CYSTINE is one of some twenty substances (amino-acids) which are produced when the proteins of food are digested in the alimentary canal. Together with its congeners it is liberated from proteins by any process which involves hydrolysis, and it is ob tained when any typical protein is boiled with mineral acids (see PROTEINS). It differs from all but one other among the constituent amino-acids of protein in that it contains sulphur, that element being present in relatively unstable combination.
Cystine is almost insoluble in neutral aqueous media, a circum stance which led to its discovery and affects its behaviour in the animal body. Long before it had been shown to be derived from protein, it was discovered as the main constituent of a urinary calculus removed from a human subject (Wollaston, 181o), and although relatively rare, calculi composed of cystine have since been many times observed. It is occasionally found in human urine when it tends to form a white crystalline deposit. In such cases a highly interesting anomaly of metabolism exists. While the body of a normal individual is capable of completely oxidizing all the amino-acids to which the breakdown of protein gives rise, there are rare cases in which this power is deficient. The failure applies especially to the oxidation of cystine. As a result consider able amounts of this substance appear unaltered in the urine. Be cause of its low solubility it may be deposited in the tissues or as calculi. Those who display this condition suffer from a definite deficiency in the chemical make-up of their bodies. A specific oxidizing mechanism fully active in normal people is in their case imperfectly developed. This is a congenital anomaly commonly transmitted (perhaps on Mendelian lines) from parents to children.
The amount of cystine contained in individual proteins varies from less than I% to more than 3%. A due supply of cystine in the diet is essential to nutrition. Proteins deficient in this con stituent may show a relatively low nutritive value. Keratin, a much modified protein, which is the basal constituent of horn and hairs, is characteristically rich in cystine, containing 8% and up wards. Natural cystine when pure is a snow-white substance con sisting of highly characteristic microscopic crystals in the form of hexagonal plates. It has an exceptionally high rotatory power on polarized light [a],,= — in normal hydrochloric acid. The structural formula and that of the related cysteine are as follows : It is characteristic of compounds containing a disulphide group ing to yield upon reduction two molecules of the corresponding compound containing the thiol or —SH group. These relations are displayed in the biological behaviour of cystine (see GLUTA THIONE).
See A. E. Garrod, Inborn Errors of Metabolism (1923).