Insulin-ricin B chain conjugate. A hybrid molecule with ricin-binding activity and insulin biological activity.

The polypeptide hormone insulin and the binding portion of ricin toxin, the B chain, were linked via a disulfide bond. This insulin-ricin B chain conjugate bound to insulin receptors with a potency one-twentieth that of native insulin. Rat HTC hepatoma cells, a cultured cell line that has relatively few insulin receptors, bound the conjugate to a much greater degree than insulin. Binding occurred predominantly via the ricin B chain portion of the conjugate since binding was not inhibited by insulin but was inhibited by galactose, a known inhibitor of the interaction of ricin B chain to its receptor. In HTC cells, the insulin-ricin B chain conjugate at 330 nM stimulated amino acid uptake to 225% of controls, a value higher than that for insulin which stimulated uptake to only 167% of controls. The conjugate also stimulated tyrosine aminotransferase activity in HTC cells with a potency value approximately one-half that of insulin. Both of these activities of the insulin-ricin B chain conjugate in HTC cells were inhibited by 100 mM galactose (90% and 80%, respectively), whereas the ability of insulin to stimulate these activities was not inhibited significantly by this sugar. The results suggest, therefore, that one can construct hybrid molecules consisting of binding proteins and polypeptide hormones and that these hybrid molecules can have binding and biological activities which are different from the parent hormone molecule.