Diminished susceptibility to proteolysis after protein modification by the lipid peroxidation product malondialdehyde: inhibitory role for crosslinked and noncrosslinked adducted proteins.

The lipid peroxidation product malondialdehyde forms adducts with proteins that are detected during routine assays for protein carbonylation. To test whether this damage alters the susceptibility of a protein to proteolysis, we treated bovine serum albumin with various concentrations of malondialdehyde and examined its susceptibility to digestion by alpha-chymotrypsin. In keeping with findings concerning the consequences of protein damage by other carbonyl products of lipid peroxidation, we found that malondialdehyde-modified protein was resistant to proteolysis. Since significant protein crosslinking occurred during modification with malondialdehyde, we investigated the possibility that crosslinked proteins were acting as proteolytic inhibitors. Malondialdehyde-modified proteins were resolved into crosslinked and noncrosslinked forms and the effectiveness of both species as proteolytic antagonists was examined. While both forms of malondialdehyde-adducted proteins were more potent proteolytic inhibitors than unmodified albumin, there were no significant differences in inhibitory potency between crosslinked and noncrosslinked proteins. Our findings suggest that malondialdehyde-modification produces protease-resistant proteins without an obligatory role for crosslinking.