What it is
Cystine is an amino acid composed of two cysteine molecules joined by a disulfide bond, and it occurs naturally in structural proteins such as keratin. In cosmetics it is used as a conditioning agent for skin and hair, where the disulfide linkages relate to the structural chemistry of keratin. It is typically included at low levels within amino-acid or conditioning blends.
Safety & warnings
Cystine is a naturally occurring amino acid and is generally well tolerated in cosmetic use.
Origin & sourcing
Can be naturally sourced or produced synthetically; cystine is found in keratin and other proteins and is also manufactured for use as a cosmetic ingredient.
Questions everyone asks
How is cystine related to cysteine?
Cystine is formed when two molecules of the amino acid cysteine join through a disulfide bond; it is essentially the oxidized, paired form of cysteine.
Why is it used in skincare and haircare?
It acts as a conditioning amino acid for skin and hair, reflecting its role in the disulfide chemistry of keratin proteins.