Cystine

Cystine is the disulfide-linked dimer of the amino acid cysteine. It forms when two cysteine molecules are oxidized and their thiol (-SH) groups form a covalent S-S bond. The molecule has formula C6H12N2O4S2. In biological contexts, cystine is the oxidized form of cysteine and is often referred to as such.

In proteins, disulfide bonds between cysteine residues create cystine crosslinks that stabilize tertiary and quaternary structure. These bonds are particularly abundant in extracellular proteins, such as keratins that make up hair, wool, nails, and the protective outer tissues.

Physiologically, cystine/cysteine interconvert via cellular redox reactions. Cystine is typically reduced to cysteine inside cells, where cysteine serves as a precursor for proteins and the synthesis of glutathione, a major antioxidant. A common transport system (system xc-) imports cystine in exchange for glutamate in many cell types.

Nutrition and disease: Cystine is not an essential amino acid; methionine can supply sulfur for cysteine biosynthesis. In humans, cystine status reflects cysteine availability. A genetic disorder called cystinuria impairs renal reabsorption of cystine, leading to elevated urinary cystine that can precipitate as kidney stones; management includes hydration and urinary alkalinization, sometimes using chelating agents.

In summary, cystine is the oxidized dimer of cysteine, central to disulfide bond formation that stabilizes proteins and contributes to the structure of keratin-containing tissues, while also playing roles in cellular redox balance and metabolism.