Low‐Molecular‐Weight Polypeptides of Vicilin from Vicia faba L. are Product of Proteolytic Breakdown

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Abstract
Vicilin, the main 7-S globulin of V. faba L., undergoes cleavage during prolonged treatment at room temperature, which can be inhibited by protease inhibitors such as 1 .mu.M leupeptin. The cleavage products show identical electrophoretic mobilities with the polypeptides normally visible after sodium dodecylsulfate gel electrophoresis of vicilin prepared from mature seeds. N-terminal amino acid analysis of electrophoretically prepared polypeptides reveals serine as common N terminus of the 2 largest polypeptides of MW .apprxeq. 50,000 and 35,000. According to serological experiments and peptide mapping the low-molecular-weight polypeptides (MW .apprxeq. 35,000; 31,000; 19,000 and below) have antigenic determinants and amino acid sequences, respectively, that are similar to each other and are all contained within the structure of the large polypeptide of MW .apprxeq. 50,000. Polypeptides of MW .apprxeq. 35,000 and below are derived by proteolysis from one (or a few closely related) polypeptide(s) of MW .apprxeq. 50,000 and that proteolysis starts soon after biosynthesis as a post-translational process within the developing seed. Some experiments indicate the existence of nicking points within the vicilin polypeptides, which are the major cleavage sites during preparation. Apparently, native vicilin is a trimeric or tetrameric globulin with polypeptides of MW .gtoreq. 50,000.