The comparative resistance to pepsin of six naturally occurring trypsin inhibitors.

The wide-spread distribution of trypsin inhibitors throughout the vegetable and animal kingdoms (1) makes elucidation of their physiological function of great interest. From this point of view, resistance to peptic digestion becomes an important property, since only an inhibitor resistant to pepsin is likely to produce any physiological effect when ingested by a normal animal. The soy bean inhibitor is the only inhibitor which has been studied in detail. Kunitz (2, 3) investigated the digestion of native and heat-denatured soy bean trypsin inhibitor by 0.01 per cent pepsin at pH 3 and pH 2 at 30”. The denatured inhibitor was rapidly digested at both pH values. The native inhibitor remained totally active after 7 hours exposure to pepsin at pH 3, whereas, at pH 2, it, was digested at, an almost constant initial rate, about 40 per cent of the inhibiting activity being lost after 150 minutes incubation. While rate measurements have not been made with other trypsin inhibitors, Fraenkel-Conrat et al. (4) incubated solutions of inhibitor preparations with 0.0025 per cent pepsin in 0.01 N HCl at 40” for 24 hours, and found that the amorphous Lima bean inhibitor retained 93 per cent of the original activity, soy bean inhibitor 1 per cent, and ovomucoid 2 per cent. Incubation with acid alone gave values of 100, 55, and 100 per cent activity retained, respectively. The present. paper describes a study of the resistance to peptic digestion of six inhibitors, four of which have not been investigated before.