THE CHEMISTRY OF THE DIGESTIVE JUICES 34 1 



or even destroyed, at temperatures which very likely would favour 

 their action were they as thermostable as the majority of catalysing 

 agents. And inorganic catalysts are known which also show the 

 phenomenon of an optimum temperature depending on changes 

 produced in their physical condition when the temperature is 

 raised above this point. Thus a colloidal solution (or ' sol/ as it is 

 called) of platinum, prepared by passing electric sparks between 

 two platinum electrodes immersed in distilled water, and containing 

 the metal in the form of ultra-microscopic particles, acts as a cata- 

 lyser of a number of reactions. As the temperature is increased 

 up to a certain ' optimum,' the velocity of the catalysed reaction 

 is increased. But beyond this, as the boiling-point is approached, 

 the colloidal platinum is precipitated, and ceases to influence the 

 reaction. 



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

 appropriate reaction, it is not easy to make any very positive statement. 

 Several possibilities are recognized, 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 tbe same volume as a leucocyte, with 

 a diameter of, say, 10 p, it would form eight thousand million of these 

 spheres, with a total surface of over i\ square metres. If the small 

 spheres were further subdivided into spherical particles, with a diameter 



only the thousandth part of that of a leucocyte, say -^-, each would 



form a thousand million of these 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 inorganic 

 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 v platinum, in the form of 

 platinum black, greatly hastens the decomposition, and the oxygen 

 bubbles off. The colloidal platinum sol is still more effective. The 

 nature of the surface effect is not entirely clear. One factor has been 

 thought to be an increase in the concentration of dissolved substances 

 or condensation of gases at the surface, and the better opportunity 

 for mutual action thus afforded to the ferment and the substrate. 

 The great extension of the surface cannot be the only factor in the 

 catalysis; otherwise any fine powder or suspension would have a cata- 

 lytic action. But kaolin, or fine sand, or colloidal solutions of ordinary 

 proteins or gelatin, have little, if any, effect on the decomposition of 

 hydrogen peroxide. 



(2) Enzymes may produce their effects by contributing to the for- 

 mation of bodies intermediate between the substrate and the end- 

 products. If the time required for the formation of a given quantity 

 of the intermediate compound and the time required for the decom- 

 position of this compound into the final products of the ferment action 

 are in sum less than the time required for the direct change of the 



