Does Risk of Hyperhomocysteinemia Depend on Thiol-Disulfide Exchange Reactions of Albumin and Homocysteine?

Significance: Increased plasma concentrations of total homocysteine (tHcy; mild-moderate hyperhomocysteinemia: 15-50 μM tHcy) are considered an independent risk factor for the onset/progression of various diseases, but it is not known about how the increase in tHcy causes pathological conditions. Recent Advances: Reduced homocysteine (HSH ∼1% of tHcy) is presumed to be toxic, unlike homocystine (∼9%) and mixed disulfide between homocysteine and albumin (HSS-ALB; homocysteine [Hcy]-albumin mixed disulfide, ∼90%). This and other notions make it difficult to explain the pathogenicity of Hcy because: (i) lowering tHcy does not improve pathological outcomes; (ii) damage due to HSH usually emerges at supraphysiological doses; and (iii) it is not known why tiny increments in plasma concentrations of HSH can be pathological. Critical Issues: Albumin may have a role in Hcy toxicity, because HSS-ALB could release toxic HSH via thiol-disulfide (SH/SS) exchange reactions in cells. Similarly, thiol-disulfide exchange processes of reduced albumin (albumin with free SH group of Cys34 [HS-ALB]) or N-homocysteinylated albumin are plausible alternatives for initiating Hcy pathological events. Adverse effects of albumin and other data reviewed here suggest the hypothesis of a role of albumin in Hcy toxicity. Future Directions: HSS-ALB might be involved in disruption of the antioxidant/oxidant balance in critical tissues (brain, liver, kidney). Since homocysteine-albumin mixed disulfide is a possible intermediate of thiol-disulfide exchange reactions, we suggest that homocysteinylated albumin could be a new pathological factor, and that studies on the redox role of albumin and mixed disulfide production via thiol-disulfide exchange reactions could offer new therapeutic insights for reducing Hcy toxicity.

Homocysteinemia control by cysteine in cerebral vascular patients after methionine loading test: evidences in physiological and pathological conditions in cerebro-vascular and multiple sclerosis patients

The toxicity risk of hyperhomocysteinemia is prevented through thiol drug administration which reduces plasma total homocysteine (tHcy) concentrations by activating thiol exchange reactions. Assuming that cysteine (Cys) is a homocysteinemia regulator, the hypothesis was verified in healthy and pathological individuals after the methionine loading test (MLT). The plasma variations of redox species of Cys, Hcy, cysteinylglycine, glutathione and albumin (reduced, HS-ALB, and at mixed disulfide, XSS-ALB) were compared in patients with cerebral small vessels disease (CSVD) (n = 11), multiple sclerosis (MS) (n = 12) and healthy controls (n = 11) at 2-4-6 h after MLT. In MLT-treated subjects, the activation of thiol exchange reactions provoked significant changes over time in redox species concentrations of Cys, Hcy, and albumin. Significant differences between controls and pathological groups were also observed. In non-methionine-treated subjects, total Cys concentrations, tHcy and thiol-protein mixed disulfides (CSS-ALB, HSS-ALB) of CSVD patients were higher than controls. After MLT, all groups displayed significant cystine (CSSC) increases and CSS-ALB decreases, that in pathological groups were significantly higher than controls. These data would confirm the Cys regulatory role on the homocysteinemia; they also explain that the Cys-Hcy mixed disulfide excretion is an important point of hyperhomocysteinemia control. Moreover, in all groups after MLT, significant increases in albumin concentrations, named total albumin (tALB) and measured as sum of HS-ALB (spectrophometric), and XSS-ALB (assayed at HPLC) were observed. tALB increases, more pronounced in healthy than in the pathological subjects, could indicate alterations of albumin equilibria between plasma and other extracellular spaces, whose toxicological consequences deserve further studies.

Platelet function tests: a comparative review

In physiological hemostasis a prompt recruitment of platelets on the vessel damage prevents the bleeding by the rapid formation of a platelet plug. Qualitative and/or quantitative platelet defects promote bleeding, whereas the high residual reactivity of platelets in patients on antiplatelet therapies moves forward thromboembolic complications. The biochemical mechanisms of the different phases of platelet activation – adhesion, shape change, release reaction, and aggregation – have been well delineated, whereas their complete translation into laboratory assays has not been so fulfilled. Laboratory tests of platelet function, such as bleeding time, light transmission platelet aggregation, lumiaggregometry, impedance aggregometry on whole blood, and platelet activation investigated by flow cytometry, are traditionally utilized for diagnosing hemostatic disorders and managing patients with platelet and hemostatic defects, but their use is still limited to specialized laboratories. To date, a point-of-care testing (POCT) dedicated to platelet function, using pertinent devices much simpler to use, has now become available (ie, PFA-100, VerifyNow System, Multiplate Electrode Aggregometry [MEA]). POCT includes new methodologies which may be used in critical clinical settings and also in general laboratories because they are rapid and easy to use, employing whole blood without the necessity of sample processing. Actually, these different platelet methodologies for the evaluation of inherited and acquired bleeding disorders and/or for monitoring antiplatelet therapies are spreading and the study of platelet function is strengthening. In this review, well-tried and innovative platelet function tests and their methodological features and clinical applications are considered.

