Some chemicals (protonophores) can transport protons or other ions (ionophores) from the intermembrane space through the membrane into the mitochondria matrix, bypassing the proton channels of ATP synthase. As a result, the electrochemical potential reduces, and ATP synthesis stops. This phenomenon is called the uncoupling of respiration and phosphorylation. Due to dissociation, the amount of ATP decreases, while ADP increases. In this case, the oxidation rate of NADH and FADH2 increases, the amount of absorbed oxygen rises, but energy is released in the form of heat, and the P/O ratio decreases sharply. Mostly, uncouplers are lipophilic substances that easily pass through the lipid layer of the membrane. In particular, 2,4-dinitrophenol is an example of uncouplers, which easily transforms from the ionized form to the non-ionized one, attaching a proton in the intermembrane space and transferring it to the matrix.
Examples of uncouplers include some drugs, for instance, dicoumarol, an anticoagulant or metabolites, which are formed in the body( i.e., bilirubin), and thyroxin. All these substances exhibit an uncoupling effect only when they are highly concentrated.