Upregulation of the antithrombotic protein C pathway at birth

Serious thrombotic complications occur in sick neonates, while healthy infants have a very low risk of thrombosis. To better understand the regulation of physiological anticoagulation at birth, components of the protein C pathway were measured in cord plasma samples from 14 full-term healthy newborns and in samples from 10 adult controls. Although zymogen protein C was significantly reduced in cord plasma (mean +/- SEM in cord vs. adult sample 37 +/- 1.4% vs. 90 +/- 5.5%, p < 0.0001), levels of the active enzyme, activated protein C (APC), were not (119 +/- 20% vs. 75 +/- 12%, p = 0.0762). Relative to the protein C level, cord plasmas had a 5.2-fold higher APC level (p < 0.01). The APC increase was partially due to slower inactivation of APC in cord plasma (half-life for APC 50 min in cord plasma vs. 27 minutes in adult plasma). Increased sensitivity of factor V to inactivation by APC in cord plasma was observed since the activated partial thromboplastin time-based APC sensitivity ratio was significantly increased for cord vs. adult plasma samples (2.28 +/- 0.09 versus 1.97 +/- 0.03, p < 0.01). Thus, despite low zymogen protein C, the protein C pathway in newborns seems to be functionally well developed and at an activated stage at birth.

Inhibition of prothrombinase by human secretory phospholipase A2 involves binding to factor Xa

Human group II secretory phospholipase A2 (hsPLA2) exhibits significant anticoagulant activity that does not require its enzymatic activity. We examined which coagulation factor was targeted by hsPLA2 and analyzed which region of the protein may be involved in this inhibition. Prothrombin time coagulation assays indicated that hsPLA2 did not inhibit activated factor V (FVa) activity, whereas activated factor X (FXa) one-stage coagulation assays suggested that FXa was inhibited. The inhibitory effect of hsPLA2 on prothrombinase activity of FXa, FV, phospholipids, and Ca2+ complex was markedly enhanced upon preincubation of hsPLA2 with FXa but not with FV. Prothrombinase activity was also strongly inhibited by hsPLA2 in the absence of PL. High concentrations of FVa in the prothrombinase generation assay reversed the inhibitory effect of hsPLA2. By using isothermal titration calorimetry, we demonstrated that hsPLA2 binds to FXa in solution with a 1:1 stoichiometry and a Kd of 230 nM. By using surface plasmon resonance we determined the rate constants, kon and koff, of the FXa/hsPLA2 interaction and analyzed the Ca2+ effect on these constants. When preincubated with FXa, synthetic peptides comprising residues 51-74 and 51-62 of hsPLA2 inhibited prothrombinase assays, providing evidence that this part of the molecule, which shares similarities with a region of FVa that binds to FXa, is likely involved in the anticoagulant interaction of hsPLA2 with FXa. In conclusion, we propose that residues 51-62 of hsPLA2 bind to FXa at a FVa-binding site and that hsPLA2 decreases the prothrombinase generation by preventing FXa.FVa complex formation.

Chemical synthesis of human protein S thrombin-sensitive module and first epidermal growth factor module

