I. EXPERIMENTAL EVIDENCE CONCERNING THE EXISTENCE OF AN INTERMEDIATE COMPOUND.

John H. Northrop ¹

¹ From the Laboratories of The Rockefeller Institute for Medical Research.

1. The rate of hydrolysis of a casein solution by trypsin is not affected by the addition of gelatin. The trypsin, therefore, is not combined with the gelatin unless there is a separate enzyme for casein and for gelatin.

2. The presence of casein protects the gelatin-splitting power of trypsin from heat inactivation, and the presence of gelatin protects the casein-splitting power from heat inactivation.

3. It does not seem possible to account for both the above results by the assumption of an intermediate compound between enzyme and substrate, since, in order to account for the first result, a different enzyme must be assumed for each protein, while, to account for the second result, it must be assumed that the same enzyme attacks both.

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