704 RADIATION BIOLOGY 



From this work it appears highly probable that proteins are the oxi- 

 dizable substrate in photodynamic action and that tyrosine, tryptophan, 

 and perhaps histidine are amino acids affected within the protein mole- 

 cule. Whether all proteins containing these amino acids are susceptible 

 or whether certain proteins are specifically attacked, what is the nature 

 of the products formed, and how alteration to the protein affects the 

 structure or function of the cell — all these are ({uestions that remain 

 unanswered. The development of partition chromatography for protein 

 chemistry may simplify the approach to these problems (but see also 

 Addendum to this chapter). 



EFFECTS OF PHOTODYNAMIC ACTION ON THE SKIN 



The effects generally observed in photosensitized skin do not depend 

 on the nature of the photodynamic agent. Local effects produced by 

 injection of the agent are similar to the triple response described by Lewis 

 (1927) — i.e., erythema followed by edema of the exposed area and the 

 subsequent spreading outward of erythema, but not edema, into the sur- 

 rounding skin. Litense itching accompanies these changes, and pigmen- 

 tation of the skin is an aftereffect. 



Alost of these reactions are probably conseciuences of cell damage initi- 

 ated by photodynamic action, but not peculiar to it, and resemble general 

 inflammatory processes, the mechanism of which has been discussed by 

 Menkin (1940). 



DISTINCTION BETWEEN SUNBURN AND PHOTODYNAMIC ACTION 



From comparison of characteristic features of these two conditions, it is 

 apparent that the mechanism responsible for photodynamic action is 

 fundamentally different from that operating in sunburn, since Blum et al. 

 (1935) have shown that sunburn can occur in the absence of oxygen. 

 The action spectrum for sunburn is limited to radiation shorter than 

 3300 A, whereas most, if not all, photodynamic action spectra are of 

 longer wave length, generally including visible light. 



INVESTIGATION OF PHOTOSENSITIVITY DISEASES 



Although there are obvious gaps in our knowledge of the fundamental 

 processes of photodynamic action, certain inferences drawn from that 

 knowledge are of value in formulating an experimental approach to the 

 subject, particularly in its more practical aspect — the study of photo- 

 sensitivity diseases. Such a study resolves essentially into proof that 

 photosensitivity is occurring, the identification of the photodynamic 

 agent, and discovery of the way in which it reaches the skin. Blum 

 (1938, 1941a) has proposed three postulates that should be fulfilled before 

 identification of the photosensitizing agent can be considered conclusive: 



