586 PRINCIPLES OF GENERAL PHYSIOLOGY 



peroxide on hydriodic acid. The direct action of the substrate on the enzyme, 

 chemically destructive or by alteration of its physical properties, has always to !>< 

 taken into account in these specific relations. 



It may be asked, what is the function of the gum in our artificial laccase? 

 Before we can answer this question, we must examine into the state of the 

 metallic salt in solution, whether it be iron, copper, or manganese. 



Salts of all these metals, especially in -the dilute solutions in question, are 

 hydrolysed in water. In other words, the solutions used consist of hydroxides ' >t 

 the metals in the colloidal state. This seems to be their active state, although it 

 is not easy to say why the colloidal hydroxide should be more active than the ion. 

 The experiments of Moore and Webster (1913) on the formation of formaldehyde 

 by ultra-violet light, in the presence of colloidal ferric hydroxide, may be called to 

 mind. Bertrand also (1897) tested the oxidising effect of a series of manganese 

 salts on hydroquinone and found those to be the most powerful which were the 

 most hydrolysed in solution. 



If, then, the colloidal state is of so much importance, it seems clear that the 

 activity must be in direct relationship to the extent of the surface. Hence the use 

 of gum, albumin, and so on. The way these "stable" colloids act in protecting a 

 suspensoid colloid, such as ferric hydroxide, from precipitation by electrolytes, thus 

 ensuring a high degree of dispersion, has been explained above (page 97). 



A peroxidase is, then, in all probability, a peculiarly active form of the 

 colloidal hydroxide of manganese, iron, or copper, preserved in this active state 

 by the presence of an emulsoid colloid, such as gum or albumin. It is to this 

 stable colloid that the enzyme owes its precipitation by heat or by alcohol, and, 

 possibly, any degree of specificity that it possesses. A view essentially the same 

 as this was suggested by Pen-in (1905, p. 103). 



ENZYMES CONCERNED WITH REDUCTION 



Although it has long been known that fresh animal and plant tissues have 

 the power of reducing nitrates to nitrites, and it was held by some that the 

 process is a catalytic one, the existence of reducing enzymes, analogous to the 

 oxidising ones, has not been generally accepted. 



Schardinger (1902), however, made the observation that fresh milk rapidly 

 reduces methylene blue, indigo, and so on, if an aldehyde, such as formic or acetic 

 aldehyde, be present ; whereas it has no such action in the absence of the aldehyde. 

 The property is abolished by boiling, and has been used as a test to distinguish 

 fresh from sterilised milk. It was clearly proved by Trommsdorff (1909) that 

 this reaction is due to an enzyme and not to bacteria. If microbes are present, 

 the milk, after some hours, acquires the property of reducing methylene blue 

 without the addition of an aldehyde. 



Now it is clear that we must have a formation of nascent or active hydrogen, 

 and that it must come from the water in the system. If a reaction is going on 

 which takes up oxygen from water, hydrogen will Ije set free. It is probable, 

 then, that we have to do with a reaction of the kind called by Bach (1913, p. 150) 

 " hydrolytic oxidative-reducing reactions." 



A reaction of this kind has been already described (page 266) in the oxidation of a-amino-acids 

 to aldehydes, as discovered by Strecker. To understand the mechanism, however, a simpler 

 system is better and we may take that of the decomposition of water by hypophosphites in 

 the presence of metallic palladium, as invested by Bach (1909). 



To begin with, it must be admitted that the mechanism of such reactions is by no means 

 clear as yet and I confess to a certain amount of misgiving as to the chemical nature of the 

 intermediate compounds supposed to be formed. The system is a heterogeneous one, palladium 

 and similar metals being insoluble, and the phenomena of adsorption or surface condensation, 

 always present in such systems, should be kept in mind. In the following description when 

 oxides of platinum, etc., are spoken of, it is very doubtful whether they really have t In- 

 definite chemical formula? assigned to them, since they have not been isolatd. Similarly, it 

 may be remembered that the permolybdic acids formed in Brode's typical case (page 324) are 

 said to be a series of this kind : 



MoO :! .l/2H 2 O 2 , MoO :t .H. 2 O.,,MoO :i .3/2H. 2 Oo, MoO 3 .2HoO,- 



