Comparison of Biochemical Properties of Natural and Recombinant Cathepsin E

Abstract

Cathepsin E (CE) (EC 3.4.23.34) is an intracellular aspartic proteinase consisting of two identical subunits with a molecular mass of about 42kDa (1–4). This enzyme is also known as a glycoprotein (1–3) and to have oligosaccharide chains of both the high-mannose-type and the complex-type (5). In contrast to cathepsin D (EC 3.4.23.5), a lysosomal aspartic proteinase, CE has a limited distribution (6, 7) and is not found in the typical lysosomal structures (8). Recent studies provide some information about its physiological significance, e.g., processing of propeptides (9), catabolism of neuropeptides (10) and participation in neuronal cell death (11). However, the precise roles of CE in cellular protein catabolism has not yet been clarified. In addition, little information about the biosynthesis and the processing and activation mechanisms of this enzyme in cells has so far been available. The major problems for these studies are the difficulty in purification of the enzyme due to its low concentration and the lack of a specific substrate that is discriminated only by CE. To solve these problems, we have tried to gain sufficient materials of CE. More recently, we have succeeded in preparing the recombinant CE (rCE) which are expressed in Chinese hamster ovary (CHO) cells at a high rate (4). In this report, we describe the purification and biochemical properties of rCE and compare these properties with those of the natural enzyme purified from human erythrocyte membranes.