Human plasma protein S is a nonenzymatic cofactor for activated protein C (APC) in the inactivation of coagulation factors Va and VIIIa, and helps to provide an essential negative feedback on blood coagulation. Previous indirect evidence suggested that the thrombin-sensitive region (TSR: residues 47-75, 1 disulfide) and the first epidermal growth factorlike region (EGF1: residues 76-116, 3 disulfides) of protein S may be functionally important for expression of its APC cofactor activity. To study the functional importance of these modules directly, access to the isolated TSR and EGF1 modules would be preferred. Recombinant expression of protein S intact TSR and correctly folded EGF1 has not been possible. Here we describe the synthesis of both TSR and EGF1 modules by stepwise solid phase peptide synthesis using the in situ neutralization/2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluron ium hexafluorophosphate activation procedure for tert-butoxycarbonyl chemistry. For the TSR, correct intramodular disulfide bonding was confirmed. To overcome folding difficulties with the EGF1, a two-step oxidation procedure was used in which the cysteines involved in the middle, crossing, disulfide bond (Cys85-Cys102) remained protected with acetamidomethyl (Acm) groups after hydrogen fluoride treatment of the peptide resin. Selective formation of the first two disulfide bonds (Cys80-Cys93 and Cys104-Cys113) was followed by release of the Acm groups and subsequent formation of the third disulfide bond (Cys85-Cys102). CD studies revealed 54% of beta-sheet/turn in the EGF1 that is characteristic for EGF modules. Deuterium exchange studies suggested a very tightly packed core in EGF1 that is not accessible to the bulk solvent, likely a result from the compact structure caused by its three disulfide bonds. The 30% beta-sheet structure observed in the TSR involved amide protons that could be readily exchanged by deuterons, likely reflecting a more flexible structure of the TSR loop in contrast to the rigid structure of EGF1. The establishment of synthetic access to the TSR and EGF1 of protein S provides a versatile tool to study interactions of these modules with the blood coagulation components of the anticoagulant plasma protein C pathway.

Binding site for blood coagulation factor Xa involving residues 311-325 in factor Va

Factor Va inactivation by activated protein C is associated with cleavages at Arg306, Arg506, and Arg679 with Arg306 cleavage causing the major activity loss. To study functional roles of the Arg306 region, overlapping 15-mer peptides representing the sequence of factor Va residues 271-345 were synthesized and screened for anticoagulant activities. The peptide containing residues 311-325 (VP311) noncompetitively inhibited prothrombin activation by factor Xa, but only in the presence of factor Va. Fluorescence studies showed that VP311 bound to fluorescence-labeled 5-dimethylaminonaphthalene-1-sulfonyl-Glu-Gly-Arg factor Xa in solution with a Kd of 70 microM. Diisopropylphosphoryl factor Xa and factor Xa but not factor VII/VIIa or prothrombin bound to immobilized VP311 with relatively high affinity. These results support the hypothesis that residues 311-325, which are positioned between the A1 and A2 domains of factor Va and likely exposed to solvent, contribute to the binding of factor Xa by factor Va. Based on this hypothesis, it is suggested that cleavage by activated protein C at Arg306 in factor Va not only severs the covalent connection between the A1 and A2 domains but also disrupts the environment and structure of residues 311-325, thereby down-regulating the binding of factor Xa to factor Va.

Involvement of amino acid residues 423-429 of human protein S in binding to C4b-binding protein

Human protein S binds to C4b-binding protein (C4BP) both in plasma and in a system using purified proteins. Amino acid residues 420-434 of the first disulfide loop of the sex hormone binding globulinlike domain of protein S are involved in the interaction of protein S with C4BP. To define the involvement of specific polar amino acids within residues 420-434, we studied in parallel synthetic protein S peptides and recombinant protein S variants containing the same amino acid replacements, K423E, E424K, Q427E and K429E. Synthetic peptide analogs of peptide PSP-420 (residues 420-434) were assayed for binding C4BP and as inhibitors of complex formation. The PSP-420 peptide and the analogous peptide with the substitution E424K, but not the peptides containing the substitutions K423E and K429E, were able to bind C4BP. Recombinant proteins with mutations of K423E, Q427E and K429E showed reduced affinity for C4BP compared to plasma protein S, recombinant wild type protein S, or E424K-protein S. These results suggest that Lys-423, Gln-427 and Lys-429 of protein S are important for normal binding to C4BP. The anti-protein S monoclonal antibody LJ-56, raised against peptide PSP-420, recognizes only free protein S and inhibits complex formation with C4BP. Antibody LJ-56 recognized the E424K and Q427E peptides but not the K423E or K429E peptides. Similarly, the E424K and Q427E protein S mutants were recognized by LJ-56, whereas the K423E and K429E protein S mutants were not recognized. This suggests that both in the peptide PSP-420 and in protein S, Lys-423 and Lys-429 significantly contribute to binding to antibody LJ-56. These results demonstrate that protein S residues 423, 427 and 429, but not residue 424, are involved in binding to both the antibody LJ-56 and to C4BP. When peptides PSP 420 and SL-6 (residues 447-460) with carboxyterminal amide or carboxylate moieties were compared to their ability to inhibit C4BP-protein S complexation, PSP-420-amide was the most potent. This finding together with the other results described here supports the hypothesis that the residues 420 and 434 in protein S provides a major binding site for C4BP.

