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ENZYMES. 51
Like the colloids the enzymes only diffuse very slowly and the dif-
fusion through membranes does not occur in most cases; only certain
membranes such as collodion tubes allow certain enzymes to pass
through. The collodion tubes can be impregnated in such a way with
lecithin or cholesterin that the diffusion is very slight. The same
applies to the filtration through collodion membranes (Bierry and
S( iiaeffer). 1
It must not be forgotten in such experiments that the
membrane can adsorb a considerable part of the enzyme (Bechhold).2
Just as it is difficult to prepare an enzyme free from non-enzymotic
contaminations, so also is it difficult to exclude the possibility that a
so-called enzyme is not a mixture of several related enzymes. In fact
the several enzymotic processes proceed step by step, and it is possible
that the various steps are caused by different enzymes. Thus the
decomposition of protein into amino-acids, with proteoses, peptones,
and polypeptides as intermediary products, may be the result of the
activity of several enzymes which are active one after another or are
parallel with one another in activity. Erepsin does not attack genuine
proteins, but completes the decomposition which has been begun by
other enzymes (pepsin, trypsin).
The enzymes are formed within the living cells. In certain cases
the cells do not secrete the complete enzyme, but substances which are
transformed first outside of the cells into active enzymes. These pre-
liminary steps or mother substances of the enzymes have been called
proenzymes or zymogens. These under certain conditions are changed
into enzymes and in certain cases this is brought about by the inter-
action of special but not well known substances which have been called
kinases (see Chapters V and VIII). In other cases the transformation
of the zymogen into the active enzyme is brought about by well defined
chemical substances. Thus the proenzymes of pepsin and of rennin are
activated by acids (see below on the retardation of enzyme action and
also Chapter VIII).
In certain other cases the presence of bodies which resist temperature
and are dialyzable and therefore not enzymes, are necessary or helpful
besides the real organic enzyme. Thus the presence of an acid is neces-
sary for the action of pepsin and hydrocyanic acid, according to Mendel
and Blood,3
favors to a high degree the action of papain (a plant pro-
tease). R. Magnus 4
has been able to separate by dialysis, from a solu-
tion of liver-lipase, a body which is necessary for the action upon amyl
^ompt. rend. soc. biol., 62, 723 (1907).
2
Zeitschr. f. physik. Chem., 60, 257 (1907).
3
Journ. of biol. Chem., 8, 177 (1910).
4
Zeitschr. f. physiol. Chem., 42, 149 (1904).
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