PHOTODYNAMIC ACTION 711 



terrestris under certain conditions of growth. In attempts to reproduce 

 various aspects of this disease, Quin (1933a) obstructed the bile flow of 

 sheep by ligation of the common bile duct and thereby produced not only 

 icterus but also photosensitivity. Rimington and Quin (1934) were able 

 to identify in the blood of these sheep the pigment phylloerythrin, which 

 was known to occur in the bile of herbivores. They were next able to 

 demonstrate phylloerythrin spectroscopically in the blood of field cases of 

 geeldikkop, and following the demonstration that phylloerythrin injected 

 intravenously produced photosensitivity in sheep, they concluded that 

 this pigment was the photodynamic agent in geeldikkop. 



Phylloerythrin is a product of degradation of chlorophyll in the diges- 

 tive tract of herbivores. Normally some phylloerythrin is absorbed into 

 the portal circulation, but it is efficiently excreted by the liver into the 

 bile. It is apparent that any damage to the liver which results in biliary 

 obstruction or otherwise interferes with the mechanism of phylloerythrin 

 excretion is likely to lead to photosensitization in herbivores, provided 

 that they are at the time consuming food containing sufficient chlorophyll. 



The substance responsible for this liver dysfunction of geeldikkop has 

 not yet been isolated from T. terrestris. However, another plant, Lippia 

 rehmanni, was shown by Quin (1933b) to produce effects similar to those 

 in geeldikkop, and from this plant Rimington and Quin (1935) isolated 

 the toxic principle, which they named "icterogenin." Later this princi- 

 ple was resolved into three isomers (Rimington and Quin, 1937a). From 

 what has been determined of their structure and properties, these sub- 

 stances appear to be of the nature of resenic acids. ^ 



The work of Quin and Rimington on geeldikkop and Lippia poisoning 

 has been followed by the recognition that a number of other photosensi- 

 tivity diseases of livestock, some of them causing considerable economic 

 losses, show a similar etiology. The variety of poisonous plants and other 

 conditions that may lead to hepatogenous photosensitivity is demon- 

 strated by the list at the end of this section. In all cases the photo- 

 dynamic aspects of these diseases appear to be identical — although in 

 comparatively few have attempts been made to confirm the presence of 

 phylloerythrin in the blood — but both the severity and type of the liver 

 lesions and the nature of the hepatotoxins show considerable diversity. 

 In some plants the toxin appears to be present always; in others it appears 

 fleetingly only under certain conditions of growth. Some plants are toxic 

 to most species of animals ; others appear to affect ruminants only. Such 

 factors greatly complicate attempts to isolate the hepatotoxins responsible 

 for diseases such as facial eczema and geeldikkop. 



Congenital Photosensitivity in Southdown Sheep. Unique among the 

 hepatogenous photosensitivity diseases is that which occurs in certain 



1 For subsequent studies on the structure of icterogenin see D. H. R. Barton and 

 P. de Mayo, J. Chem. Soc, 1954: 887-900. 



