280 THE MAINTENANCE OF THE INDIVIDUAL 



and oxidation of this simple sugar produces over 100 different sub- 

 stances. The living cell yields only a few of these, and then in a 

 regular succession. Such remarkable specificity and speed of reaction 

 in the living cell is largely due to the action of enzymes which have 

 the property of accelerating some particular reaction. As was 

 pointed out previously (p. 128), enzymes may be regarded as catalysts 

 because they are not expended and primarily serve to speed up a 

 reaction. 



While the properties of particular enzymes will be discussed in some 

 detail as they are encountered later, certain of their general char- 

 acteristics as determined by the biochemist will be briefly mentioned. 

 In this connection it is interesting to note that six enzymes have been 

 prepared in crystalline form, and all are proteins. While this evidence 

 is not conclusive it suggests the probable chemical nature of a con- 

 siderable number of these activators. Most enzymes have what the 

 chemist calls a reversible reaction and so may be capable of serving 

 as a catalyst for both hydrolyses and syntheses. However, it should 

 not be forgotten that under some conditions an action may be practi- 

 cally irreversible. Such is the case with glucose which, although 

 theoretically capable of reacting in several different ways, continues 

 to react in one direction because of the presence and concentration 

 of a particular enzyme. Nearly all enzymes appear to have an opti- 

 mum working temperature of about 40° C. (104° F.). Furthermore, 

 enzymes appear to be specialized, at least to the extent of requiring 

 a definite acidity or alkalinity of the surrounding medium. One 

 classic example is the pepsin of the stomach, which reacts only in an 

 acid environment. 



Many enzymes seem to have the common function of splitting com- 

 plex molecules into simpler ones, a process usually accomplished 

 through the addition of water, or hydrol3^sis. Enzymes acting in 

 this manner may be described as hydrolytic, the term being formed 

 by adding the suffix lytic to the Greek stem for water. The enzymes 

 themselves are designated by adding the ending ase to the name of 

 the substance upon which each acts, as, for example, maltase or lipase, 

 signifying, respectively, action upon maltose or the lipins (fats). 

 Such activators may be spoken of collectively as hydrolases since 

 they act through the addition of water. Similarly the catalyzing 

 enzymes for oxidations and reductions are spoken of collectively as 

 oxidases. A few other enzymes do not fall into either of the above 

 categories. 



