PHOTODYNAMIC ACTION 701 



sure to light, but the concentration required for this "dark action" is 

 generally much higher than is needed if the system is irradiated. That 

 photodynamic action is not merely an enhancement of this dark reaction 

 is apparent from the demonstration (Blum and McBride, 1931) that the 

 latter does not require molecular oxygen. The influence of this dark 

 action must be ehminated (by choice of suitable concentrations) or 

 allowed for in experiments on photodynamic action. 



Inhibiting Factors. Since photodynamic action is an oxidative proc- 

 ess, it is subject to inhibition by reducing agents, an effect well illus- 

 trated by Blum (1937) in experiments with sodium sulfite and thiosulfate. 

 Inhibition by substances that combine with or precipitate the dye and 

 thus prevent it from reaching the substrate was also demonstrated by 

 Blum (1937). This gives an explanation of the inhibiting effect of serum, 

 especially of the albumin fraction (Rask and Howell, 1928). 



Augmentation by Cyanides. Studies on the effect of the addition of 

 cyanides on photodynamic action have led to conflicting observations. 

 Wohlgemuth and Szorenyi (1933) found an increased oxygen uptake dur- 

 ing photooxidation of serum when cyanide was added, and Bier and 

 Rocha e Silva (1935) reported a similar effect in a hemolytic system 

 within certain limits of concentration of cyanide, but Blum (1941a) found 

 that cyanide, over a range of concentrations, did not significantly influ- 

 ence the percentage hemolysis time. The reported effect of cyanide 

 was at first thought by Blum (1935) to support the participation of 

 hydrogen peroxide in the mechanism of photodynamic action, the cyanide 

 being considered to destroy the catalase that destroyed the peroxide. 

 Blum calculated, however, that the amount of hydrogen peroxide which 

 could be formed from the total volume of oxygen taken up during photo- 

 hemolysis would not be sufficient to bring about that hemolysis. In his 

 subsequent discussion (Blum, 1941a) the conclusion was reached that 

 hydrogen peroxide is a minor product in photosensitized oxidation which 

 may augment the hemolytic process, and that cyanide prevents catalase 

 from suppressing the action of peroxide. The effect of cyanide on photo- 

 dynamic action still appears to warrant further investigation. The sub- 

 ject has been revived by Rocha e Silva (1940) in his attempt to attribute 

 photosensitization in cattle eating Holocalyx glaziovii to the enhancement 

 of the action of phylloerythrin by hydrocyanic acid in this plant. 



NATURE OF PHOTODYNAMIC AGENTS 



Photodynamic agents represent many classes of substances with widely 

 different molecular structure and properties. Most of them absorb in 

 the visible range, but this is not essential; phenothiazine sulfoxide, for 

 example, is a colorless substance that produces photosensitization in the 

 cornea of calves (Clare et al., 1947). The one property that appears to 

 be common to all photodynamic substances is the power of fluorescence, 



