278 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



an enzyme which breaks down pyruvic acid. If this vitamin is 

 deficient in the diet, the oxidase cannot function and pyruvic acid 

 accumulates in the tissues, a change which can be used in the clinical 

 diagnosis of vitamin Bi deficiency. If the vitamin is supplied in the 

 diet, the pyruvic acid soon disappears. 



Several other vitamins have now been identified as parts of the 

 structure of different co-enzymes or of the nonprotein part of en- 

 zymes, but not all vitamins have yet been associated with specific 

 enzyme systems and it is probable that some may function in other 

 ways. 



A LITTLE GOES A LONG WAY -yj 



A little enzyme goes a long way — for instance, one part of rennin 

 is capable of clotting 10 million times its weight of milk — so that 

 the absolute amounts of enzymes required for the smooth running 

 of the body may be quite small, and only small amounts of vitamins 

 will be required for building new enzymes and co-enzymes to make 

 good the small but continuous loss through general wear and tear. 

 The body cannot make the vitamins itself and without them some 

 enzyme systems cannot function. The requirements of vitamins are 

 thus small but essential, and if not met, the machinery of the whole 

 body may get disorganized and death may eventually take place. 



The substance that is transformed in the presence of an enzyme 

 is called the substrate. Some enzymes can bring about changes with 

 a large range of different but related compounds; some lipases, for 

 example, will bring about the splitting of many different fats. Other 

 enzymes, such as urease, may be so selective that they will transform 

 only a single kind of substrate. It is believed that in general before 

 an enzyme can bring about a reaction, a complex must first be formed 

 between the enzyme and the substrate. This theory is based largely 

 upon mathematical analysis of the shapes of reaction-time curves, 

 but in a few cases, direct experimental evidence to support it has been 

 obtained. The enzyme peroxidase brings about the oxidation of 

 certain compounds by hydrogen peroxide. A sharp change in color 

 of peroxidase occurs when hydrogen peroxide is added to it indicating 

 the formation of a complex; if a suitable substrate is now added, 

 oxidation takes place, the hydrogen peroxide is used up, and the 

 original color of the peroxidase reappears. It is generally accepted 

 also that this combination with the substrate can only take place at a 

 special point in the enzyme structure, called the active center. Suc- 

 cessful formation of a complex may depend on the degree to which 

 the shapes of the active center and substrate are complementary. 

 Emil Fischer used the analogy of a lock and key to illustrate this 

 point. Many substances exist in two forms whose molecular struc- 

 tures are mirror images of each other (rather like a pair of gloves). 



