The active conformation of von Willebrand factor in patients with thrombotic thrombocytopenic purpura in remission

BACKGROUND

Functional deficiency of ADAMTS13 in thrombotic thrombocytopenic purpura (TTP) patients is associated with circulating ultralarge von Willebrand factor (VWF) molecules that display spontaneous platelet-binding capacities. Upon remission, however, ADAMTS13 activity does not always return to baseline.

OBJECTIVE

To study ADAMTS13 and VWF-related features in TTP patients in remission.

METHODS

ADAMTS13 activity, anti-ADAMTS13 antibodies, VWF antigen, ultralarge VWF and levels of VWF that circulate in a glycoprotein Ibalpha-binding conformation were determined in plasma samples of 22 acquired TTP patients in remission between 1 month and 6 years after achieving remission. The composition of active multimers was investigated with a novel immunoprecipitation assay based on monoclonal antibody AU/VWF-a12, which specifically recognizes the active conformation of VWF.

RESULTS

ADAMTS13 activity was undetectable in 23% of the patients, even years after they had achieved remission, and lack of ADAMTS13 activity was associated with increased active VWF levels and the presence of ultralarge VWF multimers. Active VWF levels and ultralarge VWF were also associated with blood groups. Results from immunoprecipitation experiments revealed the full range of multimers to be present.

CONCLUSION

ADAMTS13 deficiency and the concurrent presence of ultralarge VWF and increased active VWF levels can be detected in TTP patients for years after they have achieved remission. Immunoprecipitation results suggest that the active conformation of VWF may be present in the lower molecular weight multimers, but future studies are necessary to confirm our findings.

The effect of exercise on von Willebrand factor and ADAMTS-13 in individuals with type 1 and type 2B von Willebrand disease

BACKGROUND

The effect of exercise on von Willebrand factor (VWF) and ADAMTS-13 levels in individuals with von Willebrand disease (VWD) has never been reported.

OBJECTIVES

The aim was to quantify the effect of a standardized exercise protocol on individuals with type 1 and type 2B VWD.

PATIENTS/METHODS

Thirty individuals from three groups (10 controls, 11 with type 1 VWD and 9 with type 2B VWD) completed the Standard Bruce Protocol Treadmill Test. A bleeding questionnaire was administered and blood tests were performed pre- and immediately postexercise. The groups were well matched for age, gender and body mass index (BMI).

RESULTS

There was a correlation in all groups between the metabolic equivalents (METS) achieved and the degree of change of VWF and FVIII:C levels (P < 0.002, Pearson's correlation). There was a significant postexercise increase in VWF:Ag, VWF:RCo, FVIII:C and activated VWF levels in both the control group and in the type 2B VWD group, but not in the type 1 VWD group. Specific to the type 2B VWD group was an increase in the percentage of high molecular weight multimers (P = 0.022), a decrease in the mean platelet count compared with the other groups (P < 0.001) and an increase in the ADAMTS-13 level (P = 0.001).

CONCLUSIONS

There are significant differences in the effects of exercise on individuals with type 1 and type 2B VWD compared with controls. Further clinical studies are necessary to evaluate exercise as a therapeutic option in VWD.

Coagulation parameters of thawed fresh-frozen plasma during storage at different temperatures

Once thawed, fresh-frozen plasma (FFP) should be used, according to guidelines, within 24 h. In hospital practice, this may be associated with wastage. This study has been performed to investigate the coagulation levels of thawed quarantine FFP as used in the Netherlands. Five units of quarantine FFP, obtained by plasmapheresis, were thawed and by sterile docking divided into satellite bags (SB). SB 2-4 were stored at room temperature (RT) for, respectively, 1, 3 and 6 h and SB 5-9 at 4 degrees C for 6, 12 and 24 h and 1 and 2 weeks. At each time point, activated partial thromboplastin time (APTT), prothrombin time (PT), fibrinogen, factor V (FV), factor VIII (FVIII) and ADAMTS13 activity were measured. During storage at RT for up to 6 h, no major differences were found in the levels of FV, PT, fibrinogen and ADAMTS13 activity. FVIII activity showed a decrease of 16% and the APTT was prolonged by 6%. During storage at 4 degrees C for 2 weeks, FV and FVIII were reduced by 35 and 45%, respectively. The APTT and PT were prolonged by 17 and 15%, respectively. Fibrinogen was decreased by 8%. No change in ADAMTS13 activity was found. FFP stored at RT for 6 h or at 4 degrees C for 2 weeks can provide sufficient support for adequate haemostasis except for patients with a known deficiency for FVIII and can be used for plasmapheresis in patients with thrombotic thrombocytopenic purpura (TTP).