Circulating activated protein C in subjects with heterozygous Gln506-factor V

Congenital resistance to activated protein C due to a point mutation in the factor V gene (Gln506-FV) is the most common genetic risk factor for familial venous thrombosis. Considering the central role of activated protein C as a physiological anticoagulant, the question of why the thrombotic risk associated with Gln506-FV was not more pronounced was asked. We hypothesized that in Gln506-FV heterozygotes, enhanced thrombin formation might preferentially activate protein C and thereby constitute a compensatory antithrombotic effect. We compared the circulatory level of activated protein C in twelve heterozygous carriers of Gln506-FV mutation with that in eighteen noncarriers in same families, and used prothrombin fragment 1+2 as a measure of thrombin generation. The circulating level of activated protein C was higher but not significantly different in heterozygotes compared with normal relatives. Activated protein C levels correlated strongly and positively with protein C antigen levels in both carriers (Spearman R 0.684, p < 0.05) and controls (Spearman R 0.642, p < 0.01). Correlation between activated protein C and prothrombin fragment 1+2 levels was of borderline significance (Spearman R 0.354, p = 0.055). In the current study, thrombin formation assessed by prothrombin fragment 1+2 levels was not significantly enhanced in subjects with heterozygous Gln506-FV compared with family members without the mutation. In conclusion, enhanced thrombin formation is not present in all healthy Gln506-FV heterozygotes in basal conditions. It seems that enhanced protein C activation by thrombin does not constitute a compensatory anticoagulant feedback loop in heterozygous carriers of Gln506-FV. However, the positive correlation between prothrombin fragment 1+2 and activated protein C suggests that, in healthy subjects and in basal conditions, thrombin upregulates the anticoagulant protein C pathway. Thus, it is questionable whether prothrombin fragment 1+2 can directly be used as an indicator of a hypercoagulable state.

Activated protein C resistance: What have we learned now that the dust has settled?

Activated protein C resistance, most often caused by a single point mutation in the factor V gene, is the most common hereditary coagulation abnormality associated with venous thrombosis. Recent studies have established risk estimates of thrombosis in multiple clinical settings. The impact of activated protein C resistance on the absolute thrombotic risk of a given individual is significantly influenced by the presence or absence of other acquired or congenital risk factors. In this paper, the complex interplay between different risk factors for venous thrombosis is discussed. Additionally, the potential significance for arterial thrombosis of activated protein C resistance, that is not due to the genetic variant, Glutamine 506 factor V, is discussed.

A factor IX-deficient mouse model for hemophilia B gene therapy

We have generated a mouse where the clotting factor IX (FIX) gene has been disrupted by homologous recombination. The FIX nullizygous (-/-) mouse was devoid of factor IX antigen in plasma. Consistent with the bleeding disorder, the factor IX coagulant activities for wild-type (+/+), heterozygous (+/-), and homozygous (-/-) mice were 92%, 53%, and <5%, respectively, in activated partial thromboplastin time assays. Plasma factor IX activity in the deficient mice (-/-) was restored by introducing wild-type murine FIX gene via adenoviral vectors. Thus, these factor IX-deficient mice provide a useful animal model for gene therapy studies of hemophilia B.

