Membrane-bound high molecular mass proteinases from human erythrocytes

Two high molecular mass proteinases, multicatalytic proteinase (MCP) and a new high molecular mass proteinase (HMP) with only chymotrypsin-like activity (Khan et al. (1994) J. Biol. Chem. 269, 10016-10021) from human erythrocyte membranes, have been compared. For this purpose, MCP was purified from human erythrocyte membranes in the active form towards synthetic peptide substrates; it also hydrolysed the protein substrates [14]methyl casein and [14C]oxidised insulin beta chain at 37 degrees C. MCP from plasma membranes exhibited hollow cylindrical structures also typical of cytosolic forms. Radiolabelled diisopropyl fluorophosphate, [3H]DFP, a serine proteinase inhibitor, labelled a band of Mr 23 000 in membrane MCP. By contrast, no labelling was obtained with HMP. Chymotrypsin-like activity of HMP was also found to be insensitive to DFP. On the other hand, DFP inhibited chymotrypsin-like and peptidylglutamyl peptide hydrolysing activities of membrane MCP, with no effect on its trypsin-like activity. The inhibition of MCP by DFP was concentration-dependent. These studies showed that MCP and HMP represent two distinct kinds of proteinases with chymotrypsin-like activities and can be distinguished by the serine proteinase inhibitor DFP.