ROLE OF REDUCTASE IN TISSUE RESPIRATION 61 



is found to hold. Considering the sources of error in our ex- 

 periments, the values obtained for k are sufficiently constant 

 to warrant our assuming that the decay in the activity of reduc- 

 tase follows the monomolecular or logarithmic law. In the 

 foregoing equation, a represents the initial activity (ioo per 

 cent.) of the enzyme, and x the percentage decrease in activity 

 at the end of time /. Determinations of the reducing power ot 

 cat's liver juice of different ages, carried out at 55, 50, and 40°C, 

 gave the following respective mean values for k, 0*0132, 0*0134, 

 and 0*0121. 



We are now perhaps in a position to summarise the evidence, 

 which has been accumulating to indicate that in living tissues 

 there is a ferment for internal respiration capable of effecting 

 chemical reduction. 



1. The criterion of solubility naturally occurs to one first of 

 all. Reductase is certainly not soluble if by " soluble " we mean 

 capable of entering into pure water. We have, however, found 

 that with difficulty some of it can pass from liver juice into 0*75 

 percent. NaCl, and from disintegrated muscle into glycerine and 

 saline solution; this mixture seems better than either menstruum 

 alone. Reductase is not soluble in the sense that pepsin is 

 soluble ; it leaves its association with the cell proteins with 

 great difficulty, nor will it dialyse away from them. The 

 glycerine and saline extract (muscle), or " solution," did, however, 

 reduce oxyhemoglobin in two or three minutes, while the boiled 

 control had no effect whatever. Neither reagent by itself has 

 any reducing effect. This glycerine and saline muscle extract 

 also reduced soluble Prussian blue. 



All subsequent attempts to isolate the ferment have failed. 

 Indeed, it was through finding the injurious influence of alcohol 

 and ammonium sulphate on the ferment, used with the view of 

 precipitating it, that we were led to study the poisonous effects 

 of other materials. The comparative insolubility of reductase 

 is perfectly intelligible. The role of the ferment is to obtain 

 activated oxygen at the boundary of the cell ; it would not serve 

 the interests of internal respiration if reductase were able to 

 leave the cell and circulate in the blood. It is not a secretion 

 like the exo-enzymes pepsin, ptyalin, etc. ; it is an endo-enzyme 

 as is glycogenase but, unlike glycogenase, it is insoluble, and as 

 such by no means alone in that class. 



2. The next criterion that may be applied is the manner in 



