Effect of sulfhydryl reagents and protease inhibitors on sodium dodecyl sulfate-heat induced dissociation of Ricinus communis agglutinin

Ricinus communis agglutinin dissociated to lower molecular weight forms when heated in sodium dodecyl sulfate in the absence of reducing agents, while ricin was little affected by such treatment. The data suggest that strong noncovalent bonds hold together two A-B heterodimers in the Ricinus communis agglutinin tetramer. Protease inhibitors such as diisopropylfluorophosphate, phenylmethanesulfonyl fluoride, and EDTA, did not prevent the sodium dodecyl sulfate-heat induced dissociation; however, sulfhydryl specific reagents (N-ethylmaleimide, 5,5′-dithiobis (2-nitrobenzoic acid) and p-chloromercuribenzoate) were effective. Titration of the lectins in sodium dodecyl sulfate indicated that ricin contains one sulfhydryl and Ricinus communis agglutinin four sulfhydryl groups, none of which react in the presence of 8 M urea. The sulfhydryl groups that could be titrated in the intact proteins in sodium dodecyl sulfate were on the A chains.