Kinetic studies on the interaction of TMPD with cytochrome c and cytochrome c oxidase

The kinetics of the reduction of cytochrome c by ascorbate and ascorbate plus TMPD3 have been examined. TMPD increases the reduction rate of cytochrome c by a factor of 30 relative to ascorbate alone. It is concluded that this is the principal reason why TMPD increases the activity of cytochrome c oxidase at a fixed concentration of cytochrome c and markedly diminishes the apparent Km value for cytochrome c. The reduction of cytochrome oxidase by ferrocytochrome c is identical in the presence or absence of TMPD (3.5 × 107m−1 sec−1). The oxidation of TMPD in the absence of cytochrome c shows an average maximal turnover of two electrons (heme a)−1 sec−1 when cytochrome a measured at 605 mμ is over 60% reduced. This identifies a slow rate of electron transfer between a and a3. It is suggested that in the presence of cytochrome c the reaction pathway may be directly from c to a3.