THE SEAT AND NATURE OF THE OXIDATIONS 511 



In the second place, it should be remembered that a substance 

 may be very closely allied to one of the known oxidizable bodies, and 

 still fail completely in being oxidized by the tissues. Thus, it has been 

 found that only four of the sixteen sugars, possessing the formula 

 C 6 Hi 2 O6, namely, glucose, fructose, galactose and mannose, are 

 acted upon by the cells, while the others cannot be utilized. In the 

 third place, a tissue may lose its power of reducing certain foodstuffs 

 completely, a condition met with in diabetes mellitus. Consequently, 

 the cell must possess a certain chemicophysical constitution which 

 becomes completely disarranged in the course of certain diseases 

 with the result that formerly assimilable substances are rendered non- 

 assimilable. It is evident, therefore, that the general arrangement of 

 the intracellular material constitutes the principal factor in the de- 

 termination of the manner in which the dysoxidizable foodstuffs 

 combine with the oxygen. On this account, there is imparted to the 

 oxidations a definite specificity and a limit is set to them in conformity 

 with the requirements of the different tissues. Consequently, the 

 magnitude of the oxidation is regulated by the tissue itself and not by 

 the amount of oxygen actually available. Thus, inhalations of pure 

 oxygen cannot augment the oxidations, because the tissues are already 

 acting at their fullest capacity. The oxygen which is required for 

 these processes may be furnished either in a free or bound state. 

 In the latter case, it is in combination with some of the nutritive sub- 

 stances. As bound oxygen must also be regarded the oxygen of water 

 which, on account of its wide distribution, must play a most important 

 part in biological oxidations. The latter are commonly designated 

 as hydrolytic oxidations. 



As slow combustions are the rule in living matter, the energy which 

 is required to instigate these processes must be furnished by the sub- 

 stances to be oxidized. The latter, therefore, must possess the power 

 of activating the molecular oxygen, and hence, the real purpose of 

 respiration is to allow the mechanism of the activation of oxygen to 

 be set in motion. Unfortunately, however, the nature of this process 

 is not clearly understood, although several theories have been formu- 

 lated to serve as possible explanations. 1 



The theories regarding the activation of, oxygen may be divided 

 into two groups, namely: those which assume that the oxygen is first 

 of all split into an active modification and those which hold that the 

 molecules of oxygen are used in their complete form. Among the 

 former may be mentioned: 



1. The ozone-autozone theory of Schonbein and Clausius which assumes that 

 the inert oxygen appears in the form of two different and active modifications. 



2. The ionization theory of van't Hoff which holds that the modifications of 

 the oxygen are not chemically different but only carry different electrical charges. 



1 A more detailed account will be found in Oppenheimer's Handbuch der 

 Biochemie, Jena, 1913, or in Mathews, Physiol. Chemistry, New York, 1915. Also 

 see Engler and Weissberg, Krit. Studien iiber die Vorg. der Autoxydation, Braun- 

 schweig, 1904. 



