CHAPTER IV 



SYSTEMS OF SPECIAL BIOLOGICAL INTEREST 



The wide variety of biological processes which are either oxidation-reduction 

 systems or are affected by oxidation-reduction conditions makes a complete discussion 

 of all of them impossible but the main purpose of this chapter is to discuss a number of 

 systems of biological interest and a selection of the components of various metabohc 

 processes. In later chapters the integration and interrelationships of some of these 

 oxidation-reduction reactions will be dealt with as parts of more complete cycles. 

 Despite their complexity a parallehsm and similarity frequently become apparent 

 among materials from widely divergent biological sources. 



SULPHYDRYL SYSTEMS 



Compounds containing a sulphydryl (SH) group are widely distributed in 

 biological systems and are intimately concerned in many vital processes. Sulphydryl 

 compounds give a characteristic colouration with nitroprusside. On oxidation 

 they yield disulphides which, in turn, are reducible to SH compounds : 



2 K.SH ^ K.S.S.R + H2 



A familiar sulphydryl system is that of cysteine, which is oxidised to cystine : 



2 CH2SH.CHNH2.COOH -^ COOH.CHNH2.CH2.S.S.CH2.CHNH2.COOH -f H, 



2 cysteine -> cystine 



Hopkins (1921, 1929) isolated a tripeptide containing cysteine from yeast and 

 many tissues. This tripeptide, glutathione, differs from cystine in that the oxidised 

 form as well as the reduced sulphydryl compound is soluble in water and hence 

 can continue to take part in oxidation-reduction processes when oxidised. 



Apart from the important part sulphydryl compounds play in enzyme activation, 

 which will be dealt with in a later paragraph, they possess direct oxygen carrying 

 activities themselves. Thus the " thermostable residue " of muscle and the oxidised 

 form of glutathione are separately each incapable of reducing dyes such as methylene 

 blue. When mixed, however, oxidised glutathione is reduced by muscle " ther- 

 mostable residue " : — 



G.S.S.G. -f TH2^2 G.SH + T 



oxidised glutathione + thermostable residue = reduced glutathione! 



The reduced glutathione reduces methylene blue to the colourless form, and in the 

 same way a mixture of glutathione and " thermostable residue " will absorb oxygen 

 by the following mechanism which continues until all the hydrogen donating power of 

 the " residue " is exhausted : — 



4 G.SH + 02 = 2 G.S.S.G + 2 HgO 



2 G.S.S.G + 2 TH2 = 4 G.SH + 2 T 



4 G.SH + O2 = 2 G.S.S.G + 2 H2O 

 and so on. It is unknown to what extent these reactions take place in vivo. 



In acid solution fats may be oxidised in the presence of oxygen, the fat taking the 

 place of " thermostable residue " in the above scheme. 



