160 OXIDATION-REDUCTION POTENTIALS 



The experiments were conducted in the presence of perchloric acid in order to 

 keep the mercury compound in solution. 



When a platinum micro-electrode was used instead of the mercury dropping 

 electrode, the curve obtained indicated irreversible oxidation of cysteine to cystine. 



At the cathode reduction of cystine appeared to depend upon favourable 

 orientation when adsorbed on the mercury droplets of the electrode, and to be 

 affected by surface active substances such as thymol and camphor. 



An interesting development of the behaviour of sulphydryl compoimds at the 

 polarographic electrode is found in the behaviour of proteins. 



SULPHYDRYL-CONTAINING PROTEINS 



In ammoniacal protein solutions containing a trace of a cobalt salt a characteristic 

 catalytic cathode wave was observed by Heyrovsky, Brdicka and their colleagues. 

 Sulphur containing proteins cause a double hump on the voltammetric curve obtained 

 at the cathode and the magnitude of the deflection suggests a catalytic effect of 

 the protein. The effect can be used for the quantitative determination of traces 

 of protein and has even been suggested for the diagnosis of cancer. 



The determination of SH groups in proteins by amperometric titration is 

 described by Benesch and Benesch (1948). The protein, or denatured protein, solution 

 is titrated with dilute silver nitrate. The galvanometer deflection obtained at a 

 rotating platinum electrode (the cell being completed by a salt bridge and mercury : 

 mercury iodide electrode) remains constant whilst silver mercaptide is precipitated, 

 but increases immediately excess silver nitrate is present. When all or part of the 

 SH groups are combined with p-chloromercuribenzoate the titratable SH groups 

 show a corresponding decrease. The results in the case of egg albumin were in good 

 agreement with previous amino acid analyses, but were lower in the case of serum 

 albumin. It is in line with common experience that the SH groups in serum albumin 

 are more " masked " and less " available " even after denaturation than in the case of 

 egg albumin. 



Cysteine and cystine give a single catalytic wave in ammoniacal solutions 

 containing divalent cobalt or nickel. The magnitude of the wave is much greater 

 than can be accounted for by the reduction of cystine to cysteine and appears to be 

 catalytic in nature. It is suggested that cysteine forms a complex with divalent 

 cobalt and the co-ordination bond with the sulphydryl group weakens the linkage 

 with hydrogen which is deposited at the dropping electrode. Some dithiols behave 

 similarly (Danielli et al, 1947). 



These waves have been used to determine cystine in protein hydrolysates and 

 as little as 0-001 g. of human hair is sufficient for an analysis. Some other sulphur 

 compounds have been found to behave similarly. It appears to be necessary to 

 hydrolyse glutathione and mustard gas before they react in this way. 



Apphcations of these phenomena have been the determination of protein in 

 insulin and in cerebro-spinal fluid, and in the diagnosis of cancer but other inflamma- 

 tory conditions are indistinguishable by the methods used. 



ENZYMES 



The catalytic waves of cytochrome C and other enzymes have been studied by 

 Carruthers (1947). The cytochrome is dissolved in a solution of pH 9-35 containing 



