Activation of the Multicatalytic Endopeptidase by Oxidants. Effects on Enzyme Structure

Abstract

It is well established that the functional properties of proteins can be compromised by oxidative damage and, in vivo, proteins modified by oxidants are rapidly degraded. It was hypothesized that oxidants may also affect the ability of proteases to hydrolyze peptides and proteins. We therefore examined the effect of oxidants on the endopeptidase activities of the 650 kDa 20S proteasome or multicatalytic endopeptidase (MCP), which is thought to play a central role in nonlysosomal protein breakdown. Treatment of the MCP with the oxidant system, FeSO₄−EDTA−ascorbate, stimulated the peptidase activities of the MCP while H₂O₂ treatment showed little or no stimulation. However, treatment of the MCP with FeSO₄−EDTA−ascorbate or H₂O₂ stimulated proteinase activity by 480% and 730%, respectively. An endogenous activator of the MCP, PA28, stimulated the acidic, basic, and hydrophobic peptidase activities of the MCP, but had no effect on proteolytic activity. Treatment of PA28 with oxidants in the presence of MCP or alone did not greatly affect PA28's ability to activate the peptidase activities of the MCP. Using nondenaturing polyacrylamide gel electrophoresis, structural alterations in the enzyme which may be responsible for the activation of peptidase and protease activities following exposure to oxidants were investigated. Treatment of the MCP with reagents that activate proteolysis, including H₂O₂, as well as the serine protease inhibitor 3,4-dichloroisocoumarin and the cysteine protease inhibitor p-(chloromercuri)benzenesulfonic acid, all caused dissociation of the 650 kDa MCP. However, exposure to FeSO₄−EDTA−ascorbate resulted in little or no dissociation of the complex. The MCP complex dissociated by p-(chloromercuri)benzenesulfonic acid could be reassociated upon treatment with the reducing agent dithiothreitol, but dithiothreitol failed to completely reassociate 3,4-dichloroisocoumarin- or H₂O₂-treated MCP. Therefore, chemical modification of the MCP can cause activation with varying degrees of complex dissociation. These results suggest that metabolites, such as reactive oxygen species, in addition to endogenous proteins, such as PA28, are capable of modulating MCP activity.