303 
Vines . — The Proteases of Plants . 
and the possibility that a considerable proportion of these may be attributed 
to the relatively large quantities of dried yeast added, amounting to 30 °/ o 
or more of the weight of the proteid to be digested, they appear to be open 
to the same objection as the others. 
In all the foregoing experiments unboiled fibrin was used, so that 
a possible source of error was present. In order to eliminate this, some 
experiments were made in which the fibrin had been boiled for a few 
minutes. I found that 50 cc. of both a 10 °/ o and a 20 °/ o mixture of yeast 
with toluol-water digested 0-3 grm. fibrin in four days. 
(b) Experiments with aqueous extracts. The foregoing experiments 
with solid yeast afforded no information as to the solubility of the digestive 
protease ; so I had recourse to filtered watery extracts in the first instance. 
It was soon ascertained that active extracts can be obtained under suitable 
conditions. The following experiment proves this, and gives some indica- 
tion of the effect of acidity and of alkalinity : — 
Experiment 1. i2-| grms. of dried yeast were extracted (24 hours) with 250 cc. 
distilled water (yeast = 5 %). 50 cc. of the filtered liquid were put into each of 
4 bottles, with 0-2 grm. fibrin, and treated thus : — to No. 1, nothing further added ; 
to No. 2, Na 2 C 0 3 to 0-5 % ; to No. 3, HC 1 to 0-05 % ; to No. 4, HC 1 to 0-2 %. 
After 21 hours’ digestion in the incubator the fibrin in Nos. 1 and 3 showed 
diminution, in Nos. 2 and 4 it was not affected : 23 hours later it had disappeared in 
Nos. 1 and 3, but remained unaffected in Nos. 2 and 4. 
In the next place I tested the digestive activity of stronger yeast- 
extracts, whether of natural acidity, or alkaline, or with added acid. 
Experiment 2. Extracts of 10 grms. and of 20 grms. dried yeast with 100 cc. 
of toluol-water (1 %) were prepared and filtered. 30 cc. of each extract were put into 
each of 3 bottles with 0-3 grm. fibrin, one bottle of each having nothing added, 
another bottle having Na 2 CO s added to 2 %, and the third bottle HC 1 to 0*5%. 
After 2 7 hours’ digestion in the incubator the condition of the fibrin was — 
Nat. acid. 
Na 2 CO s . 
HCL 
bottles 10 % yeast 
gone 
unaltered 
unaltered 
„ 20 % „ 
21 hours later it was — 
gone 
attacked 
gone 
bottles 10 % yeast 
— 
unaltered 
attacked 
„ 20 
— . 
attacked 
— 
48 hours later the fibrin was still unaltered in the 10 % extract with Na 2 C 0 3 , 
and had not disappeared in either the 10 % extract with HC 1 or the 20 % extract 
with Na 2 C 0 3 . 
Hence it appears that both io°/ o and 20°/ o watery yeast-extracts 
actively digest fibrin, and that, as might be expected, the latter are less 
affected by added acid and alkali than the former. Comparing the results 
of this experiment with those of the corresponding experiment (see p. 302) 
