Radioiodination of the active site of tissue plasminogen activator: a method for radiolabeling serine proteases with tyrosylprolylarginyl chloromethyl ketone

Tissue-type plasminogen activator mediates neuronal detection and adaptation to metabolic stress

Adenosine monophosphate-activated protein kinase (AMPK) is an energy sensor that regulates cellular adaptation to metabolic stress. Tissue-type plasminogen activator (tPA) is a serine proteinase found in the intravascular space, where its main role is as thrombolytic enzyme, and in neurons, where its function is less well understood. Here, we report that glucose deprivation induces the mobilization and package of neuronal tPA into presynaptic vesicles. Mass spectrometry and immunohistochemical studies show that the release of this tPA in the synaptic space induces AMPK activation in the postsynaptic terminal, and an AMPK-mediated increase in neuronal uptake of glucose and neuronal adenosine 5'(tetrahydrogen triphosphate; ATP) synthesis. This effect is independent of tPA's proteolytic properties, and instead requires the presence of functional N-methyl-D-aspartate receptors (NMDARs). In agreement with these observations, positron emission tomography (PET) studies and biochemical analysis with synaptoneurosomes indicate that the intravenous administration of recombinant tPA (rtPA) after transient middle cerebral artery occlusion (tMCAO) induces AMPK activation in the synaptic space and NMDAR-mediated glucose uptake in the ischemic brain. These data indicate that the release of neuronal tPA or treatment with rtPA activate a cell signaling pathway in the synaptic space that promotes the detection and adaptation to metabolic stress.

Bivalent binding to gammaA/gamma'-fibrin engages both exosites of thrombin and protects it from inhibition by the antithrombin-heparin complex

Thrombin exosite 1 binds the predominant gamma(A)/gamma(A)-fibrin form with low affinity. A subpopulation of fibrin molecules, gamma(A)/gamma'-fibrin, has an extended COOH terminus gamma'-chain that binds exosite 2 of thrombin. Bivalent binding to gamma(A)/gamma'-fibrin increases the affinity of thrombin 10-fold, as determined by surface plasmon resonance. Because of its higher affinity, thrombin dissociates 7-fold more slowly from gamma(A)/gamma'-fibrin clots than from gamma(A)/gamma(A)-fibrin clots. After 24 h of washing, however, both gamma(A)/gamma'- and gamma(A)/gamma(A)-fibrin clots generate fibrinopeptide A when incubated with fibrinogen, indicating the retention of active thrombin. Previous studies demonstrated that heparin heightens the affinity of thrombin for fibrin by simultaneously binding to fibrin and exosite 2 on thrombin to generate a ternary heparin-thrombin-fibrin complex that protects thrombin from inhibition by antithrombin and heparin cofactor II. In contrast, dermatan sulfate does not promote ternary complex formation because it does not bind to fibrin. Heparin-catalyzed rates of thrombin inhibition by antithrombin were 5-fold slower in gamma(A)/gamma'-fibrin clots than they were in gamma(A)/gamma(A)-fibrin clots. This difference reflects bivalent binding of thrombin to gamma(A)/gamma'-fibrin because (a) it is abolished by addition of a gamma'-chain-directed antibody that blocks exosite 2-mediated binding of thrombin to the gamma'-chain and (b) the dermatan sulfate-catalyzed rate of thrombin inhibition by heparin cofactor II also is lower with gamma(A)/gamma'-fibrin than with gamma(A)/gamma(A)-fibrin clots. Thus, bivalent binding of thrombin to gamma(A)/gamma'-fibrin protects thrombin from inhibition, raising the possibility that gamma(A)/gamma'-fibrin serves as a reservoir of active thrombin that renders thrombi thrombogenic.

