PHOTODYNAMIC ACTION 705 



1. The subject must be shown to be sensitive to sunhght transmitted 

 through window glass. 



2. A photodynamic substance must be isolated which will produce this 

 sensitivity to light when injected into experimental animals. 



3. The action spectra for postulates 1 and 2 must be identical and 

 should correspond to the absorption spectrum of the suspected photo- 

 dynamic agent. 



Sunlight is stipulated in the first postulate because the sun is the usual 

 source of Ught to which skin is exposed and it is difficult to devise light 

 sources that simulate sunlight in intensity and distribution of energy. 

 Window glass, or a similar absorber of the erythema-producing radiation 

 of about 3200 A, is required to eliminate confusion with sunburn. Appli- 

 cation of this test might have prevented a number of diseases involving 

 skin lesions from being incorrectly reported as photosensitivity. 



In the second postulate Blum has specified injection as the mode of 

 administration. He recognizes, however, that in most diseases of ani- 

 mals the photosensitizer comes from a plant or other ingredient of the 

 diet and that oral administration of the suspected substance is the ulti- 

 mate test in such cases. 



Injection tests with plant extracts should be regarded solely as a guide 

 to isolation, and then only if chlorophyll or its degradation products, 

 which are themselves photodynamically active, have been removed. The 

 tendency has been to disregard as possible photosensitizing agents the 

 chlorophyll derivatives obtained directly from plants, since there was no 

 evidence that these pigments might reach the systemic circulation unless 

 the excretory function of the liver was impaired. The photosensitization 

 of small animals by chlorophyll-type pigments obtained from Panicum 

 miliaceum, referred to earlier in this chapter, suggests that chlorophyll 

 pigments should not be entirely neglected if extracts containing them are 

 active on oral administration. 



The third postulate is frequently difficult to establish experimentally 

 because of the lack of convenient means of isolating various parts of the 

 spectrum, the wide range of wave lengths absorbed by most pigments, 

 the differences between absorption spectra of substances in vivo and in 

 vitro, and the difficulty of evaluating the response in animals. Defining 

 the action spectrum for a photosensitivity may assist in excluding a sus- 

 pected photodynamic agent but can rarely confirm its identification. 



A disease in which the determination of the action spectrumi was useful 

 in distinguishing between several possible photosensitizers is the keratitis 

 of calves dosed with phenothiazine (Clare ct al., 1947). 



An approach not specifically referred to by Blum is the examinatioii of 

 blood or tissue of photosensitive animals for photodynamic substances 

 not normally present. Such examinations have contributed materially 

 to the establishment of phylloerythrin as the sensitizer in the group of 



