Redox homeostasis of albumin in relation to alpha-lipoic acid and dihydrolipoic acid

Abstract

Albumin represents the predominant circulating antioxidant agent in plasma exposed to continuous oxidative stress and a change in serum albumin structure accounts for its antioxidant properties. Alterations in the redox status of albumin may result in impairments of its biological properties. Alpha-lipoic acid (LA), a naturally occurring thiol compound found in virtually all species, is a potent antioxidant with high efficacy which is also involved in the chelation of metal ions, regeneration of antioxidants, and repair of oxidatively damaged proteins. In human body LA is rapidly reduced to dihydrolipoic acid (DHLA) after intake into the cell. Both, LA and DHLA are amphipathic molecules which act as antioxidants both in hydrophilic and lipophilic environments. The present study aimed to investigate the antioxidant/pro-oxidant effects of LA and DHLA due to their concentrations in metal-catalyzed protein oxidation (MCO) of human serum albumin (HSA). Progressive oxidative modification of albumin was found in MCO system by an increased content of protein hydroperoxides (POOH), protein carbonyl groups (PCO) which is the former's major breakdown product, and other protein oxidation markers such as advanced oxidized protein products (AOPP) and protein thiol groups (P-SH). The possible antioxidant protective effects of LA and DHLA were observed with 25 microM and 50 microM; DHLA being more influential. Protein oxidation parameters were found to be lower and P-SH levels seemed higher. However, prooxidant effects of both LA and DHLA came on the scene with increased concentrations of 75 microM and 100 microM where the latter seemed the most hazardous with contradicted results. It is clear that the loss of biological activity of human serum albumin by MCO system appears of medical relevance and if LA exerts similar effects seen in the present study, it is possible that cellular prooxidant activity can also result consuming this unique antioxidant in certain doses.

Figure 1

Metal ion-catalyzed albumin oxidation, alpha-lipoic acid, and intravascular redox homeostasis. ROS, reactive oxygen species; MCO, metal-catalyzed protein oxidation; LA, alpha-lipoic acid; DHLA, dihydrolipoic acid; POOH, protein hydroperoxides; PCO, protein carbonyl groups; AOPP, advanced oxidation protein products.

Figure 2

The absorption spectra of in vitro metal catalyzed oxidation system at 265 nm (A) before and (B) following the albumin treatment.

Figure 3

(A) Variations of protein carbonyl groups (PCO) in albumin molecule exposed to metal catalyzed oxidation (MCO) with respect to incremental lipoic acid (LA) concentrations. Results are expressed as mean ± S.E.M. PCO levels were expressed as nmol/mg protein. Data are statistically significant at *,†,Ƒ,#p < 0.001. (B) Variations of protein carbonyl groups (PCO) in albumin molecule exposed to metal catalyzed oxidation (MCO) with respect to incremental dihydolipoic acid (DHLA) concentrations. Results are expressed as mean ± S.E.M. PCO levels were expressed as nmol/mg protein. Data are statistically significant at *,†,Ƒ,#p < 0.001 and **p < 0.01 respectively.

Figure 4

(A) Variations of protein hydroperoxides (POOH) in albumin molecule exposed to metal catalyzed oxidation (MCO) with respect to incremental lipoic acid (LA) concentrations. Results are expressed as mean ± S.E.M. POOH levels were expressed as nmol/mg protein. Data are statistically significant at *,†,Ƒ,#p < 0.001 and **,§p < 0.01 respectively. (B) Variations of protein hydroperoxides (POOH) in albumin molecule exposed to metal catalyzed oxidation (MCO) with respect to incremental dihydrolipoic acid (DHLA) concentrations. Results are expressed as mean ± S.E.M. POOH levels were expressed as nmol/mg protein. Data are statistically significant at *,†,Ƒ,#p < 0.001 and ‡p < 0.01 respectively.

Figure 5

(A) Variations of advanced oxidation protein products (AOPP) in albumin molecule exposed to metal catalyzed oxidation (MCO) with respect to incremental lipoic acid (LA) concentrations. Results are expressed as mean ± S.E.M. AOPP levels were expressed as nmol per liter of chloramin-T equivalent/mg protein. Data are statistically significant at *,†,Ƒ,#p < 0.001. (B) Variations of advanced oxidation protein products (AOPP) in albumin molecule exposed to metal catalyzed oxidation (MCO) with respect to incremental dihydrolipoic acid (DHLA) concentrations. Results are expressed as mean ± S.E.M. AOPP levels were expressed as nmol per liter of chloramin-T equivalent/mg protein. Data are statistically significant at *,†,Ƒ,#p < 0.001.

Figure 6

(A) Variations of protein thiol (P-SH) groups in albumin molecule exposed to metal catalyzed oxidation (MCO) with respect to incremental lipoic acid (LA) concentrations. Results are expressed as mean ± S.E.M. P-SH levels are expressed as nmol/mg protein. Data are statistically significant at *,†,Ƒ,#p < 0.001 and §p < 0.01 respectively. (B) Variations of protein thiol (P-SH) groups in albumin molecule exposed to metal catalyzed oxidation (MCO) with respect to incremental dihydrolipoic acid (DHLA) concentrations. Results are expressed as mean ± S.E.M. P-SH levels are expressed as nmol/mg protein. Data are statistically significant at *,†,Ƒ,#p < 0.001 and §p < 0.01 respectively.