Effects of cryoablation and radiofrequency ablation on endothelial and blood clotting activation

Cryoablation (CA) emerged as an alternative procedure to radiofrequency (RF). The aim of this study was to compare haemostatic system alterations in patients undergoing RF or CA for atrioventricular nodal reentrant tachycardia ablation. von Willebrand factor (vWF), spontaneous whole blood platelet aggregation, prothrombin fragment F1 + 2 (F1 + 2), thrombin-antithrombin complex (TAT), plasminogen activator inhibitor type-1 (PAI-1), and clot lysis time (CLT) were determined in 48 patients (27 CA; 21 RF; 19M/29F, mean age 49.6 ± 17.6 years). Blood samples were obtained before the procedure (T0), immediately after (T1), and 24 h later (T2). At T1 both procedures were associated with a significant increase in levels of the endothelial activation marker vWF. At T2 vWF levels were lower in CA than in RF group. No changes in whole blood platelet aggregation before and after ablation procedures were observed. At T1 both groups determined an increase in blood clotting activation markers, F1 + 2, TAT, and DD. At T2 F1 + 2, TAT and DD levels were similar to baseline values. The comparison between RF and CA showed no significant differences in F1 + 2 and TAT levels, whereas at T1 DD levels were higher in CA group than in RF group. Both procedures induced a significant decrease in CLT, whereas no changes in PAI-1 levels were found. There were no significant differences in CLT and PAI-1 levels. The fibrinolytic efficiency analysis showed that at T1 DD/TAT and DD/F1 + 2 ratios were lower in RF group and remained lower in RF than in CA group at T2. CA procedure may be associated with a lower degree of endothelial damage and with a higher fibrinolytic capacity respect to RF.

Assessment of platelet function: Laboratory and point-of-care methods

In the event of blood vessel damage, human platelets are promptly recruited on the site of injury and, after their adhesion, activation and aggregation, prevent blood loss with the formation of a clot. The consequence of abnormal regulation can be either hemorrhage or the development of thrombosis. Qualitative and/or quantitative defects in platelets promote bleeding, whereas the residual reactivity of platelets, despite antiplatelet therapies, play an important role in promoting arterial thrombotic complications. Platelet function is traditionally assessed to investigate the origin of a bleeding syndrome, to predict the risk of bleeding prior surgery or during pregnancy or to monitor the efficacy of antiplatelet therapy in thrombotic syndromes that, now, can be considered a new discipline. “Old” platelet function laboratory tests such as the evaluation of bleeding time and the platelet aggregation analysis in platelet-rich plasma are traditionally utilized to aid in the diagnosis and management of patients with platelet and hemostatic disorders and used as diagnostic tools both in bleeding and thrombotic diathesis in specialized laboratories. Now, new and renewed automated systems have been introduced to provide a simple, rapid assessment of platelet function including point of care methods. These new methodologies are also suitable for being used in non-specialized laboratories and in critical area for assessing platelet function in whole blood without the requirement of sample processing. Some of these methods are also beginning to be incorporated into routine clinical use and can be utilized as not only as first panel for the diagnosis of platelet dysfunction, but also for monitoring anti-platelet therapy and to potentially assess risk of both bleeding and/or thrombosis.

A structurally driven analysis of thiol reactivity in mammalian albumins

Understanding the structural basis of protein redox activity is still an open question. Hence, by using a structural genomics approach, different albumins have been chosen to correlate protein structural features with the corresponding reaction rates of thiol exchange between albumin and disulfide DTNB. Predicted structures of rat, porcine, and bovine albumins have been compared with the experimentally derived human albumin. High structural similarity among these four albumins can be observed, in spite of their markedly different reactivity with DTNB. Sequence alignments offered preliminary hints on the contributions of sequence-specific local environments modulating albumin reactivity. Molecular dynamics simulations performed on experimental and predicted albumin structures reveal that thiolation rates are influenced by hydrogen bonding pattern and stability of the acceptor C34 sulphur atom with donor groups of nearby residues. Atom depth evolution of albumin C34 thiol groups has been monitored during Molecular Dynamic trajectories. The most reactive albumins appeared also the ones presenting the C34 sulphur atom on the protein surface with the highest accessibility. High C34 sulphur atom reactivity in rat and porcine albumins seems to be determined by the presence of additional positively charged amino acid residues favoring both the C34 S⁻ form and the approach of DTNB.

