THE IMPORTANCE OF "IMPURITIES" AND TRACE SUBSTANCES 73 



active in many living things, we must envisage the possibility that 

 vitamins and hormones may, directly or indirectly, affect one 

 another. 



An interesting example of hormonal interrelationships appears from 

 recent research on the thyroid gland and the iodine catalysts of the 

 body, which are continually being assembled and released, and when 

 broken down are salvaged and reconstituted for re-use, as follows: 



(1) Within the gland a catalyst mechanism sets free or secretes a 

 mass of large, non-diffusible particles of thyroglobulin, forming visible 

 masses known as "colloid." 



(2) The colloid specifically adsorbs and stores iodine in the form of 

 iodide, diiodotyrosine, and thyroxine, and gradually releases these 

 catalysts into the circulation for service in such cells as may take 

 them up. 



(3) The thyroid-stimulating hormone of the anterior pituitary (TSP) 

 stimulates colloid formation, conversion of diiodotyrosine into thyroxin, 

 and release of the latter into the circulation. TSP is of protein nature. 



(4) Iodine-containing material restored to the circulation from the 

 cells is recaptured by the colloid and returned to the iodine cycle, apart 

 from small amounts excreted in bile, urine, etc. The actual amount of 

 iodine in circulation at any time is normally very small. 



(5) Thiouracil and similar goiter-forming substances inhibit absorp- 

 tion of iodine and its conversion into diiodotyrosine and thyroxin. 

 The latter process appears to occur in two steps: (a) oxidative libera- 

 tion of iodine to combine with tyrosyl radicles; (b) coupling of two 

 diiodotyrosyl groups to form thyroxin residues. 



In addition to serving either directly as catalysts, or as carriers 

 or prosthetic groups in enzymes, trace substances may affect the 

 permeability of membranes (septa), through which the raw mate- 

 rials and reaction products of the catalysts must diffuse, and thus 

 influence reaction velocities. Working through the endocrine, 

 nervous, or circulatory systems, such stimuli, though physically 

 small, may trigger off vitally important reactions. 



