Immunoblot analysis reveals that isopeptide antibodies do not specifically recognize the epsilon-(gamma-glutamyl)lysine bonds formed by transglutaminase activity.

Transglutaminases (TGs) posttranslationally modify proteins by transamidation of specific polypeptide bond glutamines. This reaction results in deamination, polyamine incorporation or the formation of isopeptide bonds. Transglutaminase activity in the brain is increased in several neurodegenerative diseases. Because insoluble inclusions occur in these neurodegenerative diseases, it has been hypothesized that transglutaminase contributes to the formation of the inclusions by catalyzing the formation of isopeptide bonds resulting in crosslinked, insoluble protein aggregates. To demonstrate a role for transglutaminase in the formation of these inclusions, the primary approach has been to show increased immunoreactivity with antibodies that recognize the isopeptide bonds. However, the specificity of these antibodies for isopeptide crosslinks within or between proteins has not been clearly established. In this report we demonstrate that the two most commonly used isopeptide antibodies do not specifically recognize the isopeptide bonds formed by transglutaminase when they are within or between proteins.