Various forms of plasma cysteine and its metabolites in patients undergoing hemodialysis.

The thiol tripeptide glutathione (GSH) is particularly important as an antioxidative protector in the cells. GSH is also a form of storage and transport of cysteine. Under physiological conditions, the kidney plays an essential role in GSH biodegradation to free cysteine via gamma-glutamyl cycle and subsequently in further metabolism of this sulfur amino acid. Our aim was to assess to what degree renal insufficiency affects the level of various cysteine forms and its metabolites (sulfates and sulfane sulfur compounds) in the plasma, and whole blood GSH levels. The concentrations of all the above mentioned sulfur compounds were measured in plasma of patients with end stage renal failure (ESRF) before and after dialysis and in a group of healthy controls. In plasma of patients with ESRF before dialysis tendency towards significant elevation of cystine, protein-bound cysteine and sulfates levels was evident. Simultaneously, a decrease of plasma level of sulfane sulfur compounds, products of anaerobic sulfur metabolism, and whole blood GSH concentration was found. As a consequence, the ratio between the reduced cysteine and the total cysteine concentration was markedly decreased. The dialysis session restore this ratio to the value observed in plasma of control individuals. These findings indicate disturbances in the thiol-disulfide equilibrium and show a higher oxidation status in plasma of patients with ESRF than in healthy controls.