Protein S is inducible by interleukin 4 in T cells and inhibits lymphoid cell procoagulant activity

Extravascular procoagulant activity often accompanies cell-mediated immune responses and systemic administration of pharmacologic anticoagulants prevents cell-mediated delayed-type hypersensitivity reactions. These observations suggest a direct association between coagulation and cell-mediated immunity. The cytokine interleukin (IL)-4 potently suppresses cell-mediated immune responses, but its mechanism of action remains to be determined. Herein we demonstrate that the physiologic anticoagulant protein S is IL-4-inducible in primary T cells. Although protein S was known to inhibit the classic factor Va-dependent prothrombinase assembled by endothelial cells and platelets, we found that protein S also inhibits the factor Va-independent prothrombinase assembled by lymphoid cells. Thus, protein S-mediated down-regulation of lymphoid cell procoagulant activity may be one mechanism by which IL-4 antagonizes cell-mediated immunity.

Activated protein C correlates inversely with thrombin levels in resting healthy individuals

To study whether the circulating anticoagulant, activated protein C (APC), could be a regulator of thrombin activity in basal physiological conditions, fibrinopeptide A and activated protein C levels were determined in samples from 40 healthy individuals. There was a significant inverse correlation between the fibrinopeptide A and APC levels (Spearman rank correlation R = -0.487; P = 0.0023). Because of well-known mechanisms by which decreasing APC levels could cause increased thrombin formation, we suggest that APC may downregulate thrombin activity in subjects with normal protein C levels. Regulation of thrombin formation in health is likely significant for maintaining vascular patency but its molecular mechanisms are poorly understood. The current data suggest that a single physiological anticoagulant, namely APC, may be a significant regulator of procoagulant thrombin activity.

Total chemical synthesis of enzymatically active human type II secretory phospholipase A2

Human group II secretory phospholipase A2 (sPLA2) is an enzyme found in the alpha granules of platelets and at inflammatory sites. Although its physiological function is unclear, sPLA2 can inhibit blood coagulation reactions independent of its lipolytic action. To study the molecular basis of PLA2 activities, we developed a total chemical synthesis of sPLA2 by chemical ligation of large unprotected peptides. The synthetic segments PLA2-(1-58)-alphaCOSCH2COOH and PLA2-(59-124) were prepared by stepwise solid-phase peptide synthesis and ligated to yield a peptide bond between Gly58 and Cys59. The 124-residue polypeptide product (mass: 13,920 +/- 2 Da) was folded to yield one major product (mass: 13,905 +/- 1 Da), the loss of 15 +/- 3 Da reflecting the formation of seven disulfide bonds. Circular dichroism studies of synthetic sPLA2 showed alpha-helix, beta-structure, and random coil contents consistent with those found in the crystal structure of sPLA2. Synthetic sPLA2 had kcat and Km values identical to those of recombinant sPLA2 for hydrolysis of 1,2-bis(heptanoylthio)-phosphatidylcholine. Synthetic sPLA2, like recombinant sPLA2, inhibited thrombin generation from prothrombinase complex (factors Xa, V, II, Ca2+, and phospholipids). In the absence of phospholipids, both synthetic and recombinant sPLA2 inhibited by 70% prothrombin activation by factors Xa, Va, and Ca2+. Thus, synthetic sPLA2 is a phospholipid-independent anticoagulant like recombinant or natural sPLA2. This study demonstrates that chemical synthesis of sPLA2 yields a fully active native-like enzyme and offers a straightforward tool to provide sPLA2 analogs for structure-activity studies of anticoagulant, lipolytic, or inflammatory activities.

