322 CHEMISTR Y OF THE DIGESTIVE PROCESSES. 



in glass was due to accumulation in the solution of digested products. Similar 

 experiments on the digestion of various forms of proteid, by pepsin and hydro- 

 chloric acid, dialysing into hydrochloric acid of equal concentration, have been 

 made by Chittenden and Amerman, 1 who found that removal of the products 

 of digestion did not essentially favour peptonisation or alter the relative 

 amount of albumose and peptone formed. 



Concentration of enzyme. The rapidity with which zymolysis takes 

 place naturally varies with the concentration of the enzyme in the 

 solution, as well as with the concentration of the material to be digested, 

 when this is soluble. Eoberts found in the case of conversion of starch 

 by the diastatic enzyme of the pancreas, that the amount of standard 

 starch mucilage which can be converte<J in a given time and at a given 

 temperature varies directly as the quantity of active solution employed. 



Schiitz 2 found in the digestion of proteid by pepsin, that when the 

 solutions employed were sufficiently dilute, the amount of conversion 

 was proportional to the square roots of the quantities of pepsin present. 

 Any such rule can only hold within certain limits of concentration, a 

 maximum being reached beyond which further concentration of the 

 enzyme has no effect. 



Methods of estimating the relative activity of digestive solu- 

 tions. As none of the enzymes have been isolated in a pure condition, 

 it follows that there is no means of estimating the absolute amount of an 

 enzyme in solution. This is practically never a matter of any moment, 

 but a problem which often presents itself in practical work on digestion 

 is that of estimating the relative activities of two digestive extracts. 



The activity of a diastatic enzyme can be most accurately estimated 

 by determining the amount of sugar (maltose) formed under given con- 

 ditions in a given time by a given volume of the solution, acting on a 

 measured volume of a standard solution of starch mucilage ; this, however, 

 is a tedious and troublesome process, and for most purposes a sufficiently 

 accurate process is that of observing when the starch has all disappeared, 

 as shown by the failure of the iodine reaction. 



Such a method has been introduced by Roberts. 3 He varies the amount 

 of the diastatic solution added until the " achromic point " is reached within a 

 period lying between the limits of four and six minutes. This achromic point 

 is that point at which the starch solution ceases to give a yellow tinge with 

 iodine, when accordingly the solution contains only achroodextrins and maltose. 

 Roberts defines the diastatic value of a solution (denoted by the symbol D) 

 by the volume in cubic centimetres of a standard starch mucilage which can 

 be converted to the achromic point by 1 c.c. of that solution, acting during five 

 minutes at a temperature of 40 C. 



The standard solution of starch mucilage must be prepared fresh ; it is 

 made by stirring up 5 grms. of pure potato starch with 30 c.c. of water, and 

 pouring slowly into nearly 470 c.c. of water, which is kept boiling. The 

 mixture is stirred and boiled for a few seconds, and finally accurately made up 

 to 500 c.c., thus giving a standard solution (1 per cent.) of starch. 



The solution of iodine used is made by diluting 1 part of the liq. iodi of 

 the Pharm. Brit, with 200 parts of water. 



In making a determination, one proceeds as follows : Ten c.c. of the 

 standard starch mucilage are diluted with distilled water to 100 c.c. and 



1 Journ. Physiol., Cambridge and London, 1893, vol. xiv. p. 483. 



2 Ztschr. f. physiol. Chem., Strassburg, 1885, Bd. ix. S. 577. 



3 Diastasimetry, In "Digestion and Diet," London, 1891, p. 68. 



