Cysteine is classified as a nonessential amino acid, but cysteine may be essential for infants, the elderly, and individuals with certain metabolic diseases or malabsorption syndromes. Cysteine is an important structural and functional component of many proteins and enzymes. Cysteine is named after cystine, its oxidized dimer. Cysteine is potentially toxic and is catabolized in the gastrointestinal tract and blood. In opposition, cysteine is absorbed during digestion as cystine, which is more stable in the gastrointestinal tract. It is cystine that travels to cells, where it is reduced to two cysteine molecules upon cell entry. Cysteine is used as a constituent in the food, pharmaceutical, and personal care industries. One of its largest applications is in the production of flavors.
| Micromole per liter | µmol/L = mcmol/L = umol/L = µM/L = mcM/L = uM/L = micromol/L |
| Milligram per liter | mg/L = millig/L = milligram/L = mg/liter |
| Milligram per deciliter | mg/dL = millig/dL = milligram/dL |
| Milligram per 100 milliliters | mg/100mL = millig/100mL = milligram/100mL |
| Milligram percent | mg% = millig% = milligram% |
| Microgram per milliliter | µg/mL = mcg/mL = ug/mL = microg/mL = microgram/mL |