L-cystine is the L-enantiomer of the sulfur-containing amino acid cystine. It has a role as an EC 1.2.1.11 (aspartate-semialdehyde dehydrogenase) inhibitor, a flour treatment agent, a mouse metabolite, a Saccharomyces cerevisiae metabolite and a human metabolite. It is a cystine, a non-proteinogenic L-alpha-amino acid and a L-cysteine derivative. 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. L-Cystine can improve hyperlipidemia by restoring LPL and HSL activities.L-Cystine protects against the toxicity of PQ by maintaining reduced glutathione levels in the cells. Co-administration of l-cystine and l-theanine before vaccination may enhance the immune response to influenza vaccine in elderly subjects with low serum total protein or hemoglobin.