Dos and don'ts in diagnosing antiphospholipid syndrome.

Antiphospholipid syndrome (APS) is an acquired autoimmune thrombotic tendency that is identified by the presence of abnormal antiphospholipid laboratory tests in patients who have a history of vascular thrombosis and/or pregnancy complications including recurrent spontaneous miscarriages and a group of other complications due to placental insufficiency. Diagnostic testing for APS is often problematic because of many misconceptions regarding these empirically derived assays. This chapter is intended to provide hematology-oncology consultants with practical information about the uses and limitations of assays used to diagnose APS.

Thromboembolism complicating the treatment of lupus anticoagulant hypoprothrombinemia syndrome.

Lupus anticoagulant hypoprothrombinemia syndrome (LAHPS) is a rare disorder characterized by a bleeding tendency due to factor II deficiency associated with the presence of lupus anticoagulant (LAC) autoantibodies. We describe a patient with systemic lupus erythematosus and LAHPS in whom successful treatment of central nervous system bleeding due to severe factor II deficiency was followed by a major thromboembolic complication. Literature review revealed 2 other patients with LAHPS who developed thrombosis resulting from the treatment of factor II deficiency. We suggest that factor II deficiency counterbalances the prothrombotic effect of LAC in LAHPS, and correcting this deficiency may promote thromboembolism.

A natural prothrombin mutant reveals an unexpected influence of A-chain structure on the activity of human alpha-thrombin.

We have recently identified in two unrelated patients with bleeding tendency a homozygous mutation causing a deletion of one of the two contiguous Lys(9)/Lys(10) residues in the A-chain of alpha-thrombin (DeltaK9). We used in vitro expression analysis to clarify the role of the deletion of Lys(9) or Lys(10) in the thrombin function. The k(cat)/K(m) value of the hydrolysis by DeltaK9 of the synthetic substrate Phe-Pip-Arg-p-nitroanilide (where Pip represents l-pipecolyl) and fibrinopeptide A was 18- and 60-fold lower, respectively, compared with wild type (WT). Interaction with antithrombin was also reduced in the mutant, the association rate being about 20-fold lower than in the WT thrombin. The sensitivity to sodium ion of DeltaK9 was found significantly attenuated compared with the WT form. DeltaK9 has a very weak platelet-activating capacity, attributed to a severely defective PAR1 interaction, whereas the binding to the platelet glycoprotein Ibalpha was unaffected. Likewise, the interaction with protein C was severely impaired, whereas interaction with thrombomodulin had a normal K(d) value. At variance with these findings, both low affinity (basic pancreatic trypsin inhibitor) and high affinity (N-alpha-[2-naphthylsulfonyl-glycyl]-4-amidinophenylalanine-piperidide) thrombin inhibitors displayed a better binding to DeltaK9 than to the WT form, indicating a better accommodation of these inhibitors into the catalytic pocket of DeltaK9. A molecular dynamics simulation of the DeltaK9 thrombin in full explicit water solvent provided support to the role of the A-chain in affecting conformation and catalytic properties of the B-chain, especially in some insertion loops of the enzyme, such as the 60-loop, as well as in the geometry of the catalytic triad residues.

Cefazolin administration and 2-methyl-1,3,4-thiadiazole-5-thiol in human tissue: possible relationship to hypoprothrombinemia.

Cephalosporin antibiotics with structures that include the heterocyclic leaving group 1-methyltetrazole-5-thiol (MTT) can cause hypoprothrombinemia and hemorrhage as a result of MTT-dependent inhibition of the gamma-carboxylation of glutamate. The structure of cefazolin also includes a heterocyclic thiol, 2-methyl-1,3,4-thiadiazole-5-thiol (MTD), and this compound can also inhibit the gamma-carboxylation of glutamate. However, unlike MTT, which is known to be present in vivo after the administration of drugs that include this structure, there have been no reports that MTD is present in vivo after cefazolin administration. We set out to determine whether MTD might be present in the tissues of patients treated with cefazolin prior to surgery. To do that, we took advantage of the fact that heterocyclic thiols can undergo S-methylation catalyzed by the genetically polymorphic drug-metabolizing enzyme thiopurine S-methyltransferase (TPMT). Initially, we tested recombinant human TPMT as a "reagent" to S-methylate MTD. MTD was a substrate for TPMT-catalyzed S-methylation, with an apparent K(m) value of 63 micro M. Recombinant TPMT, with [(14)C-methyl]S-adenosyl-L-methionine as a cosubstrate, was then used to radioactively label a methyl acceptor substrate present in liver and kidney cytosol preparations from patients who had been treated preoperatively with cefazolin. Pooled renal cytosol from 10 of those patients was used to purify and isolate the methylated product by reverse-phase high-performance liquid chromatography. That methylated compound coeluted with S-methyl MTD. When the methylated product was subjected to tandem mass spectrometry, it was identified as S-methyl MTD. Therefore, MTD is present in the tissues of patients treated with cefazolin. These observations also raise the possibility that the TPMT genetic polymorphism may represent a risk factor for cefazolin-induced hypoprothrombinemia since subjects who genetically lack TPMT would be unable to catalyze this MTD biotransformation pathway.

