46 OXIDATION-REDUCTION POTENTIALS 



The oxidation of carbohydrates appears not to be affected by the presence of 

 glutathione, but the oxidation of proteins apparently occurs in neutral or alkaline 

 solution by a slightly different mechanism, the protein and glutathione being 

 oxidised simultaneously according to Hopkins and his collaborators. 



Protein Denaturation 



In many cases, proteins give tests for the presence of SH groups only when 

 denatured although they contain cysteine in the protein molecule. The reason for 

 this is still under discussion (Hewitt, 1943 ; Anderson, 1945), and arguments have 

 been advanced to contradict the various explanations put forward. The SH group 

 of cysteine present in proteins may be chemically combined and denaturation may 

 liberate the free SH groups. The SH groups may not be chemically combined, 

 but may be rendered inaccessible by the folding of the polypeptide chains in the 

 protein, and the chains are unfolded during denaturation rendering the SH groups 

 accessible. Finally, the SH groups may be inactive in native proteins owing to the 

 binding together of the peptide chains by the hydrogen bonds and the rupture of 

 these bonds on denaturation may increase the reactivity of neighbouring SH groups. 

 Some proteins do not give tests for SH groups even w^hen denatured since the cystine 

 is present in the oxidised form. 



Enzyme Activation 



The enzyme glyoxalase is necessary for the conversion of methyl glyoxal to 

 lactic acid : 



+ glyoxalase 

 CH3.CO.CHO + H2O ^ CH3.CHOH.COOH 



It has been shown that SH groups are necessary for the activation of the enzyme 

 and that iodoacetic acid inhibits lactic acid production by inactivation of SH groups 

 (Lundsgaard, 1930 ; Lohmann, 1932 ; Bersin, 1932). Reduced thiol compounds have 

 been shown to be necessary for the functioning of urease and papain (Hellerman, 

 Perkins and Clark, 1933) and for bacterial hsemolysins (Schwachman, Hellerman and 

 Cohen, 1934). Iodoacetic acid, iodoacetamide and p-chloromercuribenzoate inactivate 

 enzymes by attacking SH groups, but the inactivation of bacterial hsemolysins by 

 dilute cholesterol suspensions is probably due to a different mechanism (Hewitt and 

 Todd, 1930). On the other hand, reduced SH compounds inactivate the hsemolytic 

 activity of diphtheria cultures which require conditions of aeration. (Hewitt, 1947). 



A proteinase of haemolytic streptococci has been shown to require SH activation 

 (S. D. Elliott, 1945.) 



K. A. C. Elliott (1946) gives the following list of enzymes requiring sulphydryl 

 activation : — 



Systems responsible for pyruvate oxidation, dismutation, decarboxylation 

 and acetoacetate and acetylmethyl carbinol formation ; hexosemonophosphate, 

 phosphoglyceraldehyde, glycerol, a-ketoglutaric, succinic, malic, and alcohol 

 (yeast) dehydrogenases ; synthesis of a-ketoglutarate ; adenosinetriphosphatase ; 

 phosphoglucomutase ; phosphorylase ; hexokinase ; pancreatic amylase ; 

 /i-amylase of barley ; stearate, oleate (bacteria) and acetate oxidases ; /8-hydroxy- 

 butyric dehydrogenase ; pancreatic lipase and esterase ; cerebrosidase ; d-amino 



