Figure 1. Cartoon of cystine transport. Cystine is filtered across the glomerular capillary membrane into the proximal tubule. Cystine reabsorption from the tubular lumen across the apical membranes of the S3 segments and into the cell is mediated by the amino acid transporter affected in cystinuria. Cystine is exchanged for neutral amino acids like alanine (not shown). The transporter is composed of two subunits (making it a heterodimer) and the protein products of SLC3A1 (rBAT, the heavy component) and SLC7A9 (b^0,+ AT, the light component). Inside the proximal tubule cell, cystine is reduced to two molecules of cysteine, which exit across the basolateral membrane.

Figure 2. Chemical structure of cystine, a combination of two molecules of cysteine. Two thiol (–SH) containing drugs used for treatment are penicillamine and -mercaptopropionylglycine. Both thiols reduce the disulfide bond in cystine to cysteine and chelate the latter to form a more soluble product. Only the penicillamine–cysteine complex is shown here.

Figure 3. Typical hexagonal cystine crystal in urine (Courtesy of Louis Herring Lab, Orlando, Florida).

Figure 4. A staghorn kidney stone composed of cystine (Courtesy of Louis Herring Lab, Orlando, Florida).

Figure 5. Curve of increasing cystine solubility with increasing urinary pH.