Degradation of Immunoglobulins by Intracellular Proteases in the Range of Neutral pH

A proteolytic enzyme active in the range of neutral or slightly alkaline pH has been partially purified from a lysosome-containing granule fraction of human spleen. The molecular weight of the enzyme was estimated to be in the range of 15,000 to 30,000 by gel filtration, and it has a pH optimum of approximately 7.5 with bovine hemoglobin or human IgG as substrates. Its activity is stimulated by cysteine but is strikingly inhibited by diisopropylfluorophosphate. The enzyme is active against a variety of protein substrates including human IgM, IgG, IgA, albumin, and also bovine hemoglobin. The products of IgG and IgM digestion were studied most extensively. IgG was broken down into Fab and Fc fragments, which proved to be very similar in size and immunochemical properties to fragments produced by papain digestion. In addition, a slightly degraded product with a molecular weight in the range of 110,000 to 120,000 was observed, which was not distinguishable from intact IgG by immunodiffusion analysis. IgM was extensively degraded by the neutral protease, a large proportion being broken down to small, dialyzable fragments. Relatively small quantities of large fragments containing Fab and Fc(μ) determinants were found. Evidence was obtained that both Fab and Fc regions of the IgM molecule are degraded by this enzyme.