The carbohydrate moiety of factor V modulates inactivation by activated protein C

An important risk factor for thrombosis is the polymorphism R506Q in factor V that causes resistance of factor Va to proteolytic inactivation by activated protein C (APC). To study the potential influence of the carbohydrate moieties of factor Va on its inactivation by APC, factor V was subjected to mild deglycosylation (neuraminidase plus N-glycanase) under nondenaturing conditions. The APC resistance ratio values (ratio of activated partial thromboplastin time [APTT] clotting times with and without APC) of the treated factor V were increased (2.4 to 3.4) as measured in APTT assays. O-glycanase treatment of factor V did not change the APC resistance ratio. The procoagulant activity of factor V as well as its activation by thrombin was not affected by mild deglycosylation. Treatment of factor V with neuraminidase and N-glycanase mainly altered the electrophoretic mobility of the factor Va heavy chain, whereas treatment with O-glycanase changed the mobility of the connecting region. This suggests that the removal of the N-linked carbohydrates from the heavy chain of factor Va, which is the substrate for APC, is responsible for the increase in susceptibility to inactivation by APC. Thus, variability in carbohydrate could account for some of the known variability in APC resistance ratios, including the presence of borderline or low APC resistance ratios among patients who lack the R506Q mutation.

Anticoagulant synergism of heparin and activated protein C in vitro. Role of a novel anticoagulant mechanism of heparin, enhancement of inactivation of factor V by activated protein C

Interactions between standard heparin and the physiological anticoagulant plasma protein, activated protein C (APC) were studied. The ability of heparin to prolong the activated partial thromboplastin time and the factor Xa- one-stage clotting time of normal plasma was markedly enhanced by addition of purified APC to the assays. Experiments using purified clotting factors showed that heparin enhanced by fourfold the phospholipid-dependent inactivation of factor V by APC. In contrast to factor V, there was no effect of heparin on inactivation of thrombin-activated factor Va by APC. Based on SDS-PAGE analysis, heparin enhanced the rate of proteolysis of factor V but not factor Va by APC. Coagulation assays using immunodepleted plasmas showed that the enhancement of heparin action by APC was independent of antithrombin III, heparin cofactor II, and protein S. Experiments using purified proteins showed that heparin did not inhibit factor V activation by thrombin. In summary, heparin and APC showed significant anticoagulant synergy in plasma due to three mechanisms that simultaneously decreased thrombin generation by the prothrombinase complex. These mechanisms include: first, heparin enhancement of antithrombin III-dependent inhibition of factor V activation by thrombin; second, the inactivation of membrane-bound FVa by APC; and third, the proteolytic inactivation of membrane-bound factor V by APC, which is enhanced by heparin.

Potent blood coagulant activity of human semen due to prostasome-bound tissue factor

Human semen contains very potent blood clotting activity; for example, seminal serum diluted up to 10,000-fold significantly decreased the recalcification clotting time of blood plasma. This seminal coagulant activity was dependent on factor X and calcium ions, suggesting the presence of a facto X activator. Immunoblotting analysis and immunoadsorption studies confirmed the presence of tissue factor antigen (45 kDa) in semen. Centrifugation studies suggested that tissue factor was membrane associated, and fractionation of seminal serum by gel filtration followed by immunoelectron microscopy revealed that tissue factor antigen was on the prostasome vesicle surface. Tissue factor originated from prostatic fluid and not from seminal vesicle secretions. Tissue factor antigen averaged 21 ng/ml in seminal serum. Hypothetical roles for very high levels of tissue factor in semen include several possibilities. In the event of abrasion and bleeding during intercourse, rapid blood clotting at lesion sites would prevent sperm and seminal components, including infectious agents such as human immunodeficiency virus, from entering the blood stream, generating antibodies, or promoting infectious disease. This could imply that development of infection from semen-borne agents or development of antisperm antibodies in some patients could result from impairment or absence of seminal tissue factor.

