SULPHYDRYL SYSTEMS 47 



acid oxidase ; mono-amine oxidase ; 1-glutamic dehydrogenase ; transaminase ; 

 urease ; cathepsins ; papain ; bromelin ; hsemolysins ; lysozyme ; choline oxidase ; 

 cholinesterase ; and choline acetylase. 



The activity of the following enzymes is thought not to depend upon free 

 sylphydryl groups : — 



Cytochrome oxidase ; catalase ; heart flavoprotein ; alcohol (liver), lactic and 

 isocitric dehydrogenases ; uricase ; diamine oxidase ; polyphenol oxidase ; oleic 

 oxidase (peanuts) ; carbonic anhydrase ; acid phosphatase ; arginase ; pepsin ; 

 and trypsin. 



p-Chloromercuribenzoate has a strong affinity for SH groups and combines with 

 them in low concentrations even when they are masked by oxidising agents. The 

 combination is specific for SH groups and no other group present in proteins and it is 

 reversed by addition of excess of simple mercaptans. This is not the case with certain 

 vesicants which will be dealt with in more detail in the chapter on chemotherapy. 

 When enzyme systems are inactivated by arsenic compounds the inhibition is, however, 

 reversed by dithiols which form a stable five-membered ring with the arsenic as shown 

 below, 



CHj-SH CH-S. 



I -hOAsR \ I >AsR 



CH-SH ^ CH-S-^ 



CH2OH CH2OH 



Electrode Potentials 



The electrode potentials established in sulphydryl systems do not yield to the 

 conventional theoretical treatment accorded to perfectly reversible systems. In 

 the case of the equilibrium of the truly reversible system :— 



2 R.SH -> R.SS.R + Ha 



the fundamental electrode equation defines the potential obtained in such a system, 

 at constant pH, as : — 



ET [RSH£ 

 K-K- 9F^^[RSSR] 



but in practice it is found that the potential observed is dependent only on the 

 concentration of the reduced form [RSH] and is independent of the concentration 

 of the oxidised form [RSSR]. Several explanations of this phenomenon have been 

 advanced. It was originally suggested that the real reversible equilibrium is not 

 between RSSR and RSH but between a hypothetical, intermediate oxidation product 

 say RS, and RSH. 



Thus : RSH ^ RS + H 



RS, directly it is formed, is transformed irreversibly into RSSR, so that significant 

 quantities of RS are never present (Dixon and Quastel, 1923). There are many 

 objections to this explanation. One is that RS concentration, although small, 

 must be proportional to RSSR concentration, so that the concentration of RSSR 

 ought to appear in the equation. 



