700 RADIATION BIOLOGY 



The second objection appears to assume the irreversible formation of 

 a product of the dye. Bkim (1941a) recognizes that this must be an 

 oxidation product since it is formed only when the dye is irradiated in 

 the presence of oxygen. If, however, the final lytic stage in the mecha- 

 nism involves oxidation of a cell component, as required by the mecha- 

 nism favored by Blum, an alternative process in which the photofluo- 

 rescein effects this oxidation and is simultaneously reduced to its original 

 form, thus becoming available for absorption of further radiation, would 

 appear to be equally consistent with the known facts. Such a mecha- 

 nism can be represented by the following scheme: 



D -\-hv^ D' 

 D' -\- 02^ Dox (Scheme II) 



Dox -i- S ^ D + Sox 



To determine whether such a scheme is valid and applicable to photo- 

 dynamic actions in general, further information on the nature and chemis- 

 try of photofluorescein, and on whether similar substances are formed by 

 irradiation of other photodynamic agents, is obviously required. 



An observation made by Smetana (1938) in studies on the photo- 

 dynamic action of hematoporphyrin on body fluids may be interpreted 

 as evidence in favor of Scheme I. Smetana found that after prolonged 

 exposure the uptake of oxygen gradually came to a standstill. Addition 

 of more oxygen or hematoporphyrin did not appreciably speed up the 

 reaction, whereas addition of further substrate promptly raised the oxy- 

 gen consumption. 



Another phenomenon noted by Smetana, namely, that uptake of oxy- 

 gen continued, at a decreasing rate, for some time after the light was 

 turned off, is not simply explained by either Scheme I or Scheme II. 



When the various evidence discussed in this section is considered as a 

 whole, the mechanism of photodynamic action put forward by Blum 

 (Scheme I) is probably most in accordance with the established facts. 

 Nevertheless, for the fluorescein system at least, Menke's photocompound 

 suggests, in Scheme II, a possible alternative that warrants further study. 

 It would seem that neither mechanism necessarily precludes the other; 

 the over-all action may involve a combination of both processes (but see 

 also Addendum to this chapter). 



OTHER FACTORS INFLUENCING PHOTODYNAMIC ACTION 

 Investigation on the mechanism of photodynamic action has been 



assisted by the study of a number of other factors and conditions that 



influence it. These have been reviewed and interpreted by Blum (1941a) 



and will be referred to only briefly here. 



The Dark Action. Many fluorescent substances will effect changes of 



the same type as those resulting from photodynamic action without expo- 



