Critical evaluation of plasma and LDL oxidant-trapping potential in hemodialysis patients

Background. We investigated whether the total peroxyl radical-trapping anti oxidant potential (TRAP) assay, which has recently been proposed as a gauge of oxidative stress, could serve to evaluate plasma and low density lipopr otein (LDL) antioxidant state in hemodialysis (HD) patients. Methods. TRAP was determined by the lag time of the chemiluminescence react ion induced by azo-initiator-catalyzed linoleic acid peroxidation in the pl asma and corresponding LDL preparations of 23 HD patients and 22 healthy su bjects. Antioxidant systems, including glutathione peroxidase (GSH-Px), asc orbate, vitamin E, and uric acid, oxidative stress markers including malond ialdehyde (MDA), carbonyls, and advanced oxidation protein products (AOPP), and lipids, including cholesterol and triglycerides, were also determined in the plasma. Results. Both plasma and LDL-TRAP were significantly increased in HD patien ts despite decreased GSH-Px and ascorbate and increased MDA, carbonyl, and AOPP plasma levels. Plasma TRAP values were closely related to both uric ac id and AOPP levels, and LDL-TRAP values were related to triglycerides and A OPP levels. In vitro studies showed that: (a) plasma TRAP of control plasma increased regularly with supplementation of uric acid, although not reachi ng that of I-ID plasma with similar uric acid levels; (b) the addition of h uman serum albumin-AOPP also regularly increased control plasma TRAP, but w as close to that of HD plasma with similar AOPP levels; and (c) LDL-TRAP wa s increased following LDL enrichment with triglycerides. Conclusion. Our study demonstrates that TRAP is not a relevant parameter fo r evaluating plasma or LDL antioxidant capacity in HD patients, due to the high plasma levels of uric acid, triglycerides and AOPP, which by themselve s do not exert efficient antioxidant activity in vivo, but in vitro are abl e to scavenge the peroxyl radicals involved in the TRAP assay.