Conversion of the synthetic catalase mimic precursor TAA-1 into the active catalase mimic in isolated hepatocytes

In previous studies we reported on the catalase-like activity and antioxidative properties of a non-heme Fe(III)-tetraaza[14]annulene complex, 5,4-didehydro-5,9,14,18-tetraaza-di(2,2-dimethyl-[5,6]benzo[1,3]dioxolo)[a,h]cyclotetradecene--Fe(III) chloride (TAA-1/Fe). We proposed that intracellular application of the parent, iron-free tetraaza[14]annulene ligand, TAA-1, as precursor would allow antioxidative defense along two lines, i.e. by chelation of potentially toxic cellular iron ions and, subsequently, by catalase-mimic activity. We here set out to establish whether the active catalase mimic is indeed formed intracellularly when cells are loaded with the ligand. When isolated rat hepatocytes were preloaded with TAA-1, they were protected against iron-induced cell injury and oxidative stress elicited by exposure to the membrane-permeable iron complex Fe(III)/8-hydroxyquinoline. After lysis of the cells, followed by ultrafiltration to remove endogenous catalase, the lysate exhibited catalase-like activity, while lysates of control cells not treated with TAA-1 showed no catalase-like activity. By comparison with authentic TAA-1/Fe, an intracellular formation of 2.0 +/- 0.3 microm of the active catalase mimic in native hepatocytes exposed to TAA-1 and of 6.5 +/- 1.0 microm in hepatocytes exposed to both TAA-1 and iron ions was estimated. The intracellular formation of the active catalase mimic thus renders TAA-1 an attractive compound for protection against iron- and/or hydrogen peroxide-dependent cell injuries.