Pharmacokinetics and pharmacodynamics of tenecteplase in fibrinolytic therapy of acute myocardial infarction

Tenecteplase is a novel fibrinolytic protein bioengineered from human tissue plasminogen activator (alteplase) for the therapy of acute ST-segment elevation myocardial infarction. Specific mutations at three sites in the alteplase molecule result in 15-fold higher fibrin specificity, 80-fold reduced binding affinity to the physiological plasminogen activator inhibitor PAI-1 and 6-fold prolonged plasma half-life (22 vs 3.5 minutes). Consequently, tenecteplase can be administered as a single intravenous bolus of 30-50mg (0.53 mg/kg bodyweight) over 5-10 seconds, in contrast to the 90-minute accelerated infusion regimen of alteplase. Tenecteplase plasma concentration-time profiles have been obtained from a total of 179 patients with acute myocardial infarction. Tenecteplase exhibited biphasic disposition; the initial disposition phase was predominant with a mean half-life of 17-24 minutes, and the mean terminal half-life was 65-132 min. Over the clinically relevant dose range of 30-50mg, mean clearance (CL) was 105 ml/min. The mean initial volume of distribution V(1) was 4.2-6.3L, approximating plasma volume, and volume of distribution at steady state was 6.1-9.9L, suggesting limited extravascular distribution or binding. Bodyweight and age were found to influence significantly both CL and V(1). Total bodyweight explained 19% of the variability in CL and 11% of the variability in V(1), and a 10kg increase in total bodyweight resulted in a 9.6 ml/min increase in CL. This relationship aided the development of a rationale for the weight-adjusted dose regimen for tenecteplase. Age explained only a further 11% of the variability in CL. The percentage of patients who achieved normal coronary blood flow was clearly related to AUC. More than 75% of patients achieved normal flow at 90 minutes after administration when their partial AUC(2-90) exceeded 320 microg.min/ml, corresponding to an average plasma concentration of 3.6 microg/ml. Systemic exposure to tenecteplase at all times after bolus administration of 30-50mg was higher than for alteplase 100mg. Tenecteplase has demonstrated equivalent efficacy and improved safety compared with the current gold standard alteplase in a large mortality trial (ASSENT-2). This suggests that the reduced clearance, greater fibrin specificity and higher PAI-1 resistance of tenecteplase allow higher plasma concentrations and thus a more rapid restoration of coronary patency to be attained, while providing a reduction in major non-cerebral bleeding events.

Molecular characterization, enzymatic analysis, and purification of murine proprotein convertase-1/3 (PC1/PC3) secreted from recombinant baculovirus-infected insect cells

A cDNA coding for the murine proprotein convertase-1 (mPC1 also known as mPC3 or mSPC3) was inserted into the Autographa californica nuclear polyhedrosis virus. Following infection of Spodoptera frugiperda cells, the recombinant N-glycosylated protein is secreted into the cell culture medium from which it can be purified to homogeneity as a fully enzymatically active enzyme. Two major secreted molecular forms of mPC1 with apparent molecular weights of 85 and 71 kDa, respectively, and a minor one of 75 kDa are immunodetected in the medium. Automated NH2-terminal sequencing reveals that all three forms result from processing at the predicted zymogen activation site whereas both the 75- and the 71-kDa forms are truncated at their COOH-terminus. Labeling by an active-site titrant demonstrates that the 85-kDa form is optimally labeled at near neutral pH whereas the COOH-truncated forms are optimally labeled at acidic pH. Additionally it is shown that the 85-kDa mPC1 is transformed into the COOH-truncated forms following in vitro incubation at acidic pH levels and in presence of calcium. Concomitantly, the transformation from 85 to 71 kDa is accompanied by a 10- to 40-fold increase in enzymatic activity upon assaying at pH 6.0. The 71-kDa form can be recovered after purification at a level of 1 to 1.5 mg per liter of cell culture medium and is enzymatically stable only in the pH range from 5.0 to 6.5. Cells treated with tunicamycin show a drastically reduced secretion of the convertase in the medium but are not affected by swainsonine and deoxymannojirimycin. Finally, the 85-kDa secreted mPC1 is shown to be sulfated.

