Structural Analysis of a Slowly Sedimenting Proteolytic Fragment of Human Immunoglobulin M

Abstract

Summary Human IgM was digested with crystalline papain in the presence of 2 × 10-3 M mercaptoethylamine for 4, 8, 12, 24 or 36 hr. A single, slowly sedimenting peak was observed in the schlieren patterns of mixtures after 8 or more hours of digestion. It was shown by immunoelectrophoresis that the protein initially constituted at least two components. One of them had some of the characteristics expected of Fabµ; the other may have been Fcµ. As the time of proteolysis increased, the latter was progressively destroyed, so that after 36 hr of digestion, only the former remained. The identity of the papain-resistant fragment as Fabµ was based on the following observations: a) It had a sedimentation coefficient at 6 mg/ml of 3.4 S. b) Its electrophoretic migration was very slightly cathodal at pH 8.2. c) Upon gel filtration in propionic acid, the fragment was eluted in a position between that of µ chains and light chains. d) After mild reduction, the fragment dissociated into two components when gel filtered in propionic acid. One was identified as light chain and the other appeared to be a fragment of the µ chain (Fdµ). Since Fabµ failed to dissociate in propionic acid, and was devoid of carboxymethylcysteine, the disulfide bond which joins the light chain to Fdµ remained intact during the digestive process.