — Substances in the presence of which enzymes are rendered 

 inactive, or at least markedly reduced in activity, are termed inhibitors or 

 paralyzers. The effect of certain inhibitors may be due to the actual de- 

 struction of the enzyme, of others to some effect upon the substrate that 

 renders it less readily susceptible to the action of the enzyme, and of others 

 to an influence upon the velocity of the enzymatic reaction. The salts of 

 most of the heavy metals (Au, Ag, Hg, Cu, Ni, Co, etc.) act as very effec- 

 tive inhibitors of most enzymatic reactions. Salts of the alkali and alkaline 

 earth metals, in high concentrations, also usually exert an inhibiting effect 

 upon the action of enzymes. 



Various organic substances also inhibit enzymatic reactions. Among these 

 are hydrocyanic acid and formaldehyde. Chloroform also inhibits enzy- 

 matic reactions but its retarding effect varies considerably with the specific 

 enzyme. Some of the other common organic reagents such as acetone, toluol, 

 and ethyl ether exert little or no effect upon the activity of enzymes. Ethyl 

 alcohol in solutions of approximately 50 per cent concentration is very de- 

 structive to enzymes but as its concentration is increased or decreased from 

 this value its deleterious effect becomes progressively less marked. Proto- 

 plasm is generally more sensitive to the effects of such organic reagents as 

 chloroform, ether, etc. than enzymes. Hence it is often possible to kill plant 

 cells by suitable treatment with such reagents without destroying the enzymes 

 within them. Similarly cells of most plant tissues can be killed by desicca- 

 tion or freezing without seriously affecting the properties of the enzymes 

 present. 



There is considerable evidence that living organisms themselves produce 

 substances that inactivate enzymes. Such substances are termed anti-enzymes 

 and can be classed as naturally produced enzyme inhibitors. 



9. Radiant Energy. — Certain wave lengths of radiant energy exert very 

 pronounced effects upon the activity of enzymes in vitro. Ultraviolet rays 

 usually have a marked inactivating effect upon enzymes. The shorter wave 

 lengths of the visible spectrum seem to have greater inactivating effect than 

 the longer wave lengths. It has sometimes been assumed that the well known 

 effect of the shorter wave lengths of light in checking growth rates of 

 plants is due, at least in part, to such an effect on the enzymes of the meriste- 

 matic cells, but there is practically no valid evidence in support of this 

 hypothesis. 



