A QUANTITATIVE METHOD FOR THE ESTIMATION OF PEPSIN

The quantitative estimation of ferments is beset by many difficulties which do not obtain in the case of substances which can be directly weighed or measured. Since, in the present state of our knowledge, ferments can only be recognized through their effects, and since these effects vary greatly with many conditions, it has been difficult to develop standard methods which possess reasonable accuracy and at the same time are not too cumbersome. The effects of ferment action vary with the time over which the reaction proceeds, the concentration of the substrate and the con-centration of the enzyme as well as with other factors. Most of the methods have been based on the principle of varying one factor while others are kept constant, and the strength of the enzyme is usually expressed in terms of the degree to which the substrate has been effected. The details of the many procedures which have been devised may be read in Oppenheimer's elaborate treatise (1) or in the briefer compendium of Waksman and Davison (2). With regard to pepsin the usual procedure has been to allow decreasing quantities of ferment (gastric juice) to act on a uniform amount of substrate (such as a soluble protein) over a standard length of time and to determine the smallest amount of ferment which completely digests (as digestion may be defined) the unit of substrate. Thus, if 0.01 cc. of ferment completely digests the unit of substrate it is assumed that 1 cc. of ferment has the power of digesting 100 units. As various workers have shown, however, this assumption is entirely erroneous (3) since activity does not vary in direct proportion to the concentration of ferment. Methods based on this general plan must therefore be discarded. Northrup and Hussey recently have pointed out (4) that the time required to cause a given percentage change in the viscosit