April 22, 1921] 



SCIENCE 



391 



eulphates present; and tliese indicate the 

 origin of the deposit. 



To investigate this matter, however, the 

 co'mmittee had the water collected in the 8-in. 

 rain gage on the roof of the Meteorological 

 Office analyzed for two or three months. Dur- 

 ing the month of ISEovemher the small 8-in. 

 gage collected 900 c.c. of water, the total de- 

 posit was 0.445 gramme, the total soluble 0.34 

 gramme, while in the Standard deposit gage, 

 the water collected was 783 liters, total deposit 

 1.962 grammes, total soluble 0.53 gramme. 

 There was, therefore, a large excess of soluble 

 matter in the water collected in the rain gage. 

 The same result was found in subsequent 

 months, and it was ascertained that the excess 

 of soluble matter was due to metal dissolved 

 from the rain gage. 



It was therefore useless to continue the ex-' 

 periment ujiless the solution of the metal could 

 be prevented. In order to do this the rain 

 gage was given a coating of Duroprene var- 

 nish, in the hope that this would prevent the 

 solution of the metal without any contamina- 

 tion of the water. 



i The result of the analysis of a month's rain- 

 fall showed for the 8-in. rain gage a consider- 

 ably larger proportion of soluble and insol- 

 uble matter per liter of water as compared 

 with the standard deposit gage, owing to the 

 varnish yielding to the action of the rain 

 water. It is therefore clear, if observations 

 are to be taken with small gages these must be 

 constructed of something which will not dis- 

 solve in the water, and the use of the ordinary 

 copper rain gages is therefore inadmissible. 



Alexander McAdie 



SPECIAL ARTICLES 



THE MECHANISM OF AN ENZYME REACTION 

 AS EXEMPLIFIED BY PEPSIN DIGESTIONi 



One of the most striking peculiarities of 

 living matter is the fact that nearly all the 



1 The experimental data on which this paper is 

 based may be found in J. Gen. Fhysiol., 1918-19, 

 L, 607; 1919-20, II., 113, 465, 471, 595; 1920-21, 

 III., 211. 



reactions which take place in the organism 

 are due to enzymes. The mechanism of 

 enzyme reactions is therefore very closely 

 connected with the mechanism of the living 

 cell. Many enzyme reactions, however, may 

 be studied in vitro and are therefore amenable 

 to quantitative study. The present paper is 

 an attempt to show that the peculiarities of 

 a typical enzyme reaction, pepsin digestion, 

 may be explained by the accepted laws of 

 chemical reactions and that the apparent 

 divergencies from these laws are due to the 

 fact 'that the enzyme as well as the protein 

 with which it reacts exist in solution as equi- 

 librium mixtures, consisting, in the case of 

 the protein of ionized and non-ionized protein, 

 and in the case of the pepsin of free and com- 

 bined pepsin. The influence of the various 

 factors on the digestion are primarily due to 

 changes in these equilibria. 



It is well known that enzyme reactions in 

 general have certain peculiarities wliich dis- 

 tinguish them from ordinary chemical re- 

 actions. These may be briefly stated as 

 follows : 



1. The final amount of change caused by 

 tlie enzyme is independent of the amount of 

 enzyme present. 



2. The rate of change may or may not be 

 proportional to the concentration of enzyme 

 present. 



3. The rate of change is proportional to the 

 substrate concentration in dilute solution but 

 increases less rapidly than the substrate con- 

 centration in solution of higher concentration. 



4. The amount of substrate decomposed in 

 the same time interval by varying enzyme 

 concentrations is not always proportional to 

 the concentration of enzyme but is often pro- 

 portional to the square root of this quantity 

 (Schiitz's rule). 



5. The reaction proceeds most rapidly at 

 a certain definite hydrogen ion concentration. 



It has been found in a study of pepsin 

 digestion that the above peculiarities may be 

 quantitatively accounted for on the basis of 

 the following mechanism. 



1. The protein reacts with the acid of the 



