ELECT ROPHYSIOLOGY 347 



metric method, i.e., by direct electrical measurement, and not 

 through color indicators. 



A significant result of oxidation-reduction potential measure- 

 ments on both cells and cell suspensions is that more negative 

 potentials are found under anaerobic as contrasted with aerobic 

 conditions, demonstrating that the "reducing intensity of the 

 cell" is not poised at a definite level. Cohen suggested the 

 term "reducing intensity" instead of "oxidation-reduction 

 potential," to meet the objections that there is no electric 

 potential as such in tissues and that thermodynamic equilibrium 

 does not occur. He and Chambers found the reducing intensity 

 of the aerobic cell {e.g., Amoeba) to be —0.07 volt (assuming 

 the pH to be 7). 



In addition to knowing whether the values obtained represent 

 true potentials or merely relative rates and whether or not they 

 are of thermodynamic significance, there is the almost insuperable 

 difficulty, as D. E. Green and Szent-Gyorgyi say, of relating the 

 observed potential to the multitudinous factors that determine 

 it. These are the heterogeneity of the cell and the multi- 

 phcity of reversible, semireversible, and irreversible oxidation- 

 reduction systems present; the discrepancies obtained in the 

 values themselves when indicators differing widely in chemical 

 nature are employed, owing to the fact that the state of oxidation- 

 reduction of an indicator is determined not only by factors in 

 the cell but also by the toxicity, poison action (buffering), 

 autoxidizability, and kinetic behavior of the indicator; and the 

 ease with which the values may be shifted by narcotics, pene- 

 trating acids and bases, and changes in the oxygen tension. 



Perhaps the most promising field for further investigation 

 in oxidation-reduction potentials is the isolation and study of 

 oxidation-reduction systems occurring in cells and organisms. 



Various oxidizing enzymes with their substrates, which do not 

 themselves affect electrodes, have been shown to establish 

 oxidation-reduction potentials which can be detected by using 

 reversible dyes. The cytochrome of yeast and other cells is 

 itself a reversible oxidation-reduction indicator, as also are 

 the recently discovered flavines. Various reducing substances 

 found' in Uving tissues, such as ascorbic acid (vitamin C), 

 glutathione, and other sulphhydryl compounds, have probably 

 a considerable influence in determining the oxidation-reduction 



