Pyridoxamine analogues scavenge lipid-derived gamma-ketoaldehydes and protect against H2O2-mediated cytotoxicity

Isoketals and levuglandins are highly reactive gamma-ketoaldehydes formed by oxygenation of arachidonic acid in settings of oxidative injury and cyclooxygenase activation, respectively. These compounds rapidly adduct to proteins via lysyl residues, which can alter protein structure/function. We examined whether pyridoxamine, which has been shown to scavenge alpha-ketoaldehydes formed by carbohydrate or lipid peroxidation, could also effectively protect proteins from the more reactive gamma-ketoaldehydes. Pyridoxamine prevented adduction of ovalbumin and also prevented inhibition of RNase A and glutathione reductase activity by the synthetic gamma-ketoaldehyde, 15-E2-isoketal. We identified the major products of the reaction of pyridoxamine with the 15-E2-isoketal, including a stable lactam adduct. Two lipophilic analogues of pyridoxamine, salicylamine and 5'-O-pentylpyridoxamine, also formed lactam adducts when reacted with 15-E2-isoketal. When we oxidized arachidonic acid in the presence of pyridoxamine or its analogues, pyridoxamine-isoketal adducts were found in significantly greater abundance than the pyridoxamine-N-acyl adducts formed by alpha-ketoaldehyde scavenging. Therefore, pyridoxamine and its analogues appear to preferentially scavenge gamma-ketoaldehydes. Both pyridoxamine and its lipophilic analogues inhibited the formation of lysyl-levuglandin adducts in platelets activated ex vivo with arachidonic acid. The two lipophilic pyridoxamine analogues provided significant protection against H2O2-mediated cytotoxicity in HepG2 cells. These results demonstrate the utility of pyridoxamine and lipophilic pyridoxamine analogues to assess the potential contributions of isoketals and levuglandins in oxidant injury and inflammation and suggest their potential utility as pharmaceutical agents in these conditions.