Trypsin Inhibitors of Human Serum. I. Standardization. Mechanism of Reaction, and Normal Values1

Abstract

The inhibition of crystalline trypsin has been measured, using casein as a substrate for trypsin, according to the method of Kunitz. Doubling the concentration of casein, to give a final concentration of 1% in the digestion mixture, gave a rate which was constant over the period of incubation, and a linear relation between the formation of products and the enzyme concentration. Because the activity of trypsin decreases during incubation at pH 7.6, as employed in measuring the combination with inhibitor; and because of uncertainty regarding the amount of inactive material present in the trypsin preparations, measurements have been referred to the amount of soy bean inhibitor reacting with trypsin under the same conditions. Measurements made in the presence of added Ca++ or ethylenediamine-tetracetate could be harmonized in this way. Assuming that 1 mg of soybean inhibitor should inhibit 1.2 mg of trypsin, 1 ml of normal serum inhibited 1.03 + 0.13 mg of trypsin. The reaction was reversible.