## 4 Natural Antioxidants and Their Role in Cosmetics
### Peptides, Amino Acids, Enzymes
#### 4.5.2 N-Acetyl-L-Cysteine  
N-acetyl-L-cysteine (NAC) (Figure 5) is a derivative of L-cysteine, a sulfur amino acid naturally occurring in the body. It is the most absorbable form of the amino acid and plays a key role in the production of glutathione (in combination with glutamine and glycine), responsible for antioxidant processes. NAC plays an important role in the synthesis of proteins, cofactors, and enzymes and is also a component of structural proteins [140]. Studies have shown that NAC is used in autoimmune diseases and during chronic inflammation, which is accompanied by oxidative stress. This amino acid can help  
Figure 5: Chemical structures of antioxidant peptides and amino acids, (**16**) glutathione and (**17**) N-acetylcysteine, and antioxidant hormones, (**18**) melatonin and (**19**) dehydroepiestosterone.  
reduce inflammation and slow down autoimmune tissue destruction (e.g., thyroid). By reducing oxidative stress, it affects the production of proinflammatory cytokines and has antioxidant effects [141].  
N-acetylcysteine (NAC) is both a thiol compound and a mucolytic agent. Upon entering cells, NAC is quickly metabolized into cysteine, which serves as a critical building block for the synthesis of glutathione (GSH). It plays a key role in maintaining the cell's redox balance, protecting against oxidative stress. Additionally, GSH interacts with N-methyl-D-aspartate (NMDA) receptors by binding to the glutamate recognition site, allowing it to act as a natural neuromodulator and influence cellular signaling processes. This dual functionality of NAC--supporting glutathione production and influencing neuromodulation--makes it a versatile compound in therapeutic applications. Cysteine plays a dual role, not only as a precursor for glutathione (GSH) synthesis but also as an independent free radical scavenger. It effectively neutralizes reactive oxygen species (ROS), such as hydroxyl radicals (OH), hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), and superoxide anions (O\({}^{2-}\)), which can otherwise cause damage to DNA, proteins, and lipids within cells. As a result, N-acetylcysteine (NAC) is recognized as both an antioxidant and a free radical scavenger. By boosting intracellular levels of cysteine, NAC enhances GSH production, amplifying its protective effects against oxidative stress and cellular damage. N-acetylcysteine (NAC) plays a beneficial role in managing atopic dermatitis due to its ability to reduce free radical species and mitigate oxidative stress. This reduction in oxidative damage supports the restoration of cell adhesion molecules, which are essential for maintaining a healthy skin barrier. Since atopic dermatitis is partly associated with increased TEWL and decreased skin hydration, NAC appears to address these underlying issues, making it a promising therapeutic option for improving skin health in individuals with this condition [142]. N-acetylcysteine (NAC) has shown potential in protecting melanocytes from the harmful effects of UV-induced oxidative stress and damage. In in vitro studies, NAC shields melanocytes by reducing the oxidative stress caused by UV exposure, which can otherwise lead to cellular damage and dysfunction. Additionally, in vivo studies suggest that NAC may help prevent UV-induced melanoma by minimizing oxidative damage and supporting the natural defense mechanisms of melanocytes. This dual protective role highlights NAC's potential as a therapeutic agent for skin health and as a preventive measure against UV-related skin cancers [143].  
The chemical synthesis of N-acetyl-L-cysteine involves the acetylation of L-cysteine, followed by purification steps to ensure high-quality NAC suitable for cosmetic use [144].