1 7 
Vines . — The. Proteases of Plants (VI). 
any rate one carnivorous plant, Nepenthes. This is the only one of the 
carnivorous plants which has been adequately investigated from the point 
of view of digestive activity ; but it is not improbable that the facts 
ascertained with regard to this plant will be found again when the other 
carnivorous plants are examined. 
The pitcher-liquid of Nepenthes is, under normal conditions, a clear 
colourless or yellowish liquid, either neutral or acid in reaction, containing 
very little organic matter and not more than i per cent, of mineral matter. 
It is, therefore, a fairly pure aqueous solution of the protease. The 
characteristic feature of its digestive activity upon fibrin and other complex 
proteins is that acid reaction is absolutely essential : neutral liquid has no 
action ; digestion is rapid at natural acidity, and also in the presence of 
added acid, whether organic (citric acid) or mineral (HC 1 0*3 per cent.). It 
is especially in its relation to HC 1 that ectopeptase differs essentially from 
endopeptase : for the activity of the latter, when in pure solution (thus 
corresponding nearly to the pitcher-liquid) is arrested by the presence of as 
little as 0*05 per cent. HC1. 
In considering the relation of the peptases to the acidity of the medium, 
it may be pointed out that there is some evidence (observations of Weis and 
others on Malt, quoted in No. 17, p. 291, of the list of references) to prove 
that the acid reaction which is always given by extracts of parts of plants, 
is due, at any rate in Malt, and probably also in other seeds, to the presence, 
not of free acid, but of acid phosphates. It is in an acid medium of this 
kind that endopeptase seems to be most active. Ectopeptase, on the 
contrary, is most active in the presence of free acid ; this is well established 
in the case of Nepenthes. 
It is impossible to conclude this discussion of the nature of the pro- 
teolytic enzymes of plants without some reference to those of animals. 
Certain analogies are obvious. The enzyme that I have termed £ ecto- 
peptase ’ agrees in all essential properties with animal pepsin ; this agree- 
ment is particularly interesting because it justifies the original conclusion 
(see p. 10) that the excretions of carnivorous plants contain pepsin. The 
only modification of that original view that is necessary is that these 
excretions (at least that of Nepenthes) contain ereptase in addition. Then, 
again, the ereptase of plants differs from that of animals only in that its 
reaction-range is more extensive in the direction of acidity, and is, perhaps, 
less extensive in the direction of alkalinity. 
But it is not so easy to find an animal analogue for ‘ endopeptase ’. 
It does not correspond to trypsin, because that substance is held to be 
an enzyme that both peptonizes and peptolyses ; it would, however, 
correspond fairly well with the peptonizing factor in trypsin, if that were 
regarded as separable ; and it may prove to be separable after all. 
C 
