Susceptibility of glucose-6-phosphate dehydrogenase modified by 4-hydroxy-2-nonenal and metal-catalyzed oxidation to proteolysis by the multicatalytic protease

Glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides is inactivated when exposed to metal-catalyzed oxidation or when modified by the lipid peroxidation product, 4-hydroxy-2-nonenal (HNE). Although in each case inactivation appears to be the result of the selective modification of an active site lysine residue, only the oxidized enzyme becomes more susceptible to proteolysis by purified rat liver multicatalytic protease, a multienzymatic proteolytic complex involved in the intracellular degradation of damaged proteins. The HNE-treated enzyme remains as resistant to proteolysis by the multicatalytic protease as the native enzyme. In contrast to the HNE-treated Glu-6-PDH, enzyme modified by Fe²⁺ and citrate is more thermolabile and exhibits increased binding of the hydrophobic probe 8-anilino-1-naphtalene sulfonic acid (ANSA). Heat inactivation is characterized, in part, by dissociation of the dimer to inactive subunits. No change in the secondary structure and only small variations in the fluorescence and circular dichroism of the aromatic residues are observed for the two modified forms of the enzyme as compared with the native enzyme. The increased heat sensitivity, ANSA binding, and proteolytic susceptibility are likely related to a decrease in the structural stability of oxidatively modified Glu-6-PDH. Conversely, modification of Glu-6-PDH with HNE has no apparent effect on its structural stability or proteolytic susceptibility. This finding may have important implications for the accumulation of altered protein in vivo, a process that is believed to be involved in age- and disease-related impairment of cellular function.