INTRODUCTORY 11 



(from the thyroid) hormones have not only been 

 isolated in the pure state, but they have actually 

 been synthesized in the laboratory from relatively 

 simple compounds. So far, not only have we been 

 unable to synthesize a vitamine, but we have even 

 been baffled in our attempts to isolate one in a pure 

 condition. 



It would, perhaps, be more in harmony with 

 what we know to compare hormones with amino- 

 acids, substances that are obtained when the pro- 

 teins of food are broken up by the enzymes in our 

 digestive system, or by acids used in the chemist's 

 laboratory. 1 Chemically, thyroxin shows striking 

 relations to tryptophane; and adrenaline, certain, 

 though not such striking relations to tyrosine. 

 Both tyrosine and tryptophane are among our best- 

 known amino-acids. 



One important point that is brought out in a 

 discussion of this kind is to emphasize the impor- 

 tance of the "littlest things." Enzymes (or fer- 

 ments), the substances that act on our food in the 

 digestive tract, the compounds that are largely re- 

 sponsible for much of the metabolic activity of 

 every cell, whether plant or animal, have long 

 been known to illustrate the property that certain 

 substances possess, of bringing about chemical 

 changes in a large quantity of material, though the 



'See the chapter on Ammo-Acida in the author's book on 

 Vitamines. 



