P4‐143: Mitochondrial aconitase of aged and Alzheimer brain as a target for endogenous anti‐ and pro‐oxidants

Mitochondrial aconitase (m-aconitase) is a Krebs cycle enzyme that catalyzes reversible interconversion of citrate to isocitrate through cis-aconitate. Due to susceptibility of m-aconitase [4Fe-4S]2+ cluster to redox-dependent modulation, the enzyme might serve as a potential target for anti- and pro-oxidants. Pathogenesis of Alzheimer's disease (AD) and familial AD (FAD) is related to severe oxidant-antioxidant imbalance in brain mitochondria. This study aimed at elucidation of m-aconitase susceptibility to mitochondrially active pro- and antioxidants in control (Co), AD and Swedish FAD (SFAD) frontal (FC) and occipital primary (OPC) cortex. Postmortem brain tissues were provided by Huddinge Brain Bank (Sweden). Mitochondria were isolated according to Rajapakse et al. (2001). M-aconitase activity was measured by monitoring formation of cis-aconitate at 240 nM. In the more damaged FC of SFAD and AD brain, the enzyme activity was decreased compared with Co region whereas in the less damaged OPC of SFAD and AD brain the parameter was not changed. Oxidants Ab(25-35), H2O2 and homocysteine (Hcy) dose-dependently inhibited m-aconitase. In Co FC, maximal inhibitory effect increased in sequence: 0.5 mM Hcy (15%) < 1mM H2O2 (34%) < 0.001 mM Ab(25-35) (40%). In AD FC, Ab(25-35) and H2O2 induced weaker enzyme inhibition (26%) than in Co FC whereas in SFAD FC inhibition was lowest (18%). Co and AD brain OPC showed lower inhibition of m-aconitase by oxidants than FC. SFAD brain revealed no regional difference of inhibition. In contrast to oxidants, the antioxidants melatonin (Mel), glutathione (GSH) and 17-b-estradiol (17bE) stimulated m-aconitase in Co, AD and SFAD brain. In Co FC, maximal stimulatory effect increased in sequence: 0.01 mM 17bE (21%) < 1.0 mM GSH (46 %) < 0.01 mM Mel (67%). In SFAD FC, the effects were weaker than in Co FC. In OPC, antioxidant-induced stimulation of m-aconitase did not differ significantly from that in FC. Thus, m-aconitase of human brain reveals opposite response to endogenous pro- and antioxidants. The mitochondrial oxidant-antioxidant relationship may play a principal role in the regulation of m-aconitase activity and, thereby, Krebs cycle rate in aged, AD and SFAD brain regions.