Bioactivation of selenocysteine Se-conjugates by a highly purified rat renal cysteine conjugate beta-lyase/glutamine transaminase K

Selenocysteine Se-conjugates have recently been proposed as potential prodrugs to target pharmacologically active selenol compounds to the kidney. Although rat renal cytosol displayed a high activity of beta-elimination activity toward these substrates, the enzymes involved in this activation pathway as yet have not been identified. In the present study, the possible involvement of cysteine conjugate beta-lyase/glutamine transaminase K (beta-lyase/GTK) in cytosolic activity was investigated. To this end, the enzyme kinetics of 15 differentially substituted selenocysteine Se-conjugates and 11 cysteine S-conjugates was determined using highly purified rat renal beta-lyase/GTK. The results demonstrate that most selenocysteine Se-conjugates are beta-eliminated at a very high activity by purified beta-lyase/GTK, implicating an important role of this protein in the previously reported beta-elimination reactions in rat renal cytosol. As indicated by the rapid consumption of alpha-keto-gamma-methiolbutyric acid, purified beta-lyase/GTK also catalyzed transamination reactions, which appeared to even exceed that of beta-elimination. The corresponding sulfur analogs also showed significant transamination but were beta-eliminated at an extremely low rate. Comparison of the obtained enzyme kinetic data of purified beta-lyase/GTK with previously obtained data from rat renal cytosol showed a poor correlation. By determining the activity profiles of cytosolic fractions applied to anion exchange fast protein liquid chromatography and gel filtration chromatography, the involvement of multiple enzymes in the beta-elimination of selenocysteine Se-conjugates in rat renal cytosol was demonstrated. The identity and characteristics of these alternative selenocysteine conjugate beta-lyases, however, remain to be established.

Different roles for abf1p and a T-rich promoter element in nucleosome organization of the yeast RPS28A gene

In vivo mutational analysis of the yeast RPS28A ribosomal protein (rp-)gene promoter demonstrated that both the Abf1p binding site and the adjacent T-rich element are essential for efficient transcription. In vivo Mnase and DNaseI digestion showed that the RPS28A promoter contains a 50-60 bp long nucleosome-free region directly downstream from the Abf1p binding site, followed by an ordered array of nucleosomes. Mutating either the Abf1p binding site or the T-rich element has dramatic, but different, effects on the local chromatin structure. Failure to bind Abf1p appears to cause nucleosome positioning to become disorganized as concluded from the complete disappearance of Mnase hypersensitive sites. On the other hand, mutation of the T-rich element causes the downstream nucleosomal array to shift by approximately 50 bp towards the Abf1p site, resulting in loss of the nucleosome-free region downstream of Abf1p. We conclude that Abf1p is a strong organizer of local chromatin structure that appears to act as a nucleosomal boundary factor requiring the downstream T-rich element to create a nucleosome-free region.

Protective effects of lobenzarit against allyl alcohol-induced hepatotoxicity in mice and rats

The protective effects of lobenzarit disodium against the toxicity of allyl alcohol were investigated in vitro using isolated rat hepatocytes and in vivo using mice. In mice, at i.p doses of 25, 50 and 100 mg/kg lobenzarit significantly decreased the activity of alanine amino transferase (ALT) in serum and the concentration of malondialdehyde (MDA) in liver homogenates, both of which were increased by allyl alcohol at a dose of 64 mg/kg. At concentrations of 0.2 and 0.3 mM, lobenzarit reduced the release of lactate dehydrogenase (LDH) and the levels of malondialdehyde (MDA) induced by 0.4 mM of allyl alcohol in isolated rat hepatocytes. However, lobenzarit did not increase the levels of reduced glutathione (GSH) depleted by allyl alcohol in any of the two experimental models. The protective effects of lobenzarit were dose- and concentration-dependent and they were most obvious when lobenzarit was administered 30 min before allyl alcohol. It is concluded that lobenzarit exerts the observed protective effects most likely by its antioxidant properties.

