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‘ Aequiradio-Activity.’ 
(Radio-activity of potassium in relation to its physiological action.) 
IV. Enzymes. 
In the present report, that aspect only of the action of enzymes 
which is related to their colloidal nature will be discussed. 
The fact that enzymes are present in their solutions in the colloidal 
state is shown by their non-diffusibility through parchment paper, 
and is now generally recognized. Taking this fact into consideration, 
together with the form of the relation between the concentration of 
the substrate and enzyme and the velocity of the reaction, the present 
writer (1906) put forward the hypothesis that the chemical decom- 
position of the substrate is preceded and controlled in rate by 
adsorption. Not only, however, are enzymes in the colloidal state, 
but they are able to act in a medium in which they are insoluble and 
present as coarse particles which can be filtered off, leaving the 
filtrate inactive. This has been shown in the case of lipase by 
Dietz (1907), in that of emulsin by Bourquelot et Bridel (1913), by 
myself (1915) for lactase, papain, peroxidase, catalase, urease, and 
invertase, with great probability for trypsin and pepsin. Armstrong, 
Benjamin and Horton (1913) came to the conclusion that urease acts 
by its surface. A similar conclusion may be drawn from the fact 
that invertase may be adsorbed by charcoal and nevertheless remain 
active (Nelson and Griffin, 1916). We may conclude, then, that 
enzymes belong to the class of catalysts present as a separate phase 
and acting in a heterogeneous system. 
The next question that arises concerns the mechanism of their 
action. 
There are two theories of catalysis in heterogeneous systems: 
one of these assumes the formation of an intermediate unstable 
chemical compound between the catalyst and substrate, the reaction 
following the ordinary laws of mass action; the other regards the 
chemical change as being brought about, or rather greatly accelerated 
by close approximation of the reacting substances by condensation on 
the surface of the catalyst by adsorption. An intermediate position 
is taken by those who regard the rate of the reaction as conditioned 
by the amount of reagents present on the surface at any given time, 
but hold that the actual chemical change is due to formation of a 
chemical compound with the substance of the catalyst itself, occurring 
as a second stage of the complete process. This process may be ex- 
pressed as made up, in order, of adsorption, combination with 
enzyme, decomposition of intermediate compound, with enzyme left 
finally as at first. 
As a typical instance of the first view, we may take Van Slyke and 
~Cullen’s (1914) account of the kinetics of urease. These observers 
state that the action consists of two consecutive reactions: (1) com- 
