L-cystine is the L-enantiomer of the sulfur-containing amino acid cystine. It has a role as a flour treatment agent, a human metabolite, a Saccharomyces cerevisiae metabolite, a mouse metabolite and an EC 184.108.40.206 (aspartate-semialdehyde dehydrogenase) inhibitor. It is a cystine, a L-cysteine derivative and a non-proteinogenic L-alpha-amino acid. It is a conjugate acid of a L-cystine anion. It is an enantiomer of a D-cystine. It is a tautomer of a L-cystine zwitterion.