712 RADIATION BIOLOGY 



lines of purebred Southdown sheep in New Zealand. Lambs that are 

 normal while subsisting solely on milk become severely photosensitive 

 when they begin to eat grass. There is no sign of structural damage or 

 defect in the liver (Cunningham et al., 1942)', but slight bilirubinemia 

 may be seen, and the rose-bengale excretion test also indicates defective 

 liver function (Clare, 1945). The photosensitizing agent has been shown 

 to be phylloerythrin (ibid.). Coproporphyrin I in small amounts was 

 also identified in the blood, but it was established that this porphyrin 

 does not contribute to the photosensitivity. Hancock (1950) has shown 

 that expression of the disease is inherited as a Mendehan recessive. 



This disease is of particular interest because it indicates that photo- 

 sensitization by phylloerythrin may occur as a result of a more or less 

 specific functional derangement that is not associated with structural 

 changes and not betrayed by obvious cHnical signs. It is possible that 

 among photosensitivity diseases of uncertain etiology there may be simi- 

 lar instances of permanent or sporadic impairment of the phylloerythrin- 

 excreting mechanism not accompanied by demonstrable fiver dysfunction. 



Poisoning by Holocalyx glaziovii. Photosensitivity in this disease of 

 cattle in Brazil has been attributed by Rocha e Silva (1940) to the pres- 

 ence of a cyanogenetic substance in the plant. Arguing from his obser- 

 vations that cyanides increase the uptake of oxygen in photodynamic 

 hemolysis (referred to earlier in this chapter), Rocha e Silva considered 

 that hydrogen cyanide in the blood will so enhance the activity of phyllo- 

 erythrin that small amounts present in the blood of normal animals will 

 bring about photodynamic action. There appears to be little evidence 

 to support this ingenious hypothesis, and even the nature and extent 

 of the effect of cyanides on photodynamic hemolysis are still doubtful. 

 Rocha e Silva gave cyanide in sublethal amounts over long periods to 

 one animal and claimed that photosensitivity was produced, but his 

 description of the symptoms scarcely justifies this conclusion. Photo- 

 sensitivity is not seen in other examples of poisoning by cyanogenetic 

 plants and did not occur in sheep dosed daily with sublethal amounts of 

 potassium cyanide, although these animals were kept outdoors and 

 received green feed (Van der Walt, 1944). The occurrence of phyllo- 

 erythrin in the blood of normal ruminants has never been established, 

 but, if present, it is there in very small amounts, since it has not been 

 found in tests made in the writer's laboratory (D. D. Perrin, unpublished 

 observations), although the method used would detect 0.003 mg per 

 100 ml. Furthermore sheep can be photosensitized during poisoning by 

 Panicum miliaceum, a condition in which phylloerythrin is the photo- 

 sensitizing agent (Clare, unpublished observations), before this detectable 

 level is attained in the blood. 



Some of the toxicity of Holocalyx appears to be due to a cyanogenetic 

 substance, but as the reports by Rocha e Silva definitely indicate that 



