56 ENZYMES— MODE OF ACTION 



and the effect of traces of colloidal (finely divided) platinum, 

 known as platinum black, in causing the explosive combination 

 of oxygen and hydrogen at ordinary temperatures. In either 

 case the reaction is one that would also take place in the absence 

 of the catalytic agent, but with this difference, that the rate 

 would be very much slower. Similarly, there is reason to believe 

 that the changes brought about by enzymes would also occur in 

 their absence, but at so slow a rate as not generally to be capable 

 of recognition. In this connection it is well to recall that much 

 the same effect as is produced by the enzyme can often be 

 attained by the use of other catalytic agents (e.g. boihng with 

 small quantities of mineral acids). 



Although the enzyme appears unaltered at the end of the 

 reaction, it is almost certain that combination of some kind 

 with the substances undergoing change takes place while the 

 process is proceeding. In view of the large surface which is 

 presented by colloids, it is very probable that this combination 

 is a ph5'sical one (adsorption), and it is thought by many that, 

 in the hydrolytic enzymes, for instance, the water and the com- 

 pound undergoing hydrolysis are brought into intimate contact 

 at the surface of the ultramicroscopic ferment particles. 



Enzymes differ from most other catalysers in that each is 

 usually only effective in accelerating one or few particular re- 

 actions (cf . p. 53) , and they are, moreover , very sensitive to heat and 

 light. The rate of the reaction is doubled or trebled with every rise 

 of 10° C. , but soon a temperature is reached (usually about 60' C.) 

 at which most enzymes are destroyed. It may be added that 

 heating which suffices to kill the protoplasm lea\'es ferments 

 unharmed. Strong light destroys them very quickly, an effect 

 for which the violet end of the spectrum appears to be mainly 

 responsible. Many chemical compounds [e.g. sulphuretted hy- 

 drogen, prussic acid, chloroform, etc.) arrest enzyme-action to 

 a more or less marked degree according to their concentration. 

 If the products of enzj'me activity are not removed, a retardation 

 of the process is at once manifest, continued action of diastase, 

 for extunple, being dependent upon the reiuowd of the maltose. 

 In some cases the accumulation of the products of the reaction 

 actually exerts a poisonous effect, as, for instance, that of the 

 alcohol produced by Yeast. Usually, however, the substances 