A single genetic origin for a common Caucasian risk factor for venous thrombosis

A common genetic risk factor for venous thrombosis among Caucasoid subpopulations is a polymorphism, nt G1691A, in blood coagulation factor V that replaces Arg506 with Gln and imparts resistance of factor Va to the anticoagulant, activated protein C. Haplotype analyses using six dimorphic sites in the factor V gene for 117 Caucasian subjects of Jewish, Arab, Austrian, and French origin who were homozygous for nt A1691 compared with 167 controls (nt G1691) support a single origin for this polymorphism. The nt G1691A mutation is estimated to have arisen circa 21,000 to 34,000 years ago, ie, after the evolutionary divergence of Africans from non-Africans and of Caucasoid from Mongoloid subpopulations.

Nonenzymatic anticoagulant activity of the mutant serine protease Ser360Ala-activated protein C mediated by factor Va

The human plasma serine protease, activated protein C (APC), primarily exerts its anticoagulant function by proteolytic inactivation of the blood coagulation cofactors Va and VIIIa. A recombinant active site Ser 360 to Ala mutation of protein C was prepared, and the mutant protein was expressed in human 293 kidney cells and purified. The activation peptide of the mutant protein C zymogen was cleaved by a snake venom activator, Protac C, but the "activated" S360A APC did not have amidolytic activity. However, it did exhibit significant anticoagulant activity both in clotting assays and in a purified protein assay system that measured prothrombinase activity. The S360A APC was compared to plasma-derived and wild-type recombinant APC. The anticoagulant activity of the mutant, but not native APC, was resistant to diisopropyl fluorophosphate, whereas all APCs were inhibited by monoclonal antibodies against APC. In contrast to native APC, S360A APC was not inactivated by serine protease inhibitors in plasma and did not bind to the highly reactive mutant protease inhibitor M358R alpha 1 antitrypsin. Since plasma serpins provide the major mechanism for inactivating APC in vivo, this suggests that S360A APC would have a long half-life in vivo, with potential therapeutic advantages. S360A APC rapidly inhibited factor Va in a nonenzymatic manner since it apparently did not proteolyze factor Va. These data suggest that native APC may exhibit rapid nonenzymatic anticoagulant activity followed by enzymatic irreversible proteolysis of factor Va. The results of clotting assays and prothrombinase assays showed that S360A APC could not inhibit the variant Gln 506-FVa compared with normal Arg 506-FVa, suggesting that the active site of S360A APC binds to FVa at or near Arg 506.

Impairments of the protein C system and fibrinolysis in infection-associated stroke

BACKGROUND AND PURPOSE

Infection/inflammation appears to be an important predisposing risk factor for brain infarction, but little is known regarding underlying molecular mechanisms. We examined the hypothesis that patients with brain infarction preceded by infection/inflammation within 1 week could be identified by a distinctive procoagulant laboratory profile characterized by abnormalities in the protein C system and endogenous fibrinolysis.

METHODS

We performed a case-control study examining the relationship between preceding systemic infectious/inflammatory syndromes and selected immunohematologic variables in 36 patients with acute brain infarction and 81 control subjects (community control subjects [n = 47] and hospitalized nonstroke neurological patient controls [n = 34]).

RESULTS

The stroke group had a lower mean level of the circulating antithrombotic enzyme activated protein C (APC) (4.33 +/- 0.34% [log-transformed percentage of control value, mean +/- SD]) than community control subjects (4.51 +/- 0.27%, P < .02) or hospitalized neurological patient controls (4.57 +/- 0.31%, P < .005). The lowest circulating APC levels were found in the stroke group with antecedent infection/inflammation within 1 week preceding index brain infarction (4.23 +/- 0.4%, n = 12). Within the stroke group, circulating APC levels were inversely related to IgG isotype anticardiolipin antibody titers (r = -.55, P < .001). Only the stroke group with infection/inflammation within 1 week had elevated plasma C4b binding protein compared with control subjects (141 +/- 61% versus 112 +/- 44%, P < .05). Stroke patients with antecedent infection/inflammation had a distinctively lower ratio of active tissue plasminogen activator to plasminogen activator inhibitor (0.11 +/- 0.04, n = 9) than other stroke patients (0.19 +/- 0.06, n = 9, P < .01) and control subjects (0.22 +/- 0.16, n = 17, P < .02).

