DIMINISHED SUSCEPTIBILITY TO PROTEOLYSIS AFTER PROTEIN MODIFICATION BY THE LIPID-PEROXIDATION PRODUCT MALONDIALDEHYDE - INHIBITORY ROLE FORCROSS-LINKED AND NONCROSSLINKED ADDUCTED PROTEINS

The lipid peroxidation product malondialdehyde forms adducts with prot eins that are detected during routine assays for protein carbonylation . To test whether this damage alters the susceptibility of a protein t o proteolysis, we treated bovine serum albumin with various concentrat ions of malondialdehyde and examined its susceptibility to digestion b y alpha-chymotrypsin. In keeping with findings concerning the conseque nces of protein damage by other carbonyl products of lipid peroxidatio n, we found that malondialdehyde-modified protein was resistant to pro teolysis. Since significant protein crosslinking occurred during modif ication with malondialdehyde, we investigated the possibility that cro sslinked proteins were acting as proteolytic inhibitors. Malondialdehy de-modified proteins were resolved into crosslinked and noncrosslinked forms and the effectiveness of both species as proteolytic antagonist s 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 crossli nked and noncrosslinked proteins. Our findings suggest that malondiald ehyde-modification produces protease-resistant proteins without an obl igatory role for crosslinking. (C) 1997 Academic Press.