Chloromethyl ketones are insulin-like stimulators of lipid synthesis in locust fat body

N alpha-tosyl-L-lysine chloromethyl ketone (TLCK) stimulates lipid synthesis in locust fat body in vitro, and is able to reverse the inhibitory effects of AKH-I on lipid synthesis. Effective stimulatory concentrations of TLCK were in the range of 0.2-1.0 mM. Similar stimulatory effects were also achieved with phenylalanine chloromethyl ketone (PheCK) and leucine chloromethyl ketone (LeuCK), but not with tosyl-phenylalanine chloromethyl ketone (TPCK), dansyl-glu-gly-arg-CK, chloroacetone, chloroacetic acid, chloroacetamide, chloroacetaldehyde, chloroacetyl-L-leucine or acetylated or fluorescamine-labelled TLCK, PheCK, and LeuCK. The level of stimulation caused by TLCK was dependent on incubation time, so that after a 5-h preincubation of fat body tissue with TLCK the stimulated rate was severalfold higher than the control. TLCK also increased the rate of uptake of trehalose and uridine, but not glucose, deoxyglucose or glycine. Increasing concentrations of bovine serum albumin (BSA) in the incubation medium caused a reduction in the rate of TLCK-stimulated acetate uptake, such that levels of uptake were no higher with 1% BSA than in the controls. A range of more specific protease and kinase inhibitors was tested, but none caused stimulation; thus the mode of action of TLCK on the stimulation of acetate uptake has yet to be identified. Elucidation of the mode of action of TLCK may facilitate the development of novel compounds for insect pest control.

Visualization of thrombi in pulmonary arteries with radiolabeled, enzymatically inactivated tissue-type plasminogen activator

BACKGROUND

Despite the high frequency of pulmonary thromboembolism and its significant morbidity and mortality, diagnosis remains suboptimal. We have been developing a method for prompt detection with the use of radiolabeled, inactivated tissue-type plasminogen activator (TPA) and performed the present study to determine whether its use permits rapid scintigraphic visualization of pulmonary thrombi in vivo.

METHODS AND RESULTS

The thrombolytic, but not fibrin-binding, property of TPA was inactivated with a tripeptide chloromethyl ketone (YPACK) that had already been iodinated with 123I to radiolabel the TPA. Pulmonary arterial thrombosis was induced in nine dogs with the use of guide wires modified to provide thrombogenic tips. 123I-YPACK-TPA (1.1 to 7.8 mCi, 0.5 to 7.8 mg) was infused for 5 minutes into either the systemic or the pulmonary circulation. Clearance of radioactivity from the blood was rapid and indistinguishable from that of unlabeled, thrombolytically active TPA, with only 6.7 +/- 1.0% (mean +/- SEM) of peak radioactivity remaining after 60 minutes and minimal release of labeled fragments from the liver during this interval. Thrombi were visualized with single photon emission computed tomography and/or planar imaging 40 to 120 minutes after infusion of tracer in all seven animals given at least 3.7 mCi of 123I-YPACK-TPA. Ratios of radioactivity in thrombus (wet mass, 610 +/- 64 mg) to blood were high (14 +/- 3:1).

CONCLUSIONS

The use of radiolabeled TPA in which thrombolytic activity is inactivated permits prompt scintigraphic detection of thrombi in pulmonary arteries in vivo.

A faster-acting and more potent form of tissue plasminogen activator

Current treatment with tissue plasminogen activator (tPA) requires an intravenous infusion (1.5-3 h) because the clearance of tPA from the circulation is rapid (t 1/2 approximately 6 min). We have developed a tPA variant, T103N,N117Q, KHRR(296-299)AAAA (TNK-tPA) that has substantially slower in vivo clearance (1.9 vs. 16.1 ml per min per kg for tPA in rabbits) and near-normal fibrin binding and plasma clot lysis activity (87% and 82% compared with wild-type tPA). TNK-tPA exhibits 80-fold higher resistance to plasminogen activator inhibitor 1 than tPA and 14-fold enhanced relative fibrin specificity. In vitro, TNK-tPA is 10-fold more effective at conserving fibrinogen in plasma compared to tPA. Arterial venous shunt models of fibrinolysis in rabbits indicate that TNK-tPA (by bolus) induces 50% lysis in one-third the time required by tPA (by infusion). TNK-tPA is 8- and 13-fold more potent in rabbits than tPA toward whole blood clots and platelet-enriched clots, respectively. TNK-tPA conserves fibrinogen and, because of its slower clearance and normal clot lysis activity, is effective as a thrombolytic agent when given as a bolus at a relatively low dose.