COENZYMES 41 



termined by the arrangement of the groupings in the complex 

 molecule. 



Evidence of the protein nature of enzymes rests upon such 

 observations as the following: (1) Many enzymes may be di- 

 gested by other enzymes, as occurs when pepsin in acid solution 

 is brought into contact with trypsin. (2) It has been shown that 

 certain amino acids, such as arginine, aspartic acid, cystine, glu- 

 tamic acid, histidine, lysine, tryptophane, and tyrosine, compose 

 pepsin [Calvery, Herriott, and Northrop (1936)]. 



The first enzyme to be obtained in purified crystalline form 

 was urease, extracted by Sumner in 1926 from jack bean. Since 

 then several others have been obtained in crystalline form, in- 

 cluding pepsin, trypsin, chymotrypsin, papain, catalase, carboxy- 

 polypeptidase, lipase, and the yellow respiratory enzyme of War- 

 burg. Northrop (1935) regards all of these crystalline enzymes 

 as specific proteins. In opposition to this theory it is maintained 

 that these crystalline proteins are not the enzymes themselves 

 but the adsorption compounds of the enzymatic component. In 

 answer Northrop points out that no specific prosthetic group is 

 known for pepsin, urease, trypsin, and carboxypolypeptidase. It 

 seems well established, however, that certain catalytic enzymes 

 are associated with a carrier and that the facts necessitate accept- 

 ance of both viewpoints. The catalytic activity of hematin, for . 

 example, is know T n to increase ten-millionfold when associated 

 with the colloidal carrier, which leads to a question regarding the 

 relative importance of the colloidal carrier and prosthetic group 

 in this type of enzyme. 



Coenzymes. Some enzymes, as has been indicated, are "en- 

 zyme systems," containing an assisting material that also has 

 the power of catalyzing. These are termed coenzymes. Some 

 coenzymes are organic, whereas others are inorganic. In 1905 

 Harden demonstrated that the dialysate of expressed veast juice 

 and the residue are separately inactive but, when combined, are 

 again active. In this instance the dialyzable portion is the co- 

 enzyme. Among coenzymes are cozymase (coenzyme I), co- 

 enzyme II, cocarboxylase (vitamin Bi), riboflavin, nicotinic acid, 

 and glutathione. Other vitamins and hormones have been postu- 

 lated to function as coenzymes; this theory, if valid, may explain 

 the essentiality of specific vitamins in the metabolism of certain 

 fungi. Others are so loosely held as to be able to oscillate between 



