An aspartic proteinase of erythrocyte membranes. Proposed mechanism for activation and further molecular properties.

An aspartic proteinase associated with human erythrocyte membranes was shown to be responsible for autodegradation of the membrane proteins at pH values below 5.0. When the membrane was treated with phospholipase C (Bacillus cereus) or trypsin, and simply heated at 40 degrees C, the membrane-bound latent enzyme was activated, with this being accompanied by dissociation of the enzyme from the membrane. Divalent cations such as Ca2+ and Mg2+ had an inhibitory effect on the dissociation of the membrane-bound enzyme when preincubated with the membrane. The results indicate that the activation of the membrane-bound enzyme is due probably to perturbation of the normal membrane organization. When the purified enzyme was treated with 10mM 2-mercaptoethanol at 37 degrees C, the enzyme (79-82 kDa) was converted to a low molecular mass form with 42-47 kDa without any loss of activity. With the exception of treatments by thiol-reducing reagents, no conversion was observed by a variety of procedures such as exposure to 1 M NaCl and 0.1% sodium dodecyl sulfate, treatment with trypsin and incubation at pH 3.5 for up to 15 h, indicating that the enzyme consists of two polypeptide chains held together by disulfide bonds.