Vines . — Proteolytic Enzyme of Nepenthes (III). 571 
nepenthin, bromelin, and papain (or papayotin), have essen- 
tially the same proteolytic action, which is tryptic : though, 
as I have already pointed out, they seem to differ in activity, 
bromelin being the most active, nepenthin the least. There 
is, however, a further difference between them as regards 
the media in which they are capable of acting. Nepenthin 
is only active in an acid liquid, and digests when as much 
as 0-35 per cent. HC1 has been added. On the other hand, 
bromelin, according to Chittenden, and papain, according to 
Martin, are less active in acid than in neutral liquids ; and 
their action is altogether inhibited when the liquid contains 
about o-i per cent, of free HCL Furthermore, both bromelin 
and papain can digest in liquids which are alkaline to an 
extent not exceeding 1 per cent. Na 2 CO s . Of papain-digestion 
Martin says that it is essentially a neutral one ; and in view 
of Chittenden’s results, the same description may be extended 
to bromelin-digestion. It must, however, be borne in mind 
that both bromelin and papain have to act, in the plants 
in which they respectively occur, in liquid containing organic 
acid. There can be no question as to the acidity of the 
tissues of the Pine- Apple ; and the fresh latex of the Papaw, 
like all other latices (see Molisch, 7), is acid. Moreover, I have 
found by experiment that papain can digest fibrin, to the extent 
of producing tryptophan, in liquids containing up to 0-5 per 
cent, of citric acid. In this respect these enzymes resemble 
animal trypsin, which, though most active in an alkaline liquid 
(1 per cent. Na 2 C0 3 ), can digest proteid in a liquid which is 
neutral or even slightly acidified with an organic acid. 
Including pepsin in the survey, the proteolytic enzymes 
under consideration may be classified as follows, according to 
the reaction of the media in which they act most vigorously, 
so far as is known : — 
a. Active only in acid liquids 
b. Most active in neutral liquids 
c. Most active in alkaline liquids 
Pepsin, 
Nepenthin. 
Bromelin, 
PapaYn. 
Trypsin. 
