Binding of inhibitor, NADH-ubiquinone oxidoreductase (complex I) leads to Inhibition, NADH-ubiquinone oxidoreductase (complex I)

It is well documented that binding of an inhibitor to CI inhibits its activity (see MIE). Naturally occurring and synthetic CI inhibitors have been shown to inhibit the catalytic activity of CI, leading to partial or total inhibition of its activity in a dose response manner (Degli Esposti and Ghelli, 1994; Ichimaru et al. 2008; Barrientos and Moraes, 1999; Betarbet et al., 2000). Indeed, binding of inhibitors stops the electron flow from CI to ubiquinone. Therefore, the Fe-S clusters of CI become highly reduced and no further electrons can be transferred from NADH to CI. This leads to the inhibition of the NADH oxido-reductase function, i.e. CI inhibition.

The kinetics of binding and CI inhibition by rotenone has been quantitatively evaluated in a dose-dependent manner using the sub-mitochondrial particles (Grivennikova et al., 1997). The consequences of CI inhibition were quantitatively measured by a variety of assays that are used to study mitochondrial dysfunction (see Key Event Relationship (KER): Inhibition of Complex I leads to mitochondrial dysfunction). There are also many in vitro and in vivo studies combining the quantification of CI inhibition and DA cell death (e.g. Choi et al., 2008, Betarbet et al., 2000, see KER Mitochondrial dysfunction induces degeneration of nigrostriatal pathway).

The binding of different classes of inhibitors (e.g., pesticides, drugs and other toxins) to CI has been determined quantitatively and I50, and KI values are available. Potency relative to that of rotenone has been determined under the same conditions in beef mitochondria or submitochondrial particles using the ratio of the KI values, when they were available (Degli Esposti, 1998; Okun et al., 1999). Rotenone I50 value is defined as 20 nM (Okun et al., 1999).

Example of a quantitative evaluation of concentration-dependent CI inhibition by rotenone (Fig. 1 from Barrientos and Moraes, 1999).

Fig. 1. Fig.1. Effect of CI (NADH decylubiquinone reductase) inhibition on endogenous cell respiration. Cells were treated with different concentrations of rotenone for 4 h before measuring cell respiration in whole cells and CI activity in isolated mitochondria. Complete CI inhibition was achieved with 100 nM rotenone. The cell respiration was inhibited also in a dose-dependent manner but showed different inhibition kinetics and a saturation threshold. For comparison, the genetically-altered cell line HXC had an approximately 40% CI reduced activity and an approximately 80% residual cell respiration. HXC, human xenomitochondrial cybrids.

Time- and concentration-relationship of NADH oxidase inhibition by rotenone (Fig. 2. from Grivennikova et al., 1997).

Fig. 2. Panel A and B: Time- and concentration-relationship of NADH oxidase inhibition by rotenone. The numbers on the curves indicate the final concentrations of rotenone (0, 20, 30, 40, 1000 nM). In Panel B: vo, zero-order rate of NADH oxidation in the absence of rotenone; vt, the `instant' values of the rates approximated within 10 s time intervals. Panel C: The dependence of first-order inhibition rate constant on the concentration of rotenone (for further description see Fig. 1 in Grivennikova et al., 1997).

Quantitative evaluation of the 1st KER: Binding of inhibitor to NADH-ubiquinone oxidoreductase (MIE; KE upstream) leads to its inhibition (KE downstream).

The CI is well-conserved across species from lower organism to mammals. The central subunits of CI harboring the bioenergetic core functions are conserved from bacteria to humans. CI from bacteria and from mitochondria of Yarrowia lipolytica, a yeast genetic model for the study of eukaryotic CI (Kerscher et al., 2002) was analyzed by x-ray crystallography (Zickermann et al., 2015). However, the affinity of various chemicals to cause partial or total inhibition of CI activity across species is not well studied (except rotenone).