Citrulline
CITRULLINE is a non-essential amino acid involved in the urea cycle, where it is formed from ornithine of carbon dioxide and ammonia. In combination with aspartic acid, citrulline goes on to form arginosuccinic acid and, finally, arginine. Lysine is the precursor for citrulline, and citrulline is the precursor for arginine and ornithine. Being deficient in one authomatically forces a deficiency in others.
Citrulline was first isolated from watermelon, although it does not appear to be a constituent of common proteins. It influences the production of growth hormone, and is used for the detoxification of ammonia and in the treatment of fatigue. Citrulline exists primarily in the liver. It is plentiful in such plant foods as onions and garlic.
Citrullinemia is an inherited metabolic disorder of the urea cycle, caused by the lack of the enzyme arginosuccinic acid synthetase. Characteristic symptoms are elevated levels of citrulline in the blood, urine, and cerebrospinal fluid; hyperammonemia; persistent vomiting; ataxia (loss of muscle coordination), seizures; and progressive mental retardation. Until recently, this condition was fatal in just a few weeks. Supplementation of arginine and benzoic acid, plus protein restriction has led to some long-term survival, but usually with mental retardation.
