The kinetics of trypsin digestion.

The present study of a typical proteolytic enzyme extends the quantitative investigations previously reported from this laboratory on the effects of pressure on protein systems in viva (1) and in vitro (2). This paper presents new data on the self-digestion of trypsin and the digestion of casein by trypsin; a forthcoming paper will discuss the effects of temperature and hydrostatic pressure on these processes. This particular system was chosen because of classical interest, because of the commercial availability of crystalline trypsin and purified casein, and also because some investigations of the kinetics of digestion of casein by trypsin are already to be found in the literature (3). As is frequently the case, the rate law suggested by the interpretation of the data has a complex integrated form not easily tested by the usual methods. A simple graphical technique (which we have called the slidefit technique) has proved useful in handling kinetic data in general (Powell, unpublished data). With this technique it is quite as easy to test the agreement of data with complex rate laws as with the simple classical rate laws. Section I of this paper presents an analysis of trypsin self-digestion data. The rate of this process was studied to find out whether it might proceed rapidly enough to introduce any significant error into the studies of casein digestion. The data, while not intended for kinetic analysis, are presented as a simple illustration of the slide-fit technique and because the kinetic process, second order digestion complicated by reversible inhibition of the enzyme by its digestion products, appears completely parallel to the casein digestion. Section II presents new data covering a wide range of the essential variables of casein-trypsin digestion and interprets them similarly in terms of known properties of the components.