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the amount produced being thereby much increased. In this instance, the catalytic effect may well depend upon the inter- mediate formation of a protein chloramine.

It is desirable here to call attention to certain peculiarities in the behaviour of enzymes which merit consideration in view of their action being that of particular agents. When submitted to the action of the enzyme urease best used in the form of an extract of the soya bean according to the amount of enzyme used, urea, for example, is rapidly hydrolysed, at a diminishing rate as the action proceeds. Contrasting the effect on a solution of moderate strength with that on a concentrated solution, it is noteworthy that the amount changed is considerably less in the latter: thus in an experiment in which gramme-molecular (6%) and five-gramme molecular (30%) solutions were contrasted, at the end of 10 hours, the ratio of the amounts of acid required to neutralize the ammonia formed was as 55 in the case of the stronger to 132 in that of the weaker solution.

Proof that the diminished activity of the enzyme is the con- sequence of the increase in the concentration is given by the observation that, if methylurea be, in part, substituted for urea, the amount of the latter hydrolysed is less than in the absence of the methylurea. Methylurea is not in the least affected by the enzyme, this being strictly selective in its action, attacking only urea, none of its derivatives.

Special reference is made to these observations as showing that in the case of catalysts generally the conditions at the sur- face cannot be considered independently of those in the medium. It is, however, to be noted that, even in simple solutions, in the case of interactions taking place under the influence only of determinants in the absence of a catalyst, as denned in this article the rate of change is not proportional to the concentra- tion. This is seen at a glance on reference to the accompanying graph (fig. 3) representing results obtained on hydrolysing cane- sugar with nitric acid, the sugar being the only variable. Such variations are certainly due to reciprocal variations in solvent and solute as the concentration is changed.

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0-5 10 IS 20 2-5

FIG. 3. Molecular Proportions of Sugar.

It is known that absorbents take up relatively more of a sub- stance from dilute than from concentrated solutions. That the condition of " water " at a surface differs from that in the main body of the liquid seems also to follow from the observation that wet paper does not stiffen until the temperature is reduced to o- 1 and that the water in a clay sphere does not freeze until 0-7. The observation made by Adrian Brown and Tinkler, that when barley corns are steeped in a 50% solution of acetic acid, the absorbed liquid ultimately in equilibrium with that outside the corn contains 80 % of the acid, would seem to show that the " water " of the thin film distributed over the surfaces of the starch granules is more active than ordinary water. Sub- stances so diverse and different from acetic acid as aniline and phenol behave in a similar way, accumulating in the capillary spaces. In the enzymes which act on carbohydrates, not only is the rate of change diminished by foreign substances generally but those which resemble the hydrolyte in structure exercise a retarding influence far in excess of neutral materials. Thus the hydrolysis of the glucosides, whether a or |3, is specially retarded by glucose, but not nearly to the same extent by the isomeric galactose. If the argument advanced above that the enzyme must fit the hydrolyte be accepted, it is obvious that a substance which could also be fitted upon the enzyme will neces- sarily interfere more with its activity than would a substance vhose interference would be merely mechanical by getting in

the way or that of a solute modifying the osmotic condition. A special interference with enzymes and with other catalysts which function chemically, not merely as surface condensing agents, may arise through the neutralization of the functioning radicle; hence perhaps the great influence of acids and alkalies. The accompanying graph illustrates the behaviour of urease

100

20

80

100

120

40 60

FIG. 4. Behaviour of Urease under Action of Enzyme.

when subjected to the action of the enzyme alone or in presence of either both or one or other of the products of change. Hydrol- ysis is retarded by the weakly alkaline mixed product of change. Taking the products separately, the more strongly alkaline product ammonia has a still greater retarding influence; on passing carbon dioxide into the solution, however, so that it is present in excess, the action of the ammonia is held more strongly in check and the action is greatly accelerated.

In the case of urea, under the influence of the enzyme, the interaction is complete there is no reaction or reversal. This is theoretically wrong. Cane-sugar behaves similarly. In other cases, an equilibrium point is reached and the enzyme will act reversibly in a solution if it be sufficiently concentrated of the products of change, reforming the hydrolyte.

Thus a and |3 methyl glucosides are resolved into methylic alcohol and glucose by the enzymes maltase and emulsin re- spectively; the resolution appears to be complete in dilute solu- tions but is less and less so the more concentrated the solution; and if a mixture of methylic alcohol and glucose in water be submitted to the action of either enzyme, the appropriate glu- coside is reproduced in proportion to the concentration.

The behaviour of a fatty oil (olive oil) in presence of the en- zyme lipase affords a particularly striking illustration of the manner in which change in the two possible but opposite direc- tions is balanced as the conditions are varied. On reference to the accompanying graph it will be seen that as the amount of water present is increased the amount of fat hydrolysed is in- creased; as the fat and the fatty acid are insoluble, it is to be supposed that the water acts by diluting the glycerol and it will be noted that, if glycerol be added, the extent to which hydrol- ysis takes place is diminished.

The reason why urea and cane-sugar are not reproducible from the final products of change by the respective enzymes is not clear; it is not improbable that the final are not the initial prod- ucts and that the initial products have but an ephemeral exist- ence in solution: some link in the chain of change is lost by the occurrence of an action outside the range of the enzyme.

Urea is known to undergo change reversibly in solution into ammonic cyanate: CON 2 H 4 t^NH 4 NCO. The proportion of cyanate present at ordinary temperatures is known to be very small; it is slightly increased by boiling the solution; if silver nitrate be added, which serves to fix the cyanate as insoluble silver cyanate, an almost complete conversion can be effected.