Isolation of a ribosome-inactivating protein from pearled barley and determination of its suitability for immunotoxin construction

A ribosome-inactivating protein (RIP) was isolated from pearled barley by a modification of the procedures of Roberts and Selitrennikoff (Biochim. Biophys. Acta 696, 239 (1982). A single cation exchange high-performance liquid chromatography step (Polycat A, Poly LC, Columbia, MD) was substituted for the combination of conventional carboxymethyl-Sephadex followed by gel permeation chromatography. The toxin migrated as a single 30 kDa protein band on sodium dodecylsulfate-polyacrylamide gel electrophoresis, and was judged to be $>$99% homogeneous. The toxicity of the barley toxin approximated that of gelonin and ricin A-chain. For the purpose of evaluating the barley toxin for suitability in immunotoxin construction, the thiolated toxin was coupled to a murine anti-human transferrin receptor monoclonal antibody. The cytotoxicity of this conjugate was comparable to similarly constructed conjugates using ricin A-chain and gelonin as the toxic component. On the basis of these results plus its availability and low cost, it is concluded that barley toxin represents a promising alternative for use in immunotoxin construction. (Abstract shortened with permission of author.).