(Wulf and Bautz, 1976) and human diploid fibroblasts 

 (Buchwald and Ingles, 1976) have all demonstrated similarly 

 resistant polymerase II molecules as being responsible for 

 the observed a-amanitin resistance. Thus a-amanitin can 

 be seen to be a structurally well characterized toxin that 

 possesses an extremely specific mechanism of action. The 

 properties of high specificity, known mechanism of action 

 and low concentration at which it is inhibitory make a- 

 amanitin an ideal inhibitor for examining targeting of 

 inhibitor conjugates to specific cell receptors. 



Conjugates of the amanitins and macromolecules were 

 first prepared as an attempt to produce antibodies to $- 

 amanitin as a haptenic substituent linked to rabbit serum 

 albumin (RSA) (Cessi and Fiume, 1969). Beta-amanitin, which 

 contains a free carboxyl group, was directly coupled to RSA 

 by reaction with water soluble carbodiimides. The conju- 

 gates not only failed to elicit antibody production, but 

 were found to be approximatley 10-fold more toxic to mice 

 than free 8-amanitin (Cessi and Fiume, 1969). The conju- 

 gated 8-amanitin retained its specificity of interaction 

 with RNA polymerase II but with a reduced binding affinity 

 (Fiume et al., 1971; Derenzini et al., 1973). Furthermore 

 the enhanced in vivo toxicity of the 3-amanitin conjugates 

 was determined to result from increased uptake by the sinu- 

 soidal cells of the liver and the proximal tubule cells of 

 the kidney, presumably because of the protein portion of 

 the conjugate (Fiume, 1969; Fiume et al., 1969). Cultured 



