Effects of rotenone and pyridaben on complex I electron transfer and on mitochondrial nitric oxide synthase functional activity

Rotenone and pyridaben were tested on activities and properties of rat brain mitochondria determining Ki (inhibitor concentration at half maximal inhibition) and Imax (% of inhibition at maximal inhibitor concentration). The assayed activities were complexes I, II and IV, respiration in states 3, 3u (uncoupled) and 4, biochemical and functional activities of mitochondrial nitric oxide synthase (mtNOS), and inner membrane potential. Selective inhibitions of complex I activity, mitochondrial respiration and membrane potential with malate-glutamate as substrate were observed, with a Ki of 0.28–0.36 nmol inhibitor/mg of mitochondrial protein. Functional mtNOS activity was half-inhibited at 0.70–0.74 nmol inhibitor/mg protein in state 3 mitochondria and at 2.52–2.98 nmol inhibitor/mg protein in state 3u mitochondria. This fact is interpreted as an indication of mtNOS being structurally adjacent to complex I with an intermolecular mtNOS-complex I hydrophobic bonding that is stronger at high Δψ and weaker at low Δψ.