GENERAL CONCLUSIONS 167 



The more or less qualitative results on the reduction of dyes by tissues, yeast 

 cells, bacteria, etc., have been generally corrected, extended and placed on a quantita- 

 tive basis by the investigations of many workers on biological electrode potentials. 



Many constituents of living organisms when isolated have been shown to be 

 subject to reversible oxidation-reduction reactions. In a large number this reversible 

 oxidation-reduction is essentially related to their vital function. Many biological 

 pigments are known to constitute oxidation-reduction systems and their electrode 

 potential relationships have been investigated. Respiratory functions have been 

 attributed to some of these pigments. 



A number of vitamins of the B-complex and ascorbic acid constitute oxidation- 

 reduction systems. It becomes increasingly difficult to distinguish between vitamins, 

 enzymes, coenzymes, hormones and even toxins. All appear to function as biological 

 catalysts and increasing numbers have been purified and crystalUsed. Their composi- 

 tion and mode of action have been made clearer and synthesis of some of them is 

 within sight. Chemical methods of synthesis still appear crude, however, compared 

 with the refined methods of the cell. Antibiotics such as penicilhn and streptomycin 

 are still produced commercially by the organisms themselves, but Chloromycetin, one 

 of the simpler antibiotics, is now a product of chemical synthesis. 



As Michaelis has pointed out many of the enzymes and co-enzymes of significance 

 in cellular respiration constitute two-stage oxidation-reduction systems, and it is 

 possible that this type of reaction is essential for many biological enzymic oxidations. 



The vitamins are by no means alone in their dependence upon oxidation- 

 reduction potentials, and some of the hormones, including adrenaline and the 

 oxytocic hormone of the posterior pituitary gland, may also be oxidised and reduced. 



In the latter case disulphide or sulphydryl groups appear to play an important 

 part, and in this it resembles certain of the enzymes, namely, papain and urease, 

 which function only in the presence of the reduced group — SH. 



In their turn, these enzymes resemble two bacterial products, pneumococcus 

 hgemolysin and one of the haemolysins of haemolytic streptococci, which are also 

 inactivated by oxidation, etc. Study of the reduced streptolysin has led to interesting 

 results in cases of streptococcal infection. 



Whether the interior of a cell can be said to possess a characteristic electrode 

 potential is a matter of some controversy. This, however, is an internal matter 

 which primarily concerns the cell ; a matter of more immediate interest is the study 

 of the oxidation-reduction changes effected by the cell in its environmental mediimi 

 and of the effect of different environmental conditions upon the behaviour of the 

 cell. These investigations are in a very healthy state of development, and progress 

 is being maintained, despite the difficulties inherent in the study of living organisms. 

 For exact physico-chemical measurements the investigator would prefer to deal with 

 a pure compound of known chemical constitution under fixed conditions, precluding 

 the possibility of any change during the measurement. In practice the biochemist 

 frequently can arrange for none of these ideals to be attained, and, to quote W. M. 

 Clark (1934), the investigator by necessity " makes painfully exact measurements 

 upon frightfully impure compounds." In a living organism and all its complexities we 

 have the very antithesis of a pure chemical compound and it would be difficult for a 

 chemist to prepare or even imagine a more heterogeneous and complicated mixture 



