RESUMEN
An increase in proteolytic activity is an early common feature of diabetes, and is associated with the development of vascular complications. We performed an extensive proteomic investigation on plasma of type 1 diabetic subjects to discover why some of them apparently lacked any measurable proteolytic activity. Activity was found enclosed in immune complexes in which Fab/(Fab)(2) displayed a serine-like catalytic activity. Disaggregation of complexes by means of Protein G affinity chromatography led to the separation of free subunits of Fab, showing a specific amidolytic activity, from Fab that displayed activity on casein and remained closely complexed with whole IgG. On both types of Fab the serine catalytic site appeared to be the same, being located in close vicinity to the antigen-binding site. The distinct substrate specificity was due to the different conformation adopted by the catalytic site depending on the structure of Fab/(Fab)(2), whether in complexes or as free subunits. Catalytic Fab/(Fab)(2) originated from idiotypic antibodies developed against Grp94, identified as the primary antigen covalently complexed with Fab. Whole IgG present in immune complexes were instead mostly formed with anti-idiotypic antibodies developed against the adduct of Fab/(Fab)(2) with Grp94, and were responsible for blocking any catalytic activity. In dot-blot experiments with native Grp94, we confirmed that in any diabetic plasma circulated anti-Grp94, idiotypic, and anti-idiotypic antibodies.