694 RADIATION BIOLOGY 



mals. In this chapter the nature and mechanism of photodynamic action 

 in biological systems are discussed, and the role of photodynamic action 

 in various diseases, particularly those affecting domestic animals, is 

 surveyed. 



NATURE AND MECHANISM OF PHOTODYNAMIC ACTION 



DEFINITION 



At this stage some definition of what is meant by photodynamic action 

 is desirable. When this expression was introduced (Tappeiner and 

 Jodlbauer, 1904), it was not clearly recognized that the type of process 

 discovered by Raab constituted only one of several ways in which radi- 

 ation can influence biological systems. If precedent and use had not 

 so firmly attached Tappeiner's term "photodynamic action" to this 

 restricted field of photobiology, a less general description, such as "photo- 

 sensitized oxidation," would now be preferable. It is therefore important 

 to stress that by photodynamic action is meant an oxidation by molecular 

 oxygen brought about in a biological system exposed to radiation through 

 the agency of a fluorescent substance (the photodynamic agent). 



The need for both light and an absorber of light was established in 

 Raab's initial experiments. The need for oxygen was demonstrated by 

 Straub (1904) and Jodlbauer and Tappeiner (1905) in systems such as 

 the lysis of red blood cells or the killing of infusoria, and Blum et al. 

 (1935), from experiments involving occlusion of the blood supply to an 

 area injected with hematoporphyrin, have concluded that a supply of 

 oxygen is necessary for photosensitization of human skin. 



It is conceivable that some effects brought about by radiation may be 

 similar to photodynamic action except that the final change is not oxi- 

 dation but some reaction such as the disruption of a molecule. Such a 

 process, if it were ever shown to occur, would fall outside the definition of 

 photodynamic action. 



MECHANISM OF PHOTODYNAMIC ACTION 



The resemblance between the conditions necessary for photodynamic 

 action in biological systems and conditions pertaining in photosensitized 

 oxidations in vitro suggested to Straub (1904) that the same fundamental 

 mechanism was operating in both phenomena. More elaborate expla- 

 nations have been advanced from time to time (Tappeiner, 1909; Clark, 

 1922), but Straub's original hypothesis, as developed by Blum (1941a), 

 is the only one consistent with most of the facts. In accordance with 

 this hypothesis the photochemical section of the process of photodynamic 

 action may be conceived of as follows: 



1. Energy in the form of radiation is absorbed by a molecule of the 

 photosensitizing substance, which is thereby transformed to an excited 



