48 GENERAL PHYSIOLOGY OF BACTERIA 
bohydrates or fats, to simpler compounds which are assimilable by 
the bacteria. Walker^ has shown that certain bacterial proteolytic 
enzymes may be precipitated in an active state by azine and azonium 
dyes. It is very probable that the exo-enzymes work uneconomic- 
ally in the sense that they transform more material than the organisms 
require: the phenomenon is exhibited in the extensive liquefaction of 
gelatin by proteolytic bacteria, as B. proteus. The organism which 
elaborates such an exo-enzyme probably derives but little energy 
from its activity, and, conversely, probably expends comparatively 
little energy in the elaboration and secretion of the exo-enzyme. 
Endo-enzymes.— Comparatively little is known of the endo-enzymes : 
it is generally believed that they are comparatively non-diffusible, at 
least in an active state, and that they are non- or but slightly filter- 
able. This suggests that they are relatively complex in their mole- 
cular aggregation. Their function is probably to act upon the nutrient 
substances which the cell has assimilated, partly to liberate energy 
from them, and partly to participate in the organization of the cell 
constituents. These endo-enzymes work economically in contra- 
distinction to the exo-enzymes in the sense that the substrate is 
apparently changed by them in proportion to the requirements of 
the cell. Endo-enzymes may be obtained from bacterial cells when 
the latter disintegrate, provided the rupture of the cells is not accom- 
plished by violent chemical means. Probably the phenomena of 
autolysis which many bacteria exhibit when they are placed in an 
environment free from food may be due, in part at least, to the auto- 
digestion of the organisms by their endo-enzymes. 
Classification of Enzymes.^— Enzymes are usually classified according 
to the substrate they act upon: thus, proteolytic enzymes, or pro- 
teases, split proteins or protein derivatives into simpler compounds; 
carbohydrolytic enzymes split starches or polysaccharides into simpler 
carbohydrates; fat-splitting enzymes, lipases, split fats into glycerin 
and fatty acids. The above enzymes are hydrolytic in character, 
that is, they effect cleavage of protein or carbohydrate or fat or of 
glucosides by splitting the molecule into simpler molecules which 
simultaneously take up hydrogen and oxygen in the proportions to 
form water, thus: 
1. 
CH2NH2CO— NHCH2COOH + H2O (+enzyme) = CH2NH2COOH + CH2NH2COOH. 
Glycyl-glycine. Glycine. Glycine. 
2. 
C12H22O11 + H2O ( +lactase) = CeHisOe + C6H12O6 
Lactose. Glucose. Galactose. 
3. 
CH2O— CO— CH3 CH2OH 
I I 
CHO— CO— CH3 + 3H2O ( +lipase) = CHOH + SCHjCOOH 
I I Acetic acid. 
CH2O— CO— CH3 CH2OH 
Triacetin. Glycerin. 
1 Proc. Soc. Exp. Biol, and Med., 1927, 24, 839. 
2 See Waksman and Davison: Enzymes, 1926, 
