INTERMEDIATE COMPOUNDS 399 



of observers. Thus Vernon (145) has shown that serum albumin, 

 paraglobulin and particularly egg-albumin, when added to a 

 tryptic digest, markedly delay the hydrolysis of another protein 

 by the trypsin. Obviously, if the enzyme did not enter into 

 combination with its substrates, the rate at which it attacks 

 one substrate should not be influenced by the presence of another. 

 Hedin (46) has shown that the power of egg-albumin to delay the 

 hydrolysis of another protein by trypsin is much greater if the 

 egg-albumin and the trypsin are mixed together before the trypsin 

 is added to the digest than after. Evidently the egg-albumin- 

 trypsin compound is but slowly reversible; when none of the 

 trypsin is bound by another substrate the active mass of free 

 trypsin is greater and the proportion bound by albumin is there- 

 fore greater. Once the trypsin is bound by albumin it is only very 

 slowly abstracted from the combination by another substrate.* 



Dauwe (23) has shown that trypsin can be extracted from its 

 solution by coagulated egg-white or by fibrin, and the ferment 

 can be regained from the compound by prolonged washing with 

 water. From what has been said in Chap. V, section 3, it will be 

 clearly realized that this latter fact does not in the least militate 

 against the view that the combination between the ferment and 

 the substrate is chemical in character. 



A very striking instance of the way in which the formation 

 of compounds of this type may interfere with the time- and mass- 

 relations in the main hydrolysis under observation has been 

 discovered by Hedin (49). When trypsin acts upon casein, the 

 time required for the attainment of a given degree of hydrolysis 

 (= is inversely proportional to the concentration of ferment 

 (= p). In other words pt = constant. But, if egg-albumin be 

 present, a proportion of the trypsin is bound by the albumin 

 and this proportion is greater the more dilute the ferment, so that 

 the quantity of ferment which is free to act upon the trypsin 

 is no longer proportionate to its concentration and the law no 

 longer holds good. From this it is clear that if we wish to obtain 

 readily interpretable time- and mass-relations in a protein digest 

 we must employ only pure proteins. We have seen, also, that we 



* Hedin has also shown that a greater proportion of trypsin is bound at 

 higher than at lower temperatures. But if excess of trypsin is attached to the 

 albumin by temporarily raising the temperature, on lowering the temperature 

 the trypsin is not given up again. 



