Pancreatic trypsin inhibitor. 2. Reaction with trypsin

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Abstract
The combination of pancreatic trypsin inhibitor with trypsin was investigated. The dissociation constant of the inhibitor-trypsin compound was determined, using hemoglobin substrate. The degree of inhibition of trypsin was unaffected by 4-fold variation of substrate concn. A method was developed for detn. of the extent of reaction in an inhibitor-trypsin mixture. The velocity constants of the reaction in the presence and absence of substrate were determined. The inhibitor-trypsin combination was considerably retarded by the presence of urea. The presence of denatured trypsin was without effect on the reaction. At pH''s 4 and 12.4 the degree of combination after 1 min. was negligible, at pH 3 no measurable reaction was found to occur even after several hrs., while at pH''s 10.5 and 7 combination was almost complete in 1 min. An explanation of the slow rate of reaction and its pH-dependence is advanced in terms of ionic combination, considering that both proteins have alkaline isoelectric points.