Tiopronin is an acylated sulfhydryl-containing derivative of glycine with reducing and complexing properties. Tiopronin breaks the disulphide bond of cystine and binds the sulfhydryl group of the resultant cysteine monomers to form a soluble tiopronin-cysteine-mixed disulfide, which is more water-soluble than cystine and is readily excreted. This leads to a reduction in urinary cystine concentration and subsequently reduces cystine stone formation. Kidney stones form when the solubility limit is exceeded and urine becomes supersaturated with endogenous cystine. Tiopronin is an active reducing agent which undergoes a thiol-disulfide exchange with cystine to form a water-soluble mixed disulfide complex. Thus, the amount of sparingly soluble cystine is reduced. By reducing urinary cystine concentrations below the solubility limit, tiopronin helps reduce cystine stone formation. | 
