11 



Chang and Neville (1977) synthesized conjugates of 

 diphtheria toxin and human placental lactogen that contained 

 equimolar ratios of toxin to carrier protein. The conju- 

 gates bound effectively to lactogen receptors on mammary 

 gland explants but these receptors were unable to mediate 

 entry of the toxin to the cell as the conjugate did not 

 inhibit protein synthesis in the target cells (Chang et al., 

 1977; Neville and Chang, 1978). 



Although previously described conjugates using albu- 

 mins and other proteins without defined cell binding activ- 

 ities resulted in some degree of enhanced cytotoxicity, 

 only those conjugates that could interact with specific 

 receptors on the target cell produced significant cell 

 specific toxicity. One other category of receptor specific 

 macromolecules besides antibody and hormones that may 

 mediate the targeting of inhibitors are lectins. Lectins 

 are proteins that possess specific binding sites for 

 carbohydrate moieties. They have been isolated from a 

 wide variety of plants and animals and possess a broad 

 range of individual binding specificities. Their specific 

 interaction with cell surface glycoproteins has been thor- 

 oughly documented and leads to an array of complex cellular 

 responses including agglutination, mitogenic stimulation 

 and cell toxicity (Sharon and Lis, 1972; Lis and Sharon, 

 1973; Nicolson, 1974). 



Perhaps the most widely investigated lectin is that 

 protein isolated from the jack bean, Concanavalin A. 



