634 PHYSIOLOGICAL CHEMISTRY. 



peptides, ammo-acids) obtained by proteolysis. They are successively 

 simpler, more soluble, and more diffusible as the proteolysis proceeds. 

 The constitution of many of these simpler decomposition products has 

 now been well substantiated (amino-acids). It has been found pos- 

 sible to make a beginning in the synthesis of protein by forming com- 

 binations of amino-acids, linking the carboxyl group of one to the 

 amino group of another, with the elimination of a molecule of water. 

 This procedure can be repeated, and substances have been formed 

 containing thirty-six or more amino-acid molecules. These substances 

 are apparently entirely analogous to the peptides derived from the 

 proteins, as some of them give a biuret test, are acted upon by prote- 

 olytic enzymes (erepsin), and have other common properties. It is 

 probable, therefore, that this grouping is one of the many forms of 

 combination that must be present in the protein molecule. While all 

 of these amino-acids have been found in protein, they are not neces- 

 sarily all present in any one protein, and in different proteins they 

 are present in markedly different proportions. Experimental work 

 is beginning only now to show the significance of these differences 

 in the proteins. The molecular weights of the proteins cannot be 

 determined by any known methods, and the actual structural consti- 

 tution is entirely unknown. 



As leucine and tyrosine are readily isolated (see Pancreatic Diges- 

 tion), their more important properties are stated here. 



Tyrosine, C 6 H 4 .OH.C 2 H 3 (NH 2 )CO 2 H (Para-oxyphenyl-amino-pro- 

 pionic acid). This is obtained from all proteins, except collagen and 

 reticulin, by trypsin digestion, by prolonged boiling with dilute acids 

 or alkalies, by fusion with alkaline hydroxides, and by putrefaction. 

 Tyrosine is generally found with leucine ; both these substances have 

 nearly the same physiological properties and pathological significance. 

 They occur in the intestine during the digestion of proteins, and, patho- 

 logically, they are found in atheromatous cysts, in pus, in abscess and 

 gangrene of the lung, in the urine during yellow atrophy of the liver, 

 and in phosphorus poisoning. 



Tyrosine crystallizes in colorless, fine, silky needles, often tufted 

 (Fig. 81). It is very slightly soluble in water, more so in the pres- 

 ence of alkalies and mineral acids, insoluble in alcohol and ether. 

 (For a method of preparing tyrosine and leucine from proteins, see 

 Pancreatic Digestion.) 



