702 RADIATION BIOLOGY 



itself evidence of molecular structure capable of temporary excitation by 

 absorption of radiation. Plant substances responsible for photosensi- 

 tivity in animals must, of course, also be capable of retaining an acti- 

 vatable form during absorption from the digestive tract and passage 

 through the liver to the systemic circulation. For this reason it seems 

 unlikely that the pigment phycocyanin would be a photosensitizer in 

 poisoning by certain algae, as suggested by Steyn (1943), since it would 

 be expected to undergo fission into protein and a pigment phycocyanobilin 

 (Lemberg and Legge, 1949), which because of its close similarity to bili- 

 rubin is not likely to be a photosensitizing agent. 



From the Einstein equivalence law it follows that in an ideal system 

 all photodynamic agents compared on a molarity basis would be equally 

 effective, since each would bring about the oxidation of one molecule of 

 substrate for each quantum absorbed. In actual systems photodynamic 

 effectiveness is limited by a number of factors. Blum (1941a) presented 

 evidence to indicate that the effectiveness of fluorescein dyes in photo- 

 dynamic hemolysis largely depends on the extent to which they are taken 

 up by the erythrocytes. The manner of uptake is also important, dyes 

 that penetrate the cell being less effective than dyes taken up on the cell 

 surface (Dognon, 1927). In the photosensitization of animals the appar- 

 ent photodynamic effectiveness is conditioned by many additional factors, 

 including the protective action of the outer layers of the skin, the stability 

 and solubility of the agent in body fluids, and the ease with which it is 

 excreted. 



The occurrence of chlorophyll in green plants and the fact that one of 

 its breakdown products, phylloerythrin, is the photosensitizing agent in 

 an important group of diseases of animals have given rise to some mis- 

 conceptions about the role of chlorophyll pigments in photosensitivity 

 diseases. There is no evidence that chlorophyll itself can be absorbed 

 from the digestive tract. In fact, in most animals it is readily broken 

 down, and those derivatives such as phylloerythrin which are absorbed 

 are ordinarily excreted through the bihary system. Photosensitization of 

 herbivores by phylloerythrin occurs only as a result of derangement of 

 the excretory mechanism of the liver, as will be apparent in the subse- 

 quent discussion of these diseases. 



Nevertheless work in progress in the writer's laboratory (Clare and 

 D. S. Letham, unpubHshed observations) has shown that leaves of 

 Panicum miliaceum, dried in a current of hot air, contain a pigment that 

 photosensitizes rats and guinea pigs. Severe effects have been produced 

 in rats by 200 g of the leaves fed over 18 days, and a rat of 100 g body 

 weight was killed by exposure to sunlight after a single dose of 10 mg of 

 the pigment. There was no evidence of liver dysfunction in these ani- 

 mals. The absorption spectrum of this pigment suggests that it is a 

 chlorin-type derivative. Discovery of it has raised the possibiHty that 