Protein-thiol substitution or protein dethiolation by thiol/disulfide exchange reactions: the albumin model

Dethiolation experiments of thiolated albumin with thionitrobenzoic acid and thiols (glutathione, cysteine, homocysteine) were carried out to understand the role of albumin in plasma distribution of thiols and disulfide species by thiol/disulfide (SH/SS) exchange reactions. During these experiments we observed that thiolated albumin underwent thiol substitution (Alb-SS-X+RSH<-->Alb-SS-R+XSH) or dethiolation (Alb-SS-X+XSH<-->Alb-SH+XSSX), depending on the different pK(a) values of thiols involved in protein-thiol mixed disulfides (Alb-SS-X). It appeared in these reactions that the compound with lower pK(a) in mixed disulfide was a good leaving group and that the pK(a) differences dictated the kind of reaction (substitution or dethiolation). Thionitrobenzoic acid, bound to albumin by mixed disulfide (Alb-TNB), underwent rapid substitution after thiol addition, forming the corresponding Alb-SS-X (peaks at 0.25-1 min). In turn, Alb-SS-X were dethiolated by the excess nonprotein SH groups because of the lower pK(a) value in mixed disulfide with respect to that of other thiols. Dethiolation of Alb-SS-X was accompanied by formation of XSSX and Alb-SH up to equilibrium levels at 35 min, which were different for each thiol. Structures by molecular simulation of thiolated albumin, carried out for understanding the role of sulfur exposure in mixed disulfides in dethiolation process, evidenced that the sulfur exposure is important for the rate but not for determining the kind of reaction (substitution or dethiolation). Our data underline the contribution of SH/SS exchanges to determine levels of various thiols as reduced and oxidized species in human plasma.

Administration of minor polar compound-enriched extra virgin olive oil decreases platelet aggregation and the plasma concentration of reduced homocysteine in rats

We investigated the effect of extra virgin olive oil (EVOO) on platelet aggregation and plasma concentrations of homocysteine (Hcy) redox forms in rats in relation to the minor polar compound (MPC) concentration of EVOO. We used 3 olive oil samples with similar fatty acid but different MPC concentrations: refined olive oil (RF) with traces of MPC (control oil), native EVOO with low MPC concentration (LC), and EVOO with high MPC concentration (HC) enriching LC with its own MPC. Oil samples were administered to rats by gavage (1.25 mL/kg body weight) using 2 experimental designs: acute (24-h food deprivation and killed 1 h after EVOO administration) and subacute (12-d treatment, a daily dose of oil for 12 d, and killed after 24 h of food deprivation). Platelet aggregation was induced by ADP (ex vivo tests) and a reduction in platelet reactivity occurred in cells from rats given LC in the subacute study and in cells from rats administered HC in both studies as indicated by an increase in the agonist half maximal effective concentration. HC inhibited platelet aggregation induced by low ADP doses (reversible aggregation) in cells of rats in both the acute and subacute studies, whereas LC had this effect only in the subacute experiment. Moreover, in rats administered HC in both experiments, the plasma concentration of free reduced Hcy (rHcy) was lower and Hcy bound to protein by disulfide bonds (bHcy) was greater than in RF-treated rats. bHcy was also greater in rats given LC than in RF-treated rats in the subacute experiment. Plasma free-oxidized Hcy was greater in rats given LC and HC than in those administered RF only in the subacute experiment. In conclusion, these results show that MPC in EVOO inhibit platelet aggregation and reduce the plasma rHcy concentration, effects that may be associated with cardiovascular protection.

