Schema 6.
Inhibition of CL-activated peroxidase activity of cyt c and prevention of CL oxidation in mitochondria leading to suppression of apoptosis.
Peroxidase activity of cyt c/CL complexes leads to CL oxidation and accumulation of products required for the release of pro-apoptotic factors from mitochondria. Consequently, agents and factors that inhibit the peroxidase activity and prevent CL oxidation may act as anti-apoptotic agents. A new approach to regulate the cyt c peroxidase activity is based on the use of modified CL with an oxidizable and fluorescent 7-nitro-2,1,3-benzoxadiazole (NBD) moiety (NBD-CL). NBD-CL forms high-affinity complexes with cyt c and blocks cyt c-catalyzed oxidation of several peroxidase substrates, cyt c self-oxidation, and, most importantly, inhibits cyt c-dependent oxidation of polyunsaturated CL and accumulation of CL hydroperoxides. Mitochondrial targeting of such agents may lead to discovery of new potent drugs. Several options shown on the schema include mitochondria-targeted conjugates of nitroxide radicals (TEMPO) with hemi-gramicidin S (GS) or triphenyl-phosphonium. Specifically, GS-TEMPO is selectively accumulated in mitochondria where it acts as an electron scavenger capable of preventing superoxide formation and its dismutation into H2O2 that is necessary for CL oxidation. GS-TEMPO is also an effective anti-apoptotic agent. Mitochondria-targeted donors of nitric oxide (NO˙) – such as 2-(hydroxyamino-vinyl)-triphenyl-phosphonium (HVTP) - activatable by peroxidase activity of cyt c owe their anti-apoptotic potency to the NO˙-dependent reduction of reactive intermediates of the peroxidase cycle.