147 



compares favorably with the magnitude of the inhibition 



-9 

 constant for a-amamtin (K = 1.8 x 10 M) and for ADGG 



-9 

 (K = 6.9 x 10 M) . On this basis, the targeting of a- 



amanitin by ADGG-Con A conjugates can be seen to be 



extremely effective in comparison to the total amount of 



inhibitor exposed to the cell. This can only occur as a 



result of the specific interaction of the Con A portion of 



the conjugate with cell surface carbohydrates, followed by 



internalization of at least the amanitin portion of the 



conjugate. 



In conclusion, covalent conjugates of oc-amanitin and 

 concanavalin A were synthesized that retained the biolo- 

 gical activities of both components of the conjugates: inhi- 

 bition of RNA polymerase II by a-amanitin and the carbohy- 

 drate binding specificities of Con A. These conjugates 

 resulted in receptor mediated targeting and uptake of the 

 amanitin by interaction of the Con A with specific cell 

 surface receptors. The resulting toxicity for CHO cells 

 of ADGG-Con A conjugate was minimally 50 times that achieved 

 with an equivalent concentration of free ADGG. The high 

 toxicity and well defined binding specificity of the ADGG- 

 Con A conjugates suggest the potential for specific inhibi- 

 tion of selected cells within a mixed population by these 

 conjugates. 



The amanitin-Con A conjugates investigated here pos- 

 sess a targeting potential that contains several advantages 

 over those systems discussed in the introduction. The 



