4-Hydroxy-2-nonenal-modified glyceraldehyde-3-phosphate dehydrogenase is degraded by cathepsin G

Degradation of oxidized or oxidatively modified proteins is an essential part of the antioxidant defenses of cells. 4-Hydroxy-2-nonenal (HNE), a major reactive aldehyde formed by lipid peroxidation, causes many types of cellular damage. It has been reported that HNE-modified proteins are degraded by the ubiquitin–proteasome pathway or, in some cases, by the lysosomal pathway. However, our previous studies using U937 cells showed that HNE-modified glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is degraded by an enzyme that is sensitive to a serine protease inhibitor, diisopropyl fluorophosphate (DFP), but not a proteasome inhibitor, MG-132, and that its degradation is not catalyzed in the acidic pH range where lysosomal enzymes are active. In the present study, we purified an HNE-modified GAPDH-degrading enzyme from a U937 cell extract to a final active fraction containing two proteins of 28 kDa (P28) and 27 kDa (P27) that became labeled with [3H]DFP. Using peptide mass fingerprinting and a specific antibody, P28 and P27 were both identified as cathepsin G. The degradation activity was inhibited by cathepsin G inhibitors. Furthermore, a cell extract from U937 cells transfected with a cathepsin G-specific siRNA hardly degraded HNE-modified GAPDH. These results suggest that cathepsin G plays a role in the degradation of HNE-modified GAPDH.