THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. I 153 



is quite useless for this purpose, but platinum black or so-called colloidal 

 platinum is, on the other hand, exceedingly active. Bredig (1901) has obtained 

 such colloidal solutions by electric spraying. Platinum and other metals may 

 be broken up by kathodic spraying under water into particles so fine that they 

 remain in suspension in the water, and cannot be seen even with the best micro- 

 scope ; such a fluid we term a colloidal solution. Bredig has made a number 

 of experiments with such solutions, which are of extreme interest to the phy- 

 siologist. He was able, in the first place, to estimate more exactly than pre- 

 viously the amount of platinum necessary, and showed what was the minimum 

 amount which could act catalytically on hydrogen-peroxide, viz. 0-000003 n^g- 

 From his further researches the catalytic effect of certain substances may be 

 estimated which are known to be inhibitory. Minute quantities of sulphuretted 

 hydrogen, bisulphide of carbon, hydrocyanic acid, &c., destroy the catalytic 

 power of the platinum solution, but they do not do so permanently ; after 

 removal of the substance the catalysis begins anew. 



Bredig finds a strong likeness to exist between enzymes and such colloidal 

 metal solutions, so much so, indeed, that he terms the latter ' inorganic en- 

 zymes '. This likeness is expressed in their colloidal form, their mode of action, 

 and the effect produced upon them by the substances named. We must 

 leave as a debatable question whether we may correctly designate the sus- 

 pension of platinum particles in water as a colloid corresponding to organic 

 substances capable of swelling and confine ourselves to the consideration of 

 the other two points of comparison. In addition to the catalytic action of 

 apparently all of them on hydrogen-peroxide ' enzymes have over and above 

 specific effects on definite substances, but these specific effects do not appear 

 to us to be as yet proved to be possessed by a colloidal solution of platinum 

 (compare Zeitschr. f. phys. Chem. 31, 262, note). The specific hydrolytic 

 action of the enzyme cannot have anything to do with the decomposition of 

 hydrogen-peroxide ; the one action may be differentiated from the other by 

 heating to a certain temperature (Jacobson, 1892). Pancreatic secretion 

 for example after heating to a temperature of 61° C. can still transform starch 

 into sugar, but it can no longer abstract oxygen from peroxide of hydrogen. 

 It is very obvious that by this means we have separated out a substance 

 which, in its behaviour it is true, shows a strong resemblance to colloidal 

 platinum, and that the actual enzyme remains uninjured. We arrive at similar 

 conclusions on a closer analysis of the action of the poisons mentioned above ; 

 of these, hydrocyanic acid is especially a case in point, because it acts poison- 

 ously on enzymes in the same way as it does on colloidal platinum. In reality, 

 the hydrocyanic acid affects only the activity of the impure enzyme on per- 

 oxide of hydrogen, and leaves the specific action of the enzyme quite intact 

 (Jacobson, 1892). Further differences between Bredig's * inorganic enzymes ' 

 and organic enzymes may perhaps come out on a closer study of the effect 

 of temperature on the reactions. At present, at all events, it is by no means 

 certain whether the so-called inorganic enzymes show a temperature curve 

 with minimum, optimum, and maximum points, but if this be considered of 

 secondary importance we have, at least, one other difference of greater weight, 

 i. e. the close of the reaction. The platinum solution remains active as long as 

 a trace of peroxide is present ; in other words, the reaction is complete ; in the 

 case of enzymes, however, the reaction (see p. 152) is incomplete unless the pro- 

 ducts be withdrawn. It would not be out of place here to go into the question 

 of the incompleteness of the enzyme reaction, but it is possible in the present 

 condition of the science only to draw attention to the diametrically opposite views 

 advanced on the question, and on which no decision has as yet been reached. 



The reason for the incompleteness of the reaction generally lies in this, that 

 it does not consist of one reaction only, but of two processes, which induce 

 opposite changes and which lead to an equilibrium at a definite temperature. 



