ENZYMES — KILBY 277 



zymes have not yet been successful, but this is not a serious handicap to 

 the biochemist, as he can obtain a great deal of information about the 

 properties of an enzyme without having the pure material. A solu- 

 tion or suspension containing a particular enzyme can often be made 

 quite readily from a suitable source and used to investigate what 

 changes the enzyme can bring about, how it is affected by heat, different 

 acidities, poisons, and so on. The biochemist has a wide range of 

 biological material from which to select a convenient and rich source 

 of the particular enzyme which interests him ; typical examples of the 

 materials employed are red blood cells, meal worms, mushrooms, bac- 

 teria, pigeon breast muscle, rat liver, beef pancreas, and horseradish. 

 All the enzymes that have been obtained pure and crystalline have 

 shown the properties of proteins, and it would appear that each pure 

 enzyme is a quite definite chemical compound, a protein with charac- 

 teristic and constant properties. Some enzymes have a purely protein 

 structure, but others have, in addition, some relatively simple unit of a 

 different nature built into the structure as an essential part. 



COENZYMES 



Other enzymes can function only if certain compounds, called co- 

 enzymes, are also present. If yeast juice, for example, is placed in a 

 cellophane bag and washed in a current of water, the co-enzyme passes 

 through the cellophane because it has a small molecule and is washed 

 away, while the large protein enzymes are left in the bag. This resid- 

 ual juice will be found to have lost its power to cause fermentation. If 

 another sample of yeast juice is boiled, the enzymes are destroyed but 

 not the heat-stable co-enzyme. This solution is also inactive, but if 

 the two preparations are mixed, then the combination shows biological 

 activity again as both enzyme and co-enzyme are present. 



A slightly different co-enzyme has been found in red blood cells, 

 and in 1935 both co-enzymes were shown to contain the substance 

 nicotinamide built into their structures. Almost exactly at the 

 same time, Elvehjem and his associates discovered that nicotin- 

 amide was the vitamin present in diets that would prevent and cure 

 human pellagra. Thus a memorable link-up occurred between two 

 of the major lines of biochemical study — of vitamins and nutrition 

 on the one hand and of enzymology on the other — to their mutual 

 advantage. It became possible to ascribe a definite biochemical func- 

 tion to a vitamin, and new light could be thrown on possible chemi- 

 cal mechanisms of enzyme systems by a study of the chemical prop- 

 erties of the vitamins. 



Another example of a vitamin associated with an enzyme is afforded 

 by vitamin Bi, or aneurin, which prevents beri-beri. In combination 

 with phosphoric acid this vitamin acts as the co-enzyme for an oxidase, 



