316 A MANUAL OF PHYSIOLOGY 



temperature, although more rapidly at 100 C. The effect of the 

 acid is to catalyse the reaction, to markedly accelerate it. The 

 hydrogen ions of the free acid appear to be responsible for the 

 catalysis. The same action upon cane-sugar is exerted by an 

 enzyme, invertase, found in intestinal juice, although the laws 

 governing the reaction are somewhat different. And it is 

 probable that there is no fundamental difference between the 

 action of the digestive enzymes and that of the inorganic 

 catalysers. 



Not even the markedly specific action of the digestive ferments can 

 be considered an essential distinction. It is true that invertase will 

 act upon dextrose, and not at all upon maltose or lactase. But there 

 are other sugars, e.g., raffinose, a trisaccharide with the formula 

 C le H 32 O 10 , obtained from beet-sugar residues, which it will hydrolyse. 

 Rafnnose is made up of one molecule each of dextrose, levulose, and 

 galactose. On heating with dilute acids, it is decomposed into 

 these substances. Invertase, however, only splits off the levulose 

 molecule, leaving a disaccharide isomeric, but not identical with 

 lactose. Similarly lactase, which is without action upon cane-sugar 

 or maltose, will hydrolyse the /3-galactosides, and maltase, inert as 

 regards cane-sugar or lactose, will hydrolyse the a-gmcosides. On 

 the other hand, emulsin decomposes the /3-glucosides, to which 

 group most of the natural glucosides belong, as well as the /3-galacto- 

 sides and lactose. From rafnnose emulsin splits off galactose, 

 leaving cane-sugar. Since the a and p compounds are isomeric, and 

 differ not in their composition but in their structure, it has been 

 concluded that the structure of the molecule of a substance must be 

 related to the structure of the enzyme which can act on it, in some 

 such way as a lock is related to its proper key. Thus the key lactase 

 fits in the lock lactose, but not in the lock dextrose or the lock mal- 

 tose. 



As to the manner in which an enzyme increases the velocity of its 

 appropriate reaction, it is not easy to make any very positive state- 

 ment. Several possibilities are recognised, of which two have been 

 especially discussed, (i) The existence of the enzyme in colloidal 

 solution may be important. It is characteristic of colloidal solutions, 

 in which the dissolved substance is present in the form of extremely 

 fine particles, that the total surface of the particles is very great 

 in proportion to the mass of the substance in solution. Thus, a 

 sphere of about the same volume as the eyeball, with a diameter of, 

 say, 2 centimetres, would have a surface of 12-5 square centimetres. 

 If this material were subdivided into spheres of about the same 

 volume as a leucocyte, with a diameter of, say, 10 ^, it would form 

 eight thousand million of these spheres, with a total surface of over 

 2% square metres. If the small spheres were further subdivided into 

 spherical particles, with a diameter only the thousandth part of that 



of a leucocyte, sav ~~ ~ each would form a thousand million of these 

 J J 100 



particles, and the total surface of all the particles would be about 

 2,500 square metres. 



Now, it is known that the intensity of action of some of the in- 

 organic catalysers is proportional to the surface exposed. For 

 example, hydrogen peroxide, if left to itself, is slowly decomposed into 

 water and oxygen. The addition of finely divided platinum, in the 



