72 



OXIDATION-REDUCTION POTENTIALS 



TABLE 15 

 Eo Values and Instability Factors of Various Compounds (pH 1-3; 30°C.) 



It is suggested by Heard and Welch (1935) that adrenaline is stabilised in the 

 body by ascorbic acid. 



Noradrenaline has been shown to be the sympathetic nerve impulse transmitter 

 (von Euler, Blaschko, Holton). 



OXYTOCIC HORMONE OF PITUITARY GLAND 



Gulland and Randall (1935) find that the oxytocic principle of the posterior lobe 

 of the pituitary gland is a reversible oxidation-reduction system. 



Oxidation-reduction indicator dyes were reduced in an atmosphere of nitrogen 

 and added to the hormone. Those dyes which reduced the potential to about E^ 

 — 0-llv. had the maximum inactivating effect, bringing the oxytocic activity down 

 to 50 per cent, of its original value. Reduced dyes which reduced the poteatial to 

 levels either above or below —0-llv. had less effect on the activity, and both the least 

 highly reducing dyes and the most highly reducing dyes had no effect at all upon the 

 potency. The curves relating the fall in activity to the electrode potential of the 

 system had the usual S-shape seen in percentage reduction : electrode potential 

 curves. 



Catalytic reduction with hydrogen destroys the activity irreversibly, possibly 

 owing to its intense effect, but electrolytic reduction results in a slow loss of activity 

 some of which is slowly regained on oxygenation. Hydrogen sulphide reduces the 

 activity to some 45-55 per cent, of its original value and oxygenation effects partial 

 reactivation. Sulphur dioxide and hydrogen sulphide, both at pH 3-5, lower the 

 activity, and this change is followed by reactivation, and finally by inactivation as 

 the action of the gas is prolonged. Sodium cyanide and sodium sulphite destroy the 

 activity irreversibly. This effect recalls the reaction of disulphide groups with sodium 

 cyanide : — 



RSSR -f NaCN -^ RSNa + RSCN 



The RSCN compound cannot be reoxidised to RSSR, and, therefore, its formation 

 results in disappearance of disulphide groups. 



