414 SCIENCE PROGRESS 



which certain processes take place. Much work has been done 

 on investigation of the factors which influence this velocity, 

 especially with a view to determining the exact nature of the 

 catalytic process. Repeated attempts have been made to 

 formulate laws of velocity for the action of enzymes, but no 

 general law has yet been agreed upon, the chief difficulty being 

 that the heterogeneous systems in which the reactions take 

 place so influence the velocity that it is seldom possible to be 

 certain that the reaction in its entirety obeys any of the known 

 laws of velocity, although it may in part appear to do so. 



Concentration of Substrate. — Work on tJhe effect of the con- 

 centration of the substrate shows that this factor plays little 

 part in velocity, and that the rate is constant within a wide 

 range of concentration. The early work of F. Armstrong on 

 lactase, and that of Nelson and Vosburgh (191 7) is quite 

 definite, but that of Van Slyke and Cullen (1914) on the hydro- 

 lysis of urea by urease shows that there is distinct depression 

 of enzymatic activity when the urea is over 10 per cent. It 

 is interesting to note that Colin and Chaudin (191 8) differenti- 

 ated between two stages of enzyme action, as, it will be seen 

 later, did also Van Slyke and Cullen in the work referred to above. 

 In the first stage they found that hydrolysis of saccharose by 

 sucrase was in proportion to the amount of enzyme present, but 

 that the second stage was in proportion to the amount of sub- 

 strate. In a later work, however, these observers arrived at 

 what has otherwise been shown to be the probable cause of the 

 decreased activity, viz. the increased viscosity, which appar- 

 ently was not taken into account by Van Slyke and Cullen as 

 no mention is made of it in their paper. 



Visicosty. — Colin and Chaudin found that with increasing 

 concentrations from 10 per cent, to 60 per cent, saccharose, the 

 hydrolysis by sucrase diminished steadily. Below there was 

 an upper limit to the activity of a given amount of sucrase in 

 the presence of an excess of sugar. By means of a viscosimeter 

 the retardation was shown to be due to increased viscosity, and 

 the observers conclude that when the saccharose is in excess 

 the hydrolysis is in proportion to the fluidity. A similar 

 explanation can therefore be put on the earlier work of Bayliss 

 (19 1 3), who found that the synthesis of glycero-glucoside by 

 emulsin was directly proportional to the concentration of the 

 glucose. The same observer has shown that the action of 

 trypsin on gelatine is less in the stronger solutions of gelatine ; 

 but even in 19 19, although admitting of it as a possibility, he 

 referred to it as an " obscure influence." What has already 

 been said on the dispersion of enzymes in relation to their action, 

 and on the effect on dispersion of substances which raise or 

 lower surface tension, appears to make the point clear that the 