Transient lupus anticoagulant associated with hypoprothrombinemia and factor XII deficiency following adenovirus infection.

A potent lupus anticoagulant (LA) was detected in four children, 1 week after the clinical onset of an adenovirus infection. The adenovirus infection was documented by direct virus detection in the stool of one patient and serologically in the others. None of the children had elevated titers of IgM- and only one of IgG-anticardiolipin antibodies (ACA). All patients had a marked reduction of prothrombin activity as well as antigen. Prothrombin-antibody complexes were demonstrated in the patients' plasma or mixtures of patient and normal plasma. Factor XII activity was moderately reduced in three of the patients. All coagulation abnormalities returned to normal within 4-12 weeks. Localized bleeding was observed in two cases, but there was no generalized bleeding tendency or evidence of thrombosis.

Vitamin K-dependent carboxylase: effect of endogenous microsomal protein precursors on the rate of exogenous substrate carboxylation.

Rat liver microsomes contain a Triton X-100 solubilizable vitamin K-dependent carboxylase activity that converts specific glutamyl residues of a microsomal prothrombin precursor to gamma-carboxyglutamyl residues. This activity has been studied in partially (0.25% Triton X-100) and completely (1.0% Triton X-100) solubilized rat liver microsomal preparations. The rate of vitamin K-dependent carboxylation of endogenous microsomal protein precursors was very rapid in the completely solubilized liver microsomal preparation, and carboxylation of an exogenous peptide substrate (Phe-Leu-Glu-Glu-Leu) proceeded at the same time. In the partially solubilized liver microsomal preparation, the rate of protein carboxylation was greatly reduced, and a lag in carboxylation of the exogenous substrate was observed. When microsomal preparations which were depleted of endogenous precursors were used, this lag was eliminated. These data suggest that both substrates utilize the same microsomal pool of carboxylase and that the fraction of the carboxylase bound to the endogenous precursors is not immediately available to exogenous substrates.

Regulation of vitamin K-dependent carboxylation.

Vitamin K-dependent carboxylation activity measured with pentapeptide substrate (Phe-Leu-Glu-Glu-Leu) gradually decreases upon in vivo injection of vitamin K to vitamin K-deficient rats. A decrease in pentapeptide carboxylation can also be observed by the in vitro addition of antibodies against prothrombin and other vitamin K-dependent proteins to the soluble system derived from vitamin K-deficient rat liver microsomes. In both cases, adding back in vitro partially decarboxylated vitamin K-dependent proteins or purified hepatic prothrombin precursor restores the level of pentapeptide carboxylation. After warfarin treatment, a 3-fold increase in carboxylation results, which can be abolished by giving cycloheximide along with the warfarin. However, the resulting decreased activity is restored by the in vitro addition of partially decarboxylated vitamin K-dependent proteins. These data are consistent with the hypothesis that (after warfarin treatment) increased peptide carboxylation is primarily due to activation of the system by precursor proteins, rather than synthesis of an increased amount of enzyme.

The effect of several viper venoms on prothrombin Padua.

The effect of four viper venoms (Oxyuranus scutellatus, Notechis scutatus scutatus, Echis carinatus, Naja nigricollis) on prothrombin Padua has been studied. Using Oxyuranus scutellatus venom and Notechis scutatus scutatus venom, prothrombin activity resulted to be moderately decreased similarly to what observed with other one-stage and two-stage methods. On the contrary, using Echis carinatus venom a normal level was obtained. No clotting was observed using the Naja nigricollis venom, regardless of the concentration used. The normal level of factor II obtained with Echis carinatus venom as compared with the low levels obtained with the other venoms, suggests that it acts on a different site of the prothrombin molecule.

Mechanism of action of vitamin K: demonstration of a liver precursor of prothrombin.

Extracts of sonicated liver microsomes that are prepared from rats deficient in vitamin K or from rats given vitamin K antagonists contain a factor that liberates a thrombin-like activity when it is incubated with venom from Echis carinatus. The amount of this factor is low in control rats and in hypoprothrombinemic rats given vitamin K 1 hour before they were killed. These data indicate that this factor is a protein precursor of prothrombin, which is synthesized in the liver.