336 



DIGESTION 



substrate into the end-products, the enzyme will clearly act as a cata- 

 lyser of the reaction. It has been shown that in the case of certain 

 inorganic catalysers this does occur. Thus, in the oxidation of hydriodic 

 acid by hydrogen peroxide, which has been already referred to, 

 molybdic acid has the power of acting as a catalyser. It has been proved 

 that the reaction occurs in two stages, permolybdic acid being first 

 formed by the action of the peroxide on molybdic acid. The permol- 

 ybdic acid then acts on hydriodic acid, producing iodine and water, 

 and being itself reduced again to molybdic acid, which therefore comes 

 out at the end of the reaction unchanged. The velocity of the double 

 reaction is much greater than that of the direct oxidation of hydriodic 

 acid by hydrogen peroxide. 



There is evidence that the ferment actually combines with the sub- 

 strate, the combination then breaking up to form the end-products. 

 For instance, it has been shown that the amount of lactose hydrolysed 

 by lactase in a given time, when the ferment is present in very small 

 quantity in comparison with the substrate, is proportional to the con- 

 centration of the ferment, and independent of the concentration of 

 the lactose. Also with a given small concentration of ferment the 

 amount of lactose hydrolysed is at first the same for successive equal 

 intervals of time. These facts can only be explained by the assump- 

 tion that the ferment first combines with a portion of the substrate, the 

 rest of which remains inactive as regards the reaction, and that this 

 combination then takes up water and decomposes into the end- 

 products, in this case dextrose and galactose, setting free the ferment 

 to combine with another portion of the substrate. 



The Quantitative Estimation of Ferment Action. Since we have as yet 

 no certain method of freeing the digestive ferments from impurities, 

 our only quantitative test is their digestive activity. And since a very 

 small quantity of ferment can act upon a practically indefinite amount 

 of material if allowed sufficient time, we can only make comparisons 

 when the time of digestion and all other conditions are the same. If 

 we find that a given quantity of one gastric extract, acting on a given 

 weight of fibrin, dissolves it in half the time required by an equal 

 amount of another gastric extract, or dissolves twice as much of it in a 

 given time, we conclude that the digestive activity of the pepsin is twice 

 as great in the first extract as in the second. But this does not permit 

 us to say that the one contains twice as much pepsin as the other. For 

 it has been found that the amount of digestion in a given time is not 

 directly proportional to the quantity of ferment present, but to the 

 square root of the quantity of ferment (Schutz's law). This law was 

 deduced by Schutz for pepsin, but is said to hold also for trypsin, 

 steapsin, and ptyalin (Pawlow, Vernon). To determine the amount of 

 proteolysis the nitrogen of the protein which has gone into solution may 

 be estimated (p. 514). The following table shows the results of one 

 experiment : 



