The preparation of deglycosylated ricin by recombination of glycosidase-treated A- and B-chains: effects of deglycosylation on toxicity and in vivo distribution

Deglycosylation of ricin may be necessary to prevent the entrapment of antibody-ricin conjugates in vivo by cells of the reticuloendothelial system which have receptors that recognise the oligosaccharide side chains on the A- and B-chains of the toxin. Carbohydrate-deficient ricin was therefore prepared by recombining the A-chain, which had been treated with α-mannosidase, with the B-chain, which had been treated with endoglycosidase H or α-mannosidase or both. By recombining treated and untreated chains, a series of ricin preparations was made having different carbohydrate moieties. The removal of carbohydrate from the B-chain did not affect the ability of the toxin to agglutinate erythrocytes, and α-mannosidase treatment of the A-chain did not affect its ability to inactivate ribosomes. The toxicity of ricin to cells in culture was only reduced in those preparations containing B-chain that had been treated with α-mannosidase, when a 75% decrease in toxicity was observed. The toxicity of the combined ricin preparation to mice varied from double to half that of native ricin, depending on the chain(s) treated and the enzymes used. Removal of carbohydrate greatly reduced the hepatic clearance of the toxin and the levels of toxin in the blood were corresponding y higher. These results suggest that antibody-ricin conjugates prepared from deglycosylated ricin would be cleared more slowly by the liver, inflict less liver damage, and have greater oppurtunity to reach their target.