HYSTERESIS IN THE BEHAVIOR OF BOVINE HEART MITOCHONDRIAL COMPLEX-I - KINETIC AND THERMODYNAMIC PARAMETERS OF THE SLOW REVERSIBLE ACTIVE INACTIVE TRANSITION/

Isolated Complex I exists in two forms. The active form catalyzes the rapid rotenone-sensitive and N-ethylmaleimide-insensitive NADH: Q(1)-r eductase reaction. The inactive form of the enzyme catalyzes rotenone- sensitive ubiquinone reduction with a prominent lag phase and is inhib ited by N-ethylmaleimide. 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 activa tion is temperature dependent (the activation energy is 170 kJ/mole) a nd is influenced by pH and divalent cations. The enzyme is quite stabl e (hours at 0 degrees C) but is spontaneously deactivated at high temp erature (the activation energy is 245 kJ/mole). The spontaneous deacti vation rate is not affected by pH, divalent cations, or the redox stat e of the enzyme. The active/inactive transition parameters are qualita tively and quantitatively similar for the isolated Complex I and NADH: ubiquinone-reductase of submitochondrial particles (SMP). The activat ion of isolated Complex I in the presence of NADH depends on the conce ntration of the added quinone. The concentration of quinone needed for half-maximal activation of the enzyme is one order of magnitude less than the K-m value for Q(1) (water-soluble homolog of ubiquinone) in t he steady-state rotenone-sensitive NADH: Q(1)-reductase reaction. The data suggest that the free energy of NADH oxidation by quinone in the respiratory chain is partly utilized to maintain the catalytically com petent Complex I conformation.