Cystine

Cystine’s main pharmacological action is as a precursor for Cysteine, which helps produce glutathione, a key antioxidant. This can protect cells from damage, potentially aiding skin conditions where oxidative stress is a factor.
Cystine, the oxidized dimer of the amino acid Cysteine, is a sulfur-containing compound with significant biochemical and potential pharmacological roles, particularly in dermatology and cellular health.
Cystine is formed by the oxidation of two Cysteine molecules, linked by a disulfide bond. This process is reversible, as Cystine can be converted back to Cysteine through reduction, typically by the addition of hydrogen. This interconversion is crucial, as Cysteine is a precursor for glutathione, a tripeptide (glutamate, cysteine, glycine) known for its antioxidant properties. Glutathione helps neutralize free radicals, protecting cells from oxidative damage, which is particularly relevant in skin health and conditions like acne, where oxidative stress is implicated.
Cystine serves as a major precursor for glutathione synthesis
Cystine interacts with the cystine-glutamate antiporter (system xc-), a sodium-independent transporter that exchanges extracellular cystine for intracellular glutamate. This system is critical for maintaining cellular redox balance, as it provides cystine for glutathione synthesis and regulates glutamate levels.
Cystine is a component of keratin, a structural protein found in high quantities in hair (constituting about 5% of human hair) and nails.
Its deficiency has been shown to slow skin and hair growth, suggesting a role in maintaining tissue integrity.
In skin lightening, a randomized controlled trial found that oral supplementation of L-Cystine with reduced L-Glutathione significantly lightened dark spots, suggesting a synergistic effect.

Action	Mechanism	Relevance to Dermatology
Precursor for Glutathione Synthesis	Converted to Cysteine, supports antioxidant production	May help manage oxidative stress in skin conditions like acne
Cystine-Glutamate Transporter Interaction	Exchanges cystine for glutamate, maintains redox balance	Potential influence on skin cell function, limited direct evidence
Support for Skin, Hair, Nail Health	Component of keratin, structural role	Used in supplements for hair loss, nail strength, indirect skin benefits