708 RADIATION BIOLOGY 



which is produced endogenously by an aberrant metabolic process. 

 Alternatively the aberration may be the excessive formation of a pig- 

 ment ordinarily produced only in small, harmless amounts. In this cate- 

 gory is the photosensitivity associated with congenital porphyria in cattle. 



Type III, Hepatogenous Photosensitivity. The photosensitizing agent 

 is a substance, normally absorbed and excreted, which is diverted to the 

 peripheral circulation through failure of a liver excretion or detoxication 

 mechanism. This type is represented by those photosensitivities in which 

 phylloerythrin, a product of chlorophyll digestion normally excreted by 

 the liver into the bile, accumulates in the systemic blood as a result of 

 liver dysfunction. In most diseases known to belong to this type, the 

 phylloerythrin excretion has been deranged by a plant hepatotoxin, but 

 mechanical bile-duct obstruction may also be responsible. 



This classification is proposed primarily to facilitate discussion of these 

 diseases and to emphasize the very different etiologies of diseases in which 

 photosensitivity can occur. It is possible that among diseases of obscure 

 origin still further mechanisms will be found by which a photodynamic 

 agent may reach the skin. Kidney dysfunction, for example, might lead 

 to retention of a substance normally excreted in the urine, or disturbance 

 of the selective permeability of the gut wall might allow the entry of a 

 substance not normally absorbed. 



DISEASES OF TYPE I 



Hypericism. This disease, the oldest and best-known representative 

 of the primary type of photosensitivity, occurs in sheep, cattle, goats, 

 and horses through ingestion of certain species of Hypericum, principally 

 H. perforatum and H. crispum. Cerny (1911) extracted a red fluorescent 

 pigment, hypericin,. from H. perforatum, and a number of investigators 

 subsequently demonstrated the photosensitizing properties of extracts 

 containing hypericin (Ray, 1914; Horsley, 1934; Quin, 1933c). Pace and 

 Mackinney (1941) resolved hypericin into a mixture of closely related 

 fractions and presented evidence that indicates that these substances are 

 partly reduced polyhydroxy derivatives of helianthrone. Pace (1942) 

 obtained good agreement between the action spectrum for photosensi- 

 tivity produced by oral administration of either hypericin or the plant 

 itself and the absorption spectrum of hypericin. 



Fagopyrism. Sensitivity to light of animals fed on buckwheat {Polygo- 

 num fagopyrum) has been reported frequently over the last 150 years, 

 and this was in fact the first photosensitivity disease in which the lesions 

 were attributed to photodynamic action (Busck, 1905). Evidence of the 

 presence of a photodynamic pigment in buckwheat was obtained by Busck 

 (1905), Ohmke (1908), and Chick and ElHnger (1941), and finally Wender 

 et al. (1943) isolated three crystalline substances, all of which produced 