CONCLUSIONS

Impairments in the protein C pathway and endogenous fibrinolysis may contribute to the increased risk for brain infarction after recent (< or = 1 week) infection/inflammation. A decrease in the circulating anticoagulant APC may be related to elevated antiphospholipid antibody titers.

Central venous thrombosis after cardiac operations in children

To evaluate the incidence, mortality, late outcome, and cause of central venous thrombosis after pediatric heart operations and other operations performed with cardiopulmonary bypass, we identified patients with postoperative central venous thrombosis during a 10-year period at a single pediatric hospital. There had been 1591 open heart (with bypass) and 1086 closed heart (no bypass) procedures and 13 operations with cardiopulmonary bypass for extracardiac indications. There were 20 patients with central venous thrombosis, yielding incidences of 1.1% and 0.2% after cardiopulmonary bypass and after closed heart operations, respectively. When neonates were compared with older children (1 to 119 months of age) undergoing heart procedures, a tenfold increase (5.8% vs 0.6%) (p < 0.001) in the incidence of central venous thrombosis was observed. The mortality was eight of 20 (40%). Central venous thrombosis contributed to seven deaths and it was a direct cause of one death. Ten patients were reinvestigated 5 to 108 months after central venous thrombosis. The outcome of surgery was excellent in eight. Two had residual thrombosis, but this was not hemodynamically significant to the cardiorespiratory condition of the patients. During or preceding thrombosis, low levels of antithrombin III and/or protein C and high levels of the plasminogen activator inhibitor were observed in five of the patients. A congenital thrombotic risk factor, "resistance to activated protein C," was found in two of 12 tested patients with central venous thrombosis (17%). In conclusion, central venous thrombosis, especially in neonates, is an important cause of morbidity and mortality after cardiac operations. The cause is multifactorial, with contributions from multiple acquired thrombophilic coagulation abnormalities, and resistance to activated protein C may act as a risk factor for thrombosis already during neonatal period.

Binding sites for blood coagulation factor Xa and protein S involving residues 493-506 in factor Va

Inactivation due to cleavage of Factor Va (FVa) at Arg 506 by activated protein C (APC) helps to downregulate blood coagulation. To identify potential functional roles of amino acids near Arg 506, synthetic overlapping pentadecapeptides comprising FVa heavy chain residues 481-525 were tested for their ability to inhibit prothrombin activation by prothrombinase complexes [Factor Xa (FXa):FVa:phospholipids:Ca2+]. The most potent inhibition was observed for peptide VP493 (residues 493-506), with 50% inhibition at 2.5 microM. VP493 also inhibited FXa in plasma in FXa-1-stage clotting assays by 50% at 3 microM. When the C-terminal carboxamide group of VP493 was replaced by a carboxyl group, most prothrombinase inhibitory activity was lost. VP493 preincubated with FXa inhibited prothrombinase with a pattern of mixed inhibition. Homologous peptides from Factor VIII sequences did not inhibit prothrombinase. Affinity-purified antibodies to VP493 inhibited prothrombinase activity and prolonged FXa-1-stage clotting times. VP493 also blocked the ability of protein S to inhibit prothrombinase independently of APC. Immobilized VP493 bound specifically with similar affinity to both FXa and protein S (Kd approximately 40 nM), but did not measurably bind prothrombin or APC. These studies suggest that FVa residues 493-506 contribute to binding sites for both FXa and protein S, providing a rationale for the ability of protein S to negate the protective effect of FXa toward APC cleavage of FVa. Possible loss of this FVa binding site for FXa due to cleavage at Arg 506 by APC may help explain why this cleavage causes 40% decrease in FVa activity and facilitates inactivation of FVa.

