Selenocysteine, often termed the 21st amino acid, is directly incorporated into several essential selenoproteins that play crucial roles in mitochondrial function. The primary scientific justification for its use in supporting the mitochondrial system stems from its presence in enzymes such as glutathione peroxidases (GPXs) and thioredoxin reductases (TRXRs). These selenoenzymes are critical for cellular antioxidant defense, specifically in neutralizing reactive oxygen species (ROS) generated as byproducts of mitochondrial respiration. Excessive ROS can impair mitochondrial DNA, proteins, and membranes, ultimately affecting energy production and cellular health.

The role of selenocysteine-containing enzymes in mitochondrial redox homeostasis is well-established. For example, mitochondrial GPX4 is vital for preventing lipid peroxidation within mitochondrial membranes, and its deficiency leads to increased oxidative stress and mitochondrial dysfunction. Scientific studies, including genetic knockout models, have demonstrated that loss of selenoprotein activity results in compromised mitochondrial integrity and function. Furthermore, selenium supplementation (as a precursor to selenocysteine) has been shown to support mitochondrial enzyme activity in both animal and human studies, although optimal dosing and clinical outcomes are still areas of active research.

In summary, there is robust scientific evidence supporting the importance of selenocysteine in maintaining mitochondrial health, largely due to its role in antioxidant defense and protection against oxidative damage within the mitochondria.