Cysteine: Cysteine is amino-mercaptorpropionic acid and cystine is dithio-aminopropionic acid. Metabolism of cystine is through the degradation of cysteine. Cysteine takes part in formation of pyruvic acid hydrogen sulphide and ammonia, formation of glutamic acid, and formation of taurine. It takes part in detoxication of halogenated aromatic hydrocarbons. Cysteine is essential for the proper alignment and attachment of the peptide chains forming the triple helix of tropocollagen.
While cystine itself is not an essential amino acid, but is formed from essential methionine and nonessential serine. The methionine is first converted to homocysteine and then to cysteine. The serine forms homoserine.
In this conversion sulphur of cysteine derives from methionine by transsulfuration, and the carbon skeleton is provided by serine. Homocysteine is a normal building block for protein, and too much can damage your arteries. Vitamin B-6, B-12 and Folate can lower it.
Like nitrogen and carbon, sulphur is constinuously recycled through the biosphere by means of the combined metabolic activities, leading to inorganic sulphur compounds. The urinary sulphur is derived principally from cysteine. The cysteine is converted to cysteine, catalysed by cystein reductase. The cysteine is converted to pyruvate involving intitial oxidative reaction.
An inherited metabolic disease, cystinuria, has urinary excretion of cysteine 20-30 times the normal. The excretion of lysine, arginine, and ornithine is also highly increased, indicating a renal disorder. Because of 4 amino acids involved, this disease is better known as cystine-lysinuria. The cystine being partially soluble may precipitate in the kidney, but the disulphide of homocystein and cysteine being more soluble, reduces the tendency of formation of cystine crystals, and calculi.
Another inherited disease Cystinosis, the cystine crystals are deposited in many tissues and organs, with aminoaciduria, impaired renal functions, bringing early death. A condition of Homocystinurias, though very rare, with high momocystine excretion in urine, and raised serum methionine levels involve at least 4 metabolic defects. It manifests in thrombosis, osteoporosis, dislocated eye lenses, and mental retardation. It could be vitamin B-6 responsive or monreponsive. A diet low in methionine and high in cystine may help in early stage.
Sources of Cysteine: The main dietary sources to get cystine are wheat, varagu, bajra, horse gram, soyabean, drumstick leaves, agathi, colocasia, beetroot, jack, French beans, gingelly seeds, linseed, banana, persimmon, egg, and milk.
