Epidemiological data consistently show that reduced levels of serum albumin, which is the most abundant protein in plasma, are associated with an increased mortality risk. Various biological properties evidenced by direct effects of the albumin molecule may explain its beneficial effects. The present work aimed to investigate in vitro whether glycation or free radicals or both factors would affect the antioxidant properties of bovine serum albumin (BSA). Glycation was performed by long-term incubations (60 days) of BSA with increasing concentrations of glucose (up to 500 mmol/l) at 37 degreesC. Minimally oxidized BSA was obtained after controlled incubations of dialyzed BSA samples with a water-soluble free radical generator [2,2' azo-bis(2-amidinopropane) HCl]. The glycation-mediated modifications and the free radical-induced conformational changes of BSA were monitored using intrinsic fluorescence measurements of the tryptophan residues and acrylamide as a quenching agent. Thiol groups, Amadori glycophore contents, and boronate binding were also measured. We found that the changes observed in the conformation of the BSA molecule were associated with modifications of its antioxidant properties. The latter were studied by the copper-mediated oxidation of human low density lipoproteins and the free radical-induced blood hemolysis test. Our data support the concept that oxidative-induced BSA modifications are important determinants in the antioxidant properties of BSA. Glycated BSA still behaved as an antioxidant but became pro-oxidant in the presence of copper, probably by generating oxygenated species. These data confirm the key role of metals ions in this process. Although these results warrant further in vivo investigations, we propose that, considering the poor glucose control found in diabetics as well as the key role of oxidative stress in vascular complications, glycation-mediated and free radical-induced impairment of the antioxidant properties of albumin might be important parameters in vascular complications encountered in diabetes.

SUMMARY:Based on recent retrospective, prospective, and experimental studies, mild to moderate elevation of fasting or postmethionine-load plasma homocysteine is accepted as an independent risk factor for cardiovascular disease and thrombosis in both men and women. Hyperhomocysteinemia results from an inhibition of the remethylation pathway or from an inhibition or a saturation of the transsulfuration pathway of homocysteine metabolism. The involvement of a high dietary intake of methionine-rich animal proteins has not yet been investigated and cannot be ruled out. However, folate deficiency, either associated or not associated with the thermolabile mutation of the N 5,10 -methylenetetrahydrofolate reductase, and vitamin B 6 deficiency, perhaps associated with cystathionine ␤-synthase defects or with methionine excess, are believed to be major determinants of the increased risk of cardiovascular disease related to hyperhomocysteinemia. Recent experimental studies have suggested that moderately elevated homocysteine levels are a causal risk factor for atherothrombotic disease because they affect both the vascular wall structure and the blood coagulation system. The oxidant stress that results from impaired homocysteine metabolism, which modifies the intracellular redox status, might play a central role in the molecular mechanisms underlying moderate hyperhomocysteinemia-mediated vascular disorders. Because folate supplementation can efficiently reduce plasma homocysteine levels, both in the fasting state and after methionine loading, results from further prospective cohort studies and from on-going interventional trials will determine whether homocysteine-lowering therapies can contribute to the prevention and reduction of cardiovascular risk. Additionally, these studies will provide unequivocal arguments for the independent and causal relationship between hyperhomocysteinemia and atherothrombotic disease. (Lab Invest 2001, 81:645-672).C ardiovascular diseases remain the leading cause of mortality in Western populations. Hyperlipoproteinemia, hypertension, diabetes, obesity, and tobacco smoking are the main risk factors for atherosclerosis and its thrombotic complications. However, these factors alone cannot account for all of the deaths caused by vascular pathologies.As early as 1969, clinical studies conducted in homocystinuric children revealed the importance of severe hyperhomocysteinemia in premature development of atherosclerosis and thromboembolism (McCully, 1983). According to numerous retrospective case-controlled studies, moderate increases in plasma homocysteine, which can be precisely quantitated by high performance liquid chromatography (HPLC), raise the risk for cardiovascular disease 2-fold, after adjusting for classic risk factors. Moreover, up to 20% to 40% of patients with vascular pathologies present moderate to intermediate hyperhomocysteinemia. However, results from prospective studies are inconsistent. Additionally, molecular mechanisms underlying hyperhomocysteinemia-induced vascular diseas...

