Glutathione Depletion in Mitochondrial Diseases

Author: Ishika Jaitly

Mitochondria are cellular substructures that function as energy powerhouses for almost all eukaryotic cells, including those of humans [1]. The number of mitochondria in a cell can vary widely [2]. Red blood cells, for example, have none, whereas liver cells can contain up to two thousand [2]. Mitochondria are integral to our survival due to the variety of roles they play in our biochemistry, the most important of which is the production of adenosine triphosphate (ATP) by aerobic respiration [1]. This process involves oxidative phosphorylation, which utilizes oxygen to release the energy contained in the foods we eat. The energy is initially stored in ATP, which subsequently releases this energy to power the body’s metabolic reactions. During this process, ATP is broken down into adenosine diphosphate (ADP) and inorganic phosphate (P_i).

Since mitochondria are essential to our very existence, it is unsurprising that the effects of mitochondrial diseases can be severe, and sometimes lethal, spanning a wide-ranging spectrum of physiological and biochemical symptoms [1,3,4] (see Table 1 for examples). Mitochondrial diseases in humans can arise from a myriad of mutations in mitochondrial and nuclear (cellular) DNA [4]. Collectively, mitochondrial diseases are reported to affect 1 in every 5,000 adults, making them quite prevalent in comparison to other hereditary metabolic disorders [4].

Table 1. Some well-known clinically defined primary mitochondrial diseases and their respective symptoms.

Mitochondrial diseases are characterized by both a diminished capacity to produce ATP as well as an elevated production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) [3]. Commonly known as free radicals, ROS and RNS are naturally produced during oxidative phosphorylation and serve an essential role as signalling molecules in the normal functioning of a cell [4]. However, a dysregulated increase in the level of these free radicals, as seen in mitochondrial diseases, can result in oxidative damage by overwhelming the cellular antioxidant defence mechanism, of which glutathione is the principal component [4]. The consequent depletion of glutathione [3] impedes the efficient quenching of free radicals, leading to redox imbalance and oxidative stress [4]. This, in turn, can compromise the structural and functional integrity of the cell’s nucleic acids, proteins and lipids [4]. As a result, the activity of the electron transport chain (ETC) is hindered, decreasing ATP production during oxidative phosphorylation [8]. Since the cellular synthesis of glutathione is ATP-dependent, decreased ETC activity also undermines glutathione synthesis, further exacerbating oxidative stress in a vicious cycle [8]. Hence, mitochondrial diseases present as a two-front attack on cellular glutathione levels; the excess free radicals deplete glutathione reserves, and the decreased ATP production slows glutathione replenishment, ultimately resulting in chronic glutathione deficiency [8].

The appreciably high incidence of mitochondrial diseases highlights a growing need to devise effective therapeutic strategies. With the current absence of technologies to rectify the actual mutations that underlie mitochondrial diseases, medical science can only address the resulting oxidative stress as a potential therapeutic option. Therefore, therapy primarily focuses on providing patients with supportive care to manage their symptoms using dietary supplements [9]. Commonly prescribed or recommended dietary supplements include coenzyme Q₁₀ and vitamin B, C, E and K [9,10]. Unfortunately, no concrete evidence of their efficacy exists to support their beneficial use in addressing the symptoms of mitochondrial diseases [9, 10].

Given the strong association between glutathione depletion and the progression of mitochondrial diseases, dietary supplements with the potential to increase intracellular glutathione reserves have been the subject of much research in recent years [11]. One of the prime candidates has been N-acetylcysteine (NAC), which is the gold standard antidote for acute acetaminophen (paracetamol) overdose. In this particular case, cysteine is severely depleted, and NAC helps to restore cellular glutathione levels to homeostasis by providing a source of cysteine. However, no clinical trials have been reported that provide any evidence of its effectiveness for raising the chronically low homeostatic level of glutathione that occurs in primary mitochondrial diseases [1, 11]. Additionally, in the context of lowered ATP production, which is a hallmark of mitochondrial diseases, it is important to note that NAC, essentially only being a form of cysteine, will require 2 ATP molecules to be converted to glutathione. This is because cysteine is at the very beginning of the two-step cellular synthesis of glutathione (see Figure 1). For a detailed explanation, read our article on NAC https://www.glutathionereporter.com/n-acetylcysteine-nac-cannot-increase-cellular-glutathione-levels/.

Figure 1. The two sequential ATP-dependent reactions by which glutathione is synthesized inside cells, with the enzyme catalyzing the reaction and the number of ATP molecules required for the reaction to occur, displayed above and below the arrows respectively.

Unlike NAC, dietary supplementation with gamma-glutamylcysteine (GGC), the immediate precursor to glutathione, can elevate intracellular glutathione levels above homeostasis [12]. This increased glutathione pool may help contend with the increased flow of free radicals from dysfunctional mitochondria. Moreover, the conversion of GGC to glutathione only requires 1 molecule of ATP as it is the product of the first step in glutathione synthesis, as shown in Figure 1. This low energy feature of GGC may be particularly useful for overcoming the ATP inadequacy that occurs in mitochondrial diseases. Finally, GGC may be capable of acting as an antioxidant in its own right by removing key ROS such as hydrogen peroxide and superoxide anions independently of glutathione [13]. This may be advantageous in severe cases of mitochondrial diseases where there could be insufficient ATP available to synthesize glutathione from GGC.

In summation, GGC’s potential to elevate intracellular glutathione levels above homeostasis in an energy-efficient manner, makes it a promising candidate as a potential therapeutic for mitochondrial diseases.

Reference

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