HUGO THEORELL 



formulas (1), (2), (3), and (4). When the values for ky. . . .k^,, 

 given in Table III, were inserted in (5) and (6) the agreement 

 was found to be very satisfactory, the deviations never exceeding 

 the possible experimental errors. The agreement of the kinetic 

 values, measured by initial velocities in kikz/kjcc,, with the older 

 values for D^JD^^^ obtained from equilibrium measurements, 

 was practically perfect. This proves that the two DPNH or 

 DPN molecules attached to the same molecule of ADH act 

 similarly and independently of one another. There is con- 

 vincing reason to believe that the presence or absence of sub- 

 strate does not interfere with the reactions (1) and (3), because 

 the value of ^i, measured spectrophotometrically in 1951 by the 

 aid of the velocity of the band shift from 340 to 325 m^u, in the 

 absence of substrate, agrees with the new value obtained in the 

 presence of substrate (Table III), kx = A X 10^ and 3.7 X 10^ 

 respectively. 



It was clear in 1951 that formula (2) might be an over- 

 simplification, since it does not take into account the possibility 

 that the formation and breakdown of enzyme-substrate com- 

 pounds may under some conditions be rate-limiting steps. 

 That such compounds must exist became evident in 1953 from 

 the beautiful work of Vennesland, Westheimer, and their as- 

 sociates (32), demonstrating a direct and stereospecific exchange 

 of deuterium between alcohol and DPN in the presence of ADH. 



According to Burton and Kaplan (3), it seems plausible 

 that (2) should involve the following partial reactions : 



DPNH 



ADH ' • DPNH + CHs • CHO , ADH ' \ {2a) 



\ \ 

 CH3CHO 



DPNH DPN+ 



ADH \ - ADH \ {2b) 



\ \ \ \ 



CH3— CHO GH3— CH2— O- 



302 