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A moderate elevation of plasma homocysteine is a risk factor for atherosclerosis and arterial and veinous thrombosis. However, the mechanisms leading to vascular disorders are poorly understood because studies that have investigated the potential atherothrombogenicity of hyperhomocysteinemia in vivo are scarce. Using a rat model, we were the first to show that dietary folic acid deficiency, a major cause of basal hyperhomocysteinemia, is associated with enhanced macrophage-derived tissue factor and platelet activities. We proposed that an homocysteine-induced oxidative stress may account for this hypercoagulable state. To determine the true thrombogenicity of moderate hyperhomocysteinemia and better understand its etiology, we have carried out an acute methionine load in control and folate-deficient animals. When rats were fed the control diet, a transient fourfold increase in plasma homocysteine levels was observed 2 h after the methionine administration. As with prolonged dietary folic acid deficiency, this methionine load potentiated the platelet aggregation in response to thrombin and ADP as well as the thrombin-induced thromboxane synthesis. It also stimulated the basal and lipopolysaccharide-induced tissue factor activity of peritoneal macrophages. These prothrombotic effects were associated with an increased lipid peroxidation characterized by an elevation of plasma conjugated dienes, lipid hydroperoxides, and thiobarbituric acid-reactive substances. When rats were fed a folic acid-deficient diet, the methionine load did not cause any further increase in plasma homocysteine concentration, platelet activation, macrophage tissue factor-dependent coagulation, or lipoperoxidation. Altogether, our data showed that the prethrombotic state due to both the altered remethylation and transsulfuration pathways resulted from the moderate elevation of circulating homocysteine. We conclude that moderate hyperhomocysteinemia plays a role in the development of a thrombogenic state that might be mediated by the occurrence of oxidative stress.

Vitamin E supplementation constitutes a promising strategy in the prevention of neurodegenerative diseases. Here, we show that a phospholipid transfer protein (PLTP) is widely expressed in the brain where it appears to function as a transfer factor for alpha-tocopherol, the main isomer of vitamin E. PLTP deficiency results in significant depletion of brain alpha-tocopherol in both homozygous (-30.1%, P<0.0002) and heterozygous (-18.0%, P<0.05) PLTP knocked-out mice. Alpha-tocopherol depletion in PLTP-deficient homozygotes is associated with the elevation of lipofuscin (+25% and +450% increases in cortex and substantia nigra, respectively), cholesterol oxides (+54.5%, P<0.05), and cellular peroxides (+32.3%, P<0.01) in the brain. Complete PLTP deficiency in homozygotes is accompanied by increased anxiety as shown by fewer entries (8.3% vs. 44.4% in controls, P<0.01) and less time spent (1.7% vs. 41.3% in controls, P<0.05) in the open arms of an elevated plus-maze, in the absence of locomotor deterioration. Thus, the vitamin E transfer activity of PLTP appears to be a key process in preventing oxidative damage in the brain, and PLTP-deficient mice could be a new model of the contribution of oxidative brain injury in the etiology of neurodegenerative diseases.

Antioxidant properties of human serum albumin (HSA) may explain part of its beneficial role in various diseases related to free radical attack. In the present study, the antioxidant role of Cys and Met was studied by copper-mediated oxidation of human low density lipoproteins and by free radical-induced blood hemolysis which essentially assessed metal-chelating and free radical scavenging activities, respectively. Mild conditions were set up to specifically modify Cys and Met residues by N-ethylmaleimide (NEM) and chloramine T treatments, respectively. We found that Met and Cys accounted for 40-80% of total antioxidant activity of HSA. Copper binding to HSA was decreased by about 50% with chloramine T treatment of Met whereas no change was observed after NEM treatment of Cys. Although other amino acid residues are likely to be involved in anti-/prooxidant properties of HSA, from our data, we propose that Cys chiefly works as a free radical scavenger whereas Met mainly acts as a metal chelator.

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Our results provide experimental evidence that a low dose supplementation with vitamins and minerals was able to normalize biological nutrient status as early as 6 months of treatment. In addition, our data indicate that antioxidant defense in elderly subjects was improved with low doses of vit C, vit E and beta CA as studied by means of a functional test utilizing red blood cells challenged in vitro with free radicals.