Differential sensitivity of purified isoforms of cucumber anionic virus-inducible peroxidase to exogenous proteases.

Three different molecular forms, isoforms, of the major virus-inducible anionic peroxidase (PRX) of cucumber (Cucumis sativus L.) were purified to homogeneity from crude extracts of hypersensitively reacting cotyledons and subjected to proteolysis with five exogenous endoproteinases. The PRX isoforms were fully resistant to degradation by trypsin and chymotrypsin even though at a prolonged incubation. Partial proteolysis with pepsin yielded peptides which were similar in size and serological properties. When papain was used, the peptides released from PRX1 isoform differed both in size and number but not serologically from the peptides released from isoforms PRX2 and PRX3 confirming similar primary structure of polypeptide chains. PRX3 was the only substrate giving a peptide map after incubation with protease K. Under experimental conditions used in this work, PRXI and PRX2 were degraded completely with protease K. These results indicate that PRX1, PRX2, and PRX3 contain similar antigenic determinants and indicate very similar but not identical primary structures. Several practical implications of the present study are also mentioned.