III. BIOCHEMICAL SYSTEMS 69 



catalyze the eciuilihiiuin between ethunol uiul acetaldehycle. That found 

 in animal tissues, already considered in part, was crystallized by Bon- 

 nichsen from horse li\'er.'""' The other, found in yeast, has been crystallized 

 by Negelein and WuUf" and by Racker.^'^ Unlike the animal enzyme, yeast 

 alcohol dehydrogenase is not a catalyst for the vitamin A-retinene equilib- 

 rium.*'-' 



Yeast alcohol deh^'drogenase is inhibited by monoiodoacetate, a poison 

 which does not affect the animal enzyme. For this reason it is usually said 

 that the yeast enzyme depends for its action upon sulfhydryl groups, 

 whereas the animal enzyme does not require — SH groups. We have re- 

 cently found, however, that crystalline horse liver alcohol dehydrogenase 

 is inhibited completely by the powerful *and specific sulfhydryl poison, 

 p-chloromercuribenzoate (2 X 10~^ M). This inhibition is reversed by add- 

 ing glutathione. One can be reasonably sure, therefore, that animal alcohol 

 dehydrogenase, like that of yeast, is a sulfhydryl enzyme.*'' 



The equilibrium catalyzed by the alcohol dehydrogenase system can be 

 written : 



Alcohol + DPN = Aldehyde + DPX-IT2 



For tliis, one can write the mass action expression: 



(Aldehyde)(DPX-H2) 



K = 



(Alcohol) (DPN) 



Racker'^ has measured the equilibrium between ethanol and acetalde- 

 hyde, catalyzed by yeast alcohol dehydrogenase (of course the nature of 

 the enz\Tne is of no importance here, since it has no effect upon the position 

 of the e(iuilibrium). He found that the eciuilibrium constant A^ is 1.3 X 

 10~* at pH 7. That is, in an equilibrium in which cozymase is half reduced, 

 half oxidized, the ratio of alcohol to acetaldehyde is about 8000:1. 



Racker found, however, that this equilibrium depends markedl}^ upon 

 pH. The logarithm of the e(|uilibrium constant, log K, varies linearly with 

 pH between about pll 7 and 0.(1. The same is true of the lactic acid-pyruvic 

 acid equilibrium. The reason for this relationshij) is clear if the equilil^riimi 

 is rewritten as follows: 



3« R. K. Bonnichsen, Acta Chcm. Scand. 4, 715 (1950). 



" E. Xegelein and H.-J. Wulff, Biochem. Z. 293, 351 (1937). 



'8 E. Racker, J. Biol. Chem. 184, 313 (1950). 



" R. K. Bonnichsen, Pensonal communication cited in R. Hubbard and CI. Wald, 

 Proc. Natl. Acarl. Sci . F. S. 37, 69 (1951); A. F. Bliss, Arch. Biochem. and Biophys. 

 31, 197 (1951). 



■"• R. Hubbard and G. Wald, rnpul)li.shed observations; cited as a i)ersonaI communi- 

 cation in H. Theorell and R. Bonnichsen, Acta Chem. Scand. 6, 329 (1951). 



