A comparative analysis of human IgM rheumatoid factor degradation by purified elastase and total granule extracts from human polymorphonuclear leukocytes

Abstract

A modified digestion system using radiolabeled IgM rheumatoid factors (RF) and unlabeled IgG was used to examine IgM RF digestion by human polymorphonuclear leukocyte (PMN) elastase. Upon molecular sieve chromatography, the radioactive fragments coelute with fragments produced by elastase digestion of an IgM protein having no RF activity. The fragments represent an Fab₂-like fragment, an Fab-like fragment, and small peptides. Utilizing this same system, digests were performed at both acid and neutral pH to compare the proteolytic action of purified elastase on IgM RF (Ove) to the action of the total granule extract (TGE) from human PMN. At pH 4.5, purified elastase exhibits low-level protease activity, producing a slightly degraded IgM fragment with a molecular weight of about 800,000 daltons. In contrast, TGE at pH 4.5 completely degrades IgM RF to small peptides. At pH 7.5, the fragments produced by TGE digestion of IgM (Ove) coelute with fragments produced by elastase digestion under the same conditions. Thus elastase appears to be the major granule protease active in IgM RF degradation at the pH characterizing the inflammatory site.