146 



OXIDATION-REDUCTION POTENTIALS 



It will be of interest to determine whether this analogue will inhibit the thyro- 

 trophic hormone from the anterior pituitary gland (Hewitt, 1929) or the direct action 

 of thyroxine. The value of such a preparation in controlling hyperthyroidism 

 •clinically is evident. 



Thiourea and thiouracil derivatives have been used with considerable success 

 for the treatment of hyperthyroidism, despite some toxic effects. The mechanism of 

 the effect is still obscure ; iodine excretion rapidly follows administration of the 

 drug, but it is not certain that the effect is directly on the thyroxine-producing cells. 

 It has been suggested that one or other of the following enzyme systems may be 

 inhibited by administration of the drug and the enzyme may be concerned in thyrox- 

 ine synthesis : peroxidase, cytochrome oxidase, phosphatase, tyrosinase. It is 

 difficult, however, to know why such enzymes in the thyroid tissue should be inhibited 

 without killing the patient. It has also been suggested by De Robertis that the 

 oxidation-reduction potential of the follicular cells of normal thyroids is higher than 

 that of colloid tissue and that activation of the gland increases the potential of the 

 colloid tissue. Thiourea is said to reduce the potential of both the cytoplasm and 

 colloid of activated glands. 



Some examples of inhibitory structural analogues are given in the table. 



Not all antiseptics or chemotherapeutic agents necessarily owe their effect to 

 being structural analogues of metabolites or coenzymes, but since they generally owe 



