Thiols as a Measure of Plasma Redox Status in Healthy Subjects and in Patients with Renal or Liver Failure

Abstract

Plasma thiols have been the object of growing interest because numerous studies have indicated that even a mild degree of hyperhomocysteinemia is associated with an increased risk of developing occlusive vascular diseases (1)(2)(3)(4)(5). However, the mechanism behind the vascular injuries is still unknown. Studies of the possible pathogenetic mechanism of increased plasma homocysteine concentrations are difficult because little is known about the mechanism for the formation of different homocysteine species in vivo. We recently published a method that measures reduced and total fractions of homocysteine, cysteine, glutathione, and cysteinylglycine(6). In a preliminary study, we found that patients suffering from stroke have hyperhomocysteinemia, whereas their reduced homocysteine was within the health-related reference interval(7). We hypothesized that the increased concentrations of the oxidized forms of homocysteine in plasma were attributable to a hyperoxidative state in the plasma. We also observed (8) that patients suffering from renal failure had concentrations of reduced homocysteine within the reference interval despite increased total homocysteine. The redox state of homocysteine in plasma may be influenced by other thiols (9), such as glutathione, which is involved in maintaining the intracellular thiols in reduced form. In the present study, we therefore investigated the relationships between homocysteine, cysteine, and glutathione in 29 healthy subjects and 15 patients with renal or liver failure. We used a newly developed preparation procedure, especially designed to minimize several known pitfalls that frequently influence plasma glutathione determinations. Increased hemolysis during sample collection causes falsely increased plasma glutathione measurements because of the high glutathione contents in the blood cells. Plasma glutathione also decreases with time because of the activity of γ-glutamyltransferase in plasma. Furthermore, reduced glutathione disappears within minutes in cell-free plasma because of oxidation(7)(10)(11)(12). In the present study, …