ScienceSeptember 29, 2020

Predicting Uncoupling Toxicity of Organic Acids

  Mitochondria are the powerhouse of the cell: After oxidizing nutrients, the…
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Andrea Ebert

Mitochondria are the powerhouse of the cell: After oxidizing nutrients, the energy gained is used to transport protons across the inner mitochondrial membrane. Similar to a battery, energy is stored in the resulting proton gradient. When needed, this energy is directly transformed into adenosine triphosphate (ATP)—which provides energy for cell processes.

But what happens if a so-called “uncoupler” disturbs this tightly regulated balance?

Just like an electrical short-circuit, an uncoupler may disperse the proton gradient by shuttling protons from one side to the other. The stored energy simply dissipates to heat, without the generation of ATP.

The consequences will strongly depend on the dose. At low concentrations, an increased metabolic rate can compensate for the loss of energy. At higher concentrations, the lack of ATP production, the produced heat and the collapse of the proton gradient may be detrimental.

The administration of an uncoupler can therefore be beneficial.  Take for example the famous uncoupler 2,4-dinitrophenol. In the1930s, it gained popularity as a fat burner to treat obesity, but was later taken off the market due to severe overdose side effects, including death.

Also today, the search for mild uncouplers (with less adverse effects) for pharmaceutical use is ongoing. At the same time, toxic uncoupling activity may as well be a side effect of other potential drug candidates. When we discover toxicity in the development process early, we reduce the number of rejected drug candidates and significantly lower costs.

A Mechanistic Model to predict Uncoupling Activity

It is thus of great importance to anticipate a potential uncoupling activity. While models that predict uncoupling activity already exist, they usually rely on empirical concepts. In consequence, they can only be applied to very similar chemical compounds, and are limited to the specific experimental conditions for which they were trained.

Yet, the uncoupling activity can strongly depend on the environment – an important fact to remember when testing chemicals for their uncoupling activity. An uncoupler may show toxic activity in one experimental setting or test system, while being completely harmless in the next. For example, the uncoupling activity may decrease with increasing experimental pH. Conventional models are not able to predict or explain these effects.

UFZ scientists have developed a biophysical model to predict pH-dependent uncoupling toxicity of organic acids from their chemical structure. They used BIOVIA COSMOtherm and TURBOMOLE to calculate the necessary input parameters, such as pKa, the compound specific membrane permeability and dimer stability constants. Due to its mechanistic nature and the ab initio approach of the quantum chemical and COSMO-RS calculations, the model should not be limited to specific substance classes, but should provide a universal screening tool for uncoupling toxicity assessment.

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