Purification and properties of a cathepsin D-like acid proteinase from rat gastric mucosa.

A unique acid proteinase was purified from rat gastric mucosa by DEAE-Sepharose CL-6B chromatography, gel filtration and isoelectric focusing. The purified enzyme was homogeneous by polyacrylamide disc gel electrophoresis. The molecular weight of the enzyme was estimated to be 86000 by gel filtration on a Sephadex G-100 column. In SDS-polyacrylamide gel electrophoresis, the enzyme showed a single band corresponding to a molecular weight of 42000. So it was concluded that the native enzyme is composed of two identical monomeric units. The enzyme is a glycoprotein and its isoelectric point was determined to be 4.33. The enzyme hydrolyzed bovine hemoglobin with maximum activity at pH 3.0, but it showed considerably lower activity on bovine serum albumin or casein than on bovine hemoglobin. The enzyme was strongly inhibited by pepstatin, but not by urea or thiol-reactive inhibitors. These results lead us to conclude that this enzyme is a cathepsin D-like acid proteinase.