Cystinuria, an inborn error of metabolism, is clinically characterized by the formation of l-cystine crystals and stones in the urinary tract. Sulfhydryl compounds react with the less soluble cystine to form water-soluble disulfide derivatives. l-Cystine stone formation and its undesirable side effects are thus prevented, and regulation of its metabolism results. The mechanism of action for the dissolution of l-cystine is based on a Thiol-Disulfide exchange reaction. The dissolving capacity of the sulfhydryl compounds can be enhanced by coupling them with amino acids, for example the mercaptopeptides, glutathion and Thiola. A further enhancement of the dissolving capacity of the sulfhydryl drugs may be achieved by adding a second sulfhydryl group to their molecule, resulting in a Dithiol-Disulfide exchange reaction, which accelerates cystine dissolution.