[Hysteresis behavior of complex I from bovine heart mitochondria: kinetic and thermodynamic parameters of retarded reverse transition from the inactive to active state].

Isolated Complex I exists in two forms. The active form catalyzes the rapid rotenone-sensitive, N-ethylmaleimide-insensitive NADH : Q1 reductase reaction. The inactive form is inhibited by N-ethylmaleimide and catalyzes the rotenone-sensitive ubiquinone reduction with a prominent lag phase. The inactive enzyme is transformed into its active form after rapid reduction by NADH and slow (compared with the steady-state turnover number) oxidation by quinone. The rate of activation is strongly temperature-dependent (the activation energy is 170 kJ/mol) and influenced by pH and divalent cations. The active enzyme is quite stable (hours at 0 degrees C) but it is spontaneously deactivated at high temperature (the activation energy is 245 kJ/mol). The active/inactive transition parameters are qualitatively and quantitatively similar for the isolated and membrane-bound Complex I. The extent of the isolated Complex I activation in the presence of NADH depends on the concentration of the added quinone. The concentration of quinone needed for the half-maximal activation is 10 times less than the KQ1m value in the steady-state NADH : Q1 reductase reaction. The data obtained suggest that the free energy of NADH oxidation in the respiratory chain is partly utilized to maintain the catalytically competent Complex I conformation.