PRINCIPLES OF GENERAL PHYSIOLOGY 



concentration of substrate, the active surface available will be "saturated," so that 

 further increase in concentration will not result in more adsorption and therefore 

 in no increase in the rate of the reaction. If the " concentration " of the active- 

 surface is increased relatively to the substrate, there will be increase until tins 

 surface is saturated. 



This circumstance, however, seems capable of explaining only the fact of eqwility of rate 

 above certain relative proportions of enzyme and substrate, and is well illustrated by tin- 

 following two experiments by E. F. Armstrong (1904, 1, p. 508). A very small amount \ 

 lactase acted on different concentrations of lactose for forty-six hours ; it was found that 

 the amount hydrolysed was the same in all, although the reaction wa,s by no means at an 

 end, thus : 



Lactose. Amount Hv<ln.l\ --il. 



10 per cent. - '2~*2 



20 2-18 



30 2-21 



When the proportion of enzyme to substrate was large, a different result was obtained : 



The amount of hydrolysis is in direct ratio to the concentration of the substrate and tin- 

 velocity constant is practically identical in all. 



Another factor which comes into play in such cases as proteins or glycerol is 

 viscosity, which, as we have seen, retards the access of substrate to enzyme. The 

 fact that gelatine shows actual retardation above a certain concentration and in a 

 more marked degree than does caseinogen, supports this view, since gelatine forms 

 solutions of a higher degree of viscosity than those of caseinogen. The following 

 numbers show the change of electrical conductivity in twenty-five minutes in 

 solutions of gelatine of different concentrations : 



10 per cent. - - 130 reciprocal megohms. 



8 - 170 



4 - 240 



2 - 280 



We see that there is a progressive increase in the rate as the solution is more 

 dilute. Similar facts apply to the synthesis of glycerol-glucoside by emulsin ; the 

 rate, is diminished when the glycerol present exceeds about 65 per cent. It is 

 important to note, however, that in this case, where it is possible to test the effect 

 on the total amount of products when the reaction has attained equilibrium, this 

 final amount is found to be in direct ratio to the concentration of the substrate, 

 although the higher the viscosity, the longer the time taken to reach equilibrium. 



Effect of Temperature. Like all processes, the action of enzymes is increased 

 in rate by rise of temperature, in some cases very considerably, more than trebled 

 by a rise of 10. The fact indicates that the controlling factor of this particular kind 

 of heterogeneous reaction is the chemical reaction proper, since both diffusion and 

 adsorption, as physical processes, have a low temperature coefficient. 



As the temperature is raised, it is found that, above a particular temperature, 

 the rate begins to fall off, and at a further rise of temperature all effect is abolished. 

 The temperature at which the maximum rate is shown has been called the optimum 

 temperature. 



It is merely due to the fact that enzymes are injured more or less rapidly by 

 rise of temperature, and the optimum temperature is that at which the acceleration 

 due to rise of temperature is in greatest excess over the simultaneous destruction 

 of the enzyme. The process has been worked out by Frost Blackman (1905) and 

 a complete explanation given. Attention should also be directed to the time 

 factor in this connection. The lethal effect of raised temperature is not a sudden 

 thing, so that the slowing of the reaction will be more and more apparent the 

 longer the time that has elapsed since the commencement of the exposure to a 



