KINETICS OF THE INHIBITION OF MITOCHONDRIAL RESPIRATION BY NO

The kinetics of the inhibition of mitochondrial respiration by NO was examined in isolated mitochondria (here obtained from rat brown adipos e tissue). The Kj of NO for the inhibition was similar to 27 nM; the I C50 of NO increased in proportion to the square of an Increase in O-2 tension. The K-m of O-2 for respiration was similar to 16 mu M; in the presence of NO, the dependence of respiration on O-2 tension had a Hi ll coefficient of similar to 2. The unusual kinetics is probably relat ed to the ability of cytochrome c oxidase to use 2 NO or 1 O-2 as elec tron acceptor. The interaction between NO and O-2 in the control of re spiration could be described by the formula V-o2(O-2, NO) = V(o2)max . ([O-2](2)/((16 mu M .(1+[NO]/27 nM))(2) + [O-2](2))). Thus, the kineti cs is such that respiration in the presence of physiological levels of NO is very sensitive to decreasing O-2 tension. (C) 1997 Federation o f European Biochemical Societies.