1916] on Chemical Change in Living Organisms 641 



position to explain all the facts met with in the action of enzjmes 

 themselves, bnt it is remarkable how many receive a satisfactory 

 account. We are at once confronted by the difficulty of the consider- 

 able number of different enzymes. But we must not for^^et that 

 adsorption is controlled by a great number of factors in addition to 

 mechanical surface tension. All those properties which suffer 

 modification at phase boundaries play their part— electrical charge, 

 solubility, compressibility, even chemical reaction itself, may be men- 

 tioned. Moreover, as Hardy has pointed out, the act of condensation 

 in itself may well be accompanied by the manifestation of molecular 

 forces which result in increased chemical potential of the reacting 

 substances. It is clear that experimental decision of the questions 

 involved is almost impossible until we have in our hands pure pre- 

 parations of enzymes. We cannot as yet exclude the possibility of 

 the formation of intermediate chemical compounds between enzyme 

 and substrate, but their existence has not been demonstrated, and 

 what I may venture to call Faraday's view has the advantage of 

 simplicity, and thus the support of William of Occam's " razor." 



The important question of the synthetic action of enzymes 

 demands a little attention at this point. All reactions may be 

 regarded as being, in principle, reversible or balanced, and the 

 greater part of those of the living organism are found experimentally 

 to be so. If we take for consideration those enzymes the action of 

 which consists in the addition or removal of the elements of water, 

 we find that, as would be expected from the law of mass action, the 

 position of equilibrium in the presence of a large excess of water is 

 very near to that of complete hydrolysis, and this is the state of 

 affairs in the usual laboratory experiments. On the other hand, the 

 less water is present, the greater is the preponderance of the opposite — 

 synthetic— aspect. Take the classical case of ethyl acetate. If the 

 ester and water are mixed in molecular proportions, hydrolysis to 

 acid and alcohol occurs until two-thirds of the ester are decomposed. 

 Moreover, the same final composition is obtained if we commence 

 with acid and alcohol, and so work in the other direction. But 

 these reactions proceed by themselves with extreme slowness, taking 

 months before coming to an end. But the presence of a catalyst, 

 uch as mineral acid, brings about equilibrium in an hour or so, and 

 we notice that it is the same as the spontaneous one. An enzyme, 

 known as lipase, also brings about equilibrium rapidly. The important 

 point in respect of tlie mechanism of living cells is that by changing 

 the available amount of water, the reaction may be made to proceed 

 in either direction at will. The series of curves given by Armstrong 

 and Gosney (Proc. Roy. Soc, 88B, p. 176) show this fact very 

 clearly. Further, if the equihbrium is brought about rapidly, even 

 if to any position except that of complete change in one or the other 

 direction, the enzyme must accelerate both reactions, and any hypothesis 

 of special " svnthesising " enzymes is superfluous. This is essentially 

 Vol. XXI. (Xo. 110) 2 u 



