1 74 
Vines . — The Proteases of Plants (///). 
reason why leucin, tyrosin, and tryptophane were not found by Mendel and 
Underhill was that sodium fluoride (NaF i °/ o ), which they employed as 
the antiseptic, prejudicially affects the digestive activity of most samples 
of papain in that it retards or inhibits peptolysis but not peptonization. 
I was thus able to confirm the accuracy of Mendel and Underhill’s observa- 
tions, though I showed that they had been somewhat misinterpreted. 
Their observations struck me as being of singular importance ; in fact 
they suggested to me those further investigations into the nature of plant- 
proteases which have led me to the new view that I have already expressed 
with reference to certain special cases (see 5 , p. 1 57), and that I now 
propound more comprehensively and in fuller detail as an advance upon 
the trypsin-theory. 
The actual starting-point of the train of thought was the discovery of 
erepsin, to which I have already alluded (see p. 171). The fact that this 
protease was found to be present in various parts of different plants, 
unassociated with a fibrin-digesting enzyme, suggested the possibility that 
in the cases in which a juice or an extract both peptonized and peptolysed, 
this complete proteolysis might be effected, not by a single * tryptic ’ 
enzyme, as was generally supposed, but by two distinct proteases ; the 
one essentially peptolytic, erepsin in fact ; the other purely fibrin-digesting 
or peptonizing, a pepsin. In a word, my idea was that the assumed 
‘ vegetable trypsin ’ might be a mixture of an ereptic with a peptic enzyme. 
The known facts of digestion could be accounted for equally well on either 
hypothesis : the process and the products of digestion would be the same 
in either case. What was wanted for a decision between the two hypotheses 
was some means of analysing this supposed mixture, so as to separate the 
two proteases, either actually or, at any rate, in their action ; and I seemed 
to have obtained a clue to the latter mode of analysis in the observations 
of Mendel and Underhill as explained by the results of my re-examination 
of them. The behaviour of papain in presence of NaF may be explained, 
on the assumption of a single ‘ tryptic ’ enzyme, by supposing that this 
protease is so acted upon by NaF that its peptolysing activity is inhibited ; 
its peptonizing, fibrin-digesting, activity remaining unimpaired. But it 
may be urged against this supposition, and with considerable force, that 
when a protease is prejudicially affected by any agent or condition, 
presumably all forms of its activity suffer equally. This supposition is, 
in fact, less reasonable than the assumption that two distinct proteases 
are present, and that the arrest of the peptolytic action of papain by NaF 
is due to the inhibition of the peptolytic enzyme (erepsin). This clue 
I have endeavoured to follow up. 
In the first place I sought for other instances of the differential action 
of various antiseptics, but without adequate success ; papain is so far the 
only clear case of the kind that I have observed. I next tried the method 