GSH depletion, protein S-glutathionylation and mitochondrial transmembrane potential hyperpolarization are early events in initiation of cell death induced by a mixture of isothiazolinones in HL60 cells

We recently described that brief exposure of HL60 cells to a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one (CMI) and 2-methyl-4-isothiazolin-3-one (MI) induces apoptosis at low concentrations (0.001-0.01%) and necrosis at higher concentrations (0.05-0.1%). In this study, we show that glutathione (GSH) depletion, reactive oxygen species generation, hyperpolarization of mitochondrial transmembrane potential (DeltaPsim) and formation of protein-GSH mixed disulphides (S-glutathionylation) are early molecular events that precede the induction of cell death by CMI/MI. When the cells exhibit common signs of apoptosis, they show activation of caspase-9, reduction of DeltaPsim and, more importantly, decreased protein S-glutathionylation. In contrast, necrosis is associated with severe mitochondrial damage and maximal protein S-glutathionylation. CMI/MI-induced cytotoxicity is also accompanied by decreased activity of GSH-related enzymes. Pre-incubation with L-buthionine-(S,R)-sulfoximine (BSO) clearly switches the mode of cell death from apoptosis to necrosis at 0.01% CMI/MI. Collectively, these results demonstrate that CMI/MI alters the redox status of HL60 cells, and the extent and kinetics of GSH depletion and S-glutathionylation appear to determine whether cells undergo apoptosis or necrosis. We hypothesize that S-glutathionylation of certain thiol groups accompanied by GSH depletion plays a critical role in the molecular mechanism of CMI/MI cytotoxicity.

Biochemical and biological aspects of protein thiolation in cells and plasma

Protein thiolation is elicited by oxidation by different mechanisms and is involved in a variety of biological processes. Thiols, protein SH (PSH) and non-protein SH groups (NPSH, namely GSH), are in competition in all biological environments in the regulation of oxidant homeostasis because oxidants thiolate proteins, whereas GSH dethiolates them (e.g., GSSG + PSH --> GSSP + GSH). Although poorly investigated, the elimination of disulfides from thiolated proteins to regenerate critical PSH is important. These aspects are poorly known in cells, where glutaredoxin and peroxiredoxin operate as enzymes or potential chaperones to accelerate dethiolation. On the contrary, studies with plasma or albumin have highlighted the importance of protein conformation in dethiolation processes and have clarified the reason why homocysteine (thiol with potential toxicity) is preferentially bound to albumin as protein-thiol mixed disulfide with respect to other NPSH. Here we provide an overview of protein thiolation/dethiolation processes, with an emphasis on recent developments and future perspectives in this field.

The effects of age and hyperhomocysteinemia on the redox forms of plasma thiols

We assayed the redox forms of cysteine (reduced [CSH], oxidized [CSSC], and bound to protein [CS-SP]), cysteinylglycine (CGSH; cysteinylgycine disulfide [CGSSGC] and cysteinylglycine-protein mixed disulfide [CGS-SP]), glutathione (GSH; glutathione disulfide [GSSG] and glutathione-protein mixed disulfide [GS-SP]), homocysteine (Hcy; homocystine [HcyS] and homocystine-protein mixed disulfides [bHcy]), and protein sulfhydryls in the plasma of healthy subjects (divided into 8 groups ranging in age from birth to 70 years) and patients with mild hyperhomocysteinemia associated with cardiovascular disease (heart-transplant patients) or vascular atherosclerosis, with or without renal failure. In healthy individuals, levels of disulfides and protein-mixed disulfides were more abundant than those of thiols, and those of protein-thiol mixed disulfides were higher than disulfides. Concentrations of CSH, GSH, and CGSH in the various groups had profiles characterized by a maximum over time. The concentration of Hcy was unchanged up to the age of 30 years, after which it increased. CSSC concentration increased gradually with age, whereas concentrations of the other disulfides were essentially unchanged. By contrast, the concentrations of all protein-thiol mixed disulfides, especially those with CSH, increased gradually with age. Ranks of distribution of the reduced forms changed with age (at birth, CSH > CGSH > GSH > Hcy; in 1- to 2-year-olds, CSH > GSH > CGSH > Hcy; and in 51- to 70-year-olds, CSH > CGSH = GSH > Hcy), whereas those of disulfides and protein-thiol mixed disulfides were substantially unchanged (in all age groups, CSSC > CGSSGC > GSSG = HcyS and CS-SP > CGS-SP > bHcy > GS-SP). In patients with pathologic conditions, plasma levels of disulfide forms CSSC, HcyS, CS-SP, and bHcy were significantly increased, whereas other redox forms of thiols were unchanged or showed variations opposite (increasing or decreasing) to control values. Maximal increases in disulfides and protein-thiol mixed disulfides were associated with renal failure. Our data suggest that increases in plasma bHcy concentrations in subjects with pathologic conditions were more likely the result of activation of thiol-disulfide exchange reactions between free reduced Hcy and CS-SP than of a direct action of reactive oxygen species.