Venous thromboembolism associated with double heterozygosity for R506Q mutation of factor V and for T298M mutation of protein C in a large family of a previously described homozygous protein C-deficient newborn with massive thrombosis

It is remarkable that certain patients with heterozygous protein C (PC) deficiency manifest venous thromboembolism (VTE), whereas others, particularly those belonging to families with homozygous PC deficiency, remain asymptomatic. The goals of the present study of a family, in which the proband had homozygous PC deficiency, were to identify members with and without VTE, to determine the mutation causing PC deficiency, and to search for the R506Q mutation of factor V (FV) causing activated PC resistance. Heterozygosity for a T298M mutation in exon 9 of the PC gene was found in the father of the homozygous proband who died of massive thrombosis. Based on analysis of a three-dimensional molecular model of PC, we speculate that this mutation causes type I deficiency due to disruption of packing of hydrophobic side chains and loss of an H-bond between Q184 and T298. Forty-six family members were examined for the T298M mutation by polymerase chain reaction (PCR) amplification of exon 9 and restriction analysis using Mae III and for the FV R506Q mutation by PCR amplification of exon 10 and restriction analysis using Mnl I. VTE was observed in five of 11 members who were heterozygous for both PC and FV mutations. In contrast, VTE was not observed for the PC mutation in 13 heterozygotes who had normal FV, including the parents of the deceased proband, 10 heterozygotes for the FV mutation who had normal PC, and 12 individuals bearing neither mutation. These observations extend recent evidence of an increased thrombotic risk conferred by the coexistence of heterozygous PC deficiency and heterozygous activated PC resistance and support the paradigm in which hereditary thrombophilia is often a multigenic disease.

Activated protein C resistance in ischemic stroke not due to factor V arginine506-->glutamine mutation

BACKGROUND AND PURPOSE

Resistance to activated protein C (APC), a natural plasma anticoagulant, is the most common identifiable risk factor for venous thromboembolic disease. One point mutation in coagulation factor V that renders it APC-resistant is found in >90% of APC-resistant venous thrombosis patients. To determine the prevalence of APC resistance and of this factor V mutation in stroke, we screened a group of ischemic stroke patients.

METHODS

Hispanic ischemic stroke patients were screened using two different activated partial thromboplastin time-based assays. One assay using neat patient plasma determined APC resistance, and the other assay using patient plasma diluted into factor V-deficient plasma determined APC-resistant factor V, including the Arg506-->Gln mutation. Results were compared with those in 31 Hispanic control subjects of similar ages.

RESULTS

Six of 63 (9.5%) stroke patients had APC resistance compared with none of 31 (0%) control subjects. No patient or control subject had APC-resistant factor V, ie, the factor V Arg506-->Gln mutation.

CONCLUSIONS

In Hispanic patients with ischemic stroke, the incidence (approximately 10%) of APC resistance is not caused by the factor V Arg506-->Gln mutation. APC resistance not caused by this factor V mutation may be a risk factor for ischemic stroke in this population.

The scrotal hitch for hemostasis and edema prevention in scrotal surgery

Postoperative hematoma, edema, and echymosis are the most common complications in scrotal surgery. Without proper hemostasis intraoperatively or appropriate scrotal compression postoperatively, significant morbidity, including re-exploration and prolonged convalescence, can occur. The scrotal hitch provides hemostasis, minimizes edema and hematomas, and reaffirms other clinicians' findings.

Primary leiomyosarcoma of the ureter

We report the thirteenth case of primary leiomyosarcoma of the ureter, as well as a summary of previous cases. It is the first case reported to be studied by computer tomography and immunohistochemical procedure. Further evaluation included intravenous pyelogram, cystoscopy with retrograde pyelogram, cell block for cytology, and electron microscopy. Leiomyosarcoma is a very rare disease that is difficult to diagnose. It has a very poor 5-year disease-specific survival.