Novel piperidine derivatives: inhibitory properties towards cytochrome P450 isoforms, and cytoprotective and cytotoxic characteristics

The ability of a series of eight piperidine derivatives, substituted at positions 1, 3 and 4, to inhibit P450-dependent metabolism of specific substrates, is reported. Five different P450 isoforms (1A1, 1A2, 2B1, 2E1 and 3A1) in differentially induced rat liver microsomes were used for this purpose. From the results it is concluded that compound 2 was the most potent and moreover, highly selective inhibitor for P4502B1 with an IC(50) of 2.5 μM. Compound 3 appeared to have high selectivity for P4501A1 but not for P4501A2 (IC(50)s 80 and > 1000 μM, respectively). P4502B1 was found to be the most susceptible P450 isoform for inhibition by compounds 2, 3 and 6, while P4502E1 was largely insensitive to the inhibitory properties of all piperidine derivatives. A preliminary SAR study for the cytotoxicity, cytoprotective and P450 inhibitory properties of the piperidine derivatives, was also attempted. Using freshly isolated rat hepatocytes, the toxicity of the compounds was estimated and expressed as lactate dehydrogenase (LDH) leakage, lipid peroxidation (LPO) levels and GSH depletion. Considering the P450 inhibition and cytotoxicity results, compounds 2 and 3 were tested for possible protective activity against paracetamol-induced cytotoxicities. It was found that compound 2 protects completely against LDH leakage and LPO caused by paracetamol in rat hepatocytes isolated from β-naphthoflavone (β-NF) pretreated rats. It is concluded that the piperidine structures studied proved to be potentially valuable lead compounds for the design of potent and selective P450 inhibitors and for non-toxic cytoprotective agents as well.

Mechanism of protection of lobenzarit against paracetamol-induced toxicity in rat hepatocytes

The protective effects of lobenzarit, an antioxidative agent and antirheumatic drug, on the cytotoxicity of paracetamol in rat hepatocytes were studied, as well as the inhibitory effects of lobenzarit on cytochrome P-450s and glutathione S-transferases (GSTs) in rat liver. Paracetamol was selected as a model toxin, since it is known to be bioactivated by specific cytochrome P-450s presumably to N-acetyl-p-benzoquinoneimine, a reactive metabolite which upon overdosage of paracetamol causes protein and non-protein thiol depletion, lipid peroxidation and cytotoxicity measurable as LDH leakage. At concentrations of lobenzarit of 0.2 and 0.3 mM, added 30 min before paracetamol, the drug prevented paracetamol-induced leakage of lactate dehydrogenase (LDH) almost completely and lipid peroxidation (LPO) and depletion of glutathione (GSH) substantially and also the formation of the 3-glutathionyl conjugate of paracetamol. However, at a concentration of 0.05 mM Lobenzarit did not protect anymore against the paracetamol toxicity, When added to the hepatocytes 1 h and 2 h before paracetamol, 0.05 and 0.2 and 0.3 mM concentrations of lobenzarit did not protect against the cytotoxicity induced by paracetamol either. Lobenzarit did not inhibit cytochromes P-450 1A1/1A2, 2B1/2B2 and 2E1 which were measured as ethoxyresorufin O-deethylation (EROD) activity in beta-naphthoflavone-induced rat liver microsomes, as pentoxyresorufin de-pentylation (PROD) activity in phenobarbital-induced microsomes and as p-nitrophenol hydroxylation (PNPH) activity in pyrazol-induced microsomes. Lobenzarit did not show inhibition of glutathione S-transferase (GST) activity towards 1-chloro-2,4-dinitrobenzene (CDNB) in cytosol from liver of rats treated with phenobarbital, pyrazol and beta-naphthoflavone either. It is concluded that the cytoprotective effect of lobenzarit is most likely due to its antioxidant effects and/or to its ability to stimulate GSH reductase.