Active-site-selective labeling of blood coagulation proteinases with fluorescence probes by the use of thioester peptide chloromethyl ketones. I. Specificity of thrombin labeling.

In a new strategy for labeling the active sites of serine proteinases with fluorescence probes (Bock, P. E. (1988) Biochemistry 27, 6633-6639), a thioester peptide chloromethyl ketone inhibitor is incorporated into the enzyme active center and used to produce a unique thiol group which provides a site for selective chemical modification with any one of many thiol-reactive fluorescence probes. This approach was developed to increase the opportunities for identifying fluorescent proteinase derivatives that act as reporters of binding interactions by allowing a large number of derivatives, representing a broad range of probe spectral properties, to be readily prepared. In the studies described here, the specificity of the labeling approach was evaluated quantitatively for the labeling of human alpha and beta/gamma-thrombin with the thioester peptide chloromethyl ketones, N alpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-CH2Cl and N alpha-[(acetylthio)acetyl]-D-Phe-Phe-Arg-CH2Cl, and the thiol-reactive fluorescence probe, 5-(iodoacetamido)fluorescein. Irreversible inactivation of thrombin by the inhibitors was accompanied by incorporation of 0.98 +/- 0.06 mol/mol of the thioester group into the active site, independent of a 470-fold difference between the thioester peptide chloromethyl ketones in the bimolecular rate constants of alpha-thrombin affinity labeling. Subsequent mild treatment of the covalent thrombin-inhibitor complexes with NH2OH in the presence of 5-(iodoacetamido)fluorescein resulted in generation of the thiol group together with its selective modification and incorporation of 0.96 +/- 0.07 mol of probe/mol of active sites. The incorporated label was localized to a 9000 molecular weight region of alpha and beta/gamma-thrombin containing the catalytic-site histidine residue. Evaluation of competing, side reactions showed that they did not significantly compromise the active site specificity of labeling. These results demonstrated equivalent, active-site-selective fluorescence probe labeling of alpha and beta/gamma-thrombin by use of either of the thioester peptide chloromethyl ketones, with a site specificity of greater than or equal to 94%.     

Zymogen/enzyme discrimination using peptide chloromethyl ketones

Glutamylglycinylarginyl chloromethyl ketone, tyrosylglycinylarginyl chloromethyl ketone, and phenylalanylprolylarginyl chloromethyl ketone have been labeled at their amino termini using fluorescein, rhodamine-X, lissamine-rhodamine, pyrene, and the 1,5-, 2,5-, and 2,6-dimethylaminonaphthalene-1-sulfonyl moieties. These peptidyl chloromethyl ketones have also been modified by incorporation of biotin and epsilon-amino caproyl biotin. The ability of these various chloromethyl ketones to be incorporated into a collection of zymogen-enzyme pairs has been evaluated using a variety of coagulation and fibrinolytic proteins. All labeled chloromethyl ketones were efficiently incorporated into the proteases tested, with the exception of urokinase which was refractory to inhibition by phenylalanylprolylarginyl chloromethyl ketone derivatives. No modification of any zymogen species was observed even under conditions designed to detect minimal reactivity. When enzymes were modified using chloromethyl ketones labeled with epsilon-amino caproyl biotin, the modified proteins readily reacted with avidin under a variety of different conditions. The observed reactivity with avidin was used in enzyme "blotting" following electrophoretic resolution of polypeptide chains and to remove active enzyme present in enzyme-zymogen mixtures. These reagents have been used to evaluate the potential for active site expression by the single-chain human factor VII molecule. Studies conducted with tissue factor, phospholipids, and calcium using factor X as substrate demonstrate that no activity can be obtained without initial activation of either factor X to factor Xa or factor VII to factor VIIa by an external source. We thus conclude that factor VII is a true zymogen, inert in the blood clotting process prior to its cleavage to factor VIIa.     

Active-site-selective labeling of blood coagulation proteinases with fluorescence probes by the use of thioester peptide chloromethyl ketones. II. Properties of thrombin derivatives as reporters of prothrombin fragment 2 binding and specificity of the labeling approach for other proteinases.

The behavior of an array of fluorescent human alpha-thrombin derivatives in reporting binding of the fragment 2 domain of prothrombin was characterized as a representative application of the active-site-selective labeling approach to studies of blood coagulation proteinase regulatory interactions. An array of 16 thrombin derivatives was prepared by affinity labeling of the proteinase active site with the thioester peptide chloromethyl ketones, N alpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-CH2Cl or N alpha-[(acetylthio)acetyl]-D-Phe-Phe-Arg-CH2Cl, followed by selective modification of the NH2OH-generated thiol group on the covalently incorporated inhibitors with each of eight thiol-reactive fluorescence probes. The changes in probe fluorescence intensity of the derivatives, signaling changes in the environment of the catalytic site associated with fragment 2 binding, appeared to be a unique and unpredictable function of the structure of the probe and the connecting peptide. These results demonstrated the utility of the labeling approach for overcoming the problem of not being able to predict which fluorescent label will provide the most useful proteinase derivative for investigating an interaction by enabling a greater variety of them to be prepared and screened for those with the most desirable properties. To determine whether the approach could be extended to other proteinases, the specificity of labeling with the fluorescence probe iodoacetamide, 5-(iodoacetamido)fluorescein, by use of the two thioester inhibitors was evaluated for several other blood coagulation proteinases and related trypsin-like enzymes. All of the proteinases were labeled in an active-site-selective manner. The combined results of quantitating the labeling reactions for the proteinase and inhibitor combinations studied thus far showed active-site-specific incorporation of 0.98 +/- 0.10 mol of inhibitor/mol of active sites and 0.92 +/- 0.11 mol of probe/mol of active sites, representing an overall greater than or equal to 93% site-specificity of labeling. These results demonstrated the broad applicability of the labeling approach for fluorescence studies of proteinases that differ greatly in their catalytic specificities.     

Template-independent poly(A) x poly(U) synthesizing activity of different forms of Bacillus subtilis RNA polymerase

The steady-state kinetic parameters of the tripeptides D-Val-Leu-Lys-, Ala-Phe-Lys-, and < Glu-Phe-Lys- in which the free carboxyl group was substituted with p-nitroaniline (substrate) or chloromethane (inhibitor), towards the serine proteinases plasmin (EC 3.4.21.7), thrombin (EC 3.4.21.5), urokinase, factor Xa, and trypsin (EC 3.4.21.4) were investigated. The p-nitroanilide derives were found to be very good substrates for plasmin, 2.5--40-times less efficient towards trypsin and very poor (100--10 000-times less efficient) substrates for thrombin, factor Xa and urokinase. The chloromethyl ketone derivatives were comparably efficient inhibitors of plasmin and trypsin and in general very poor (100--10 000-times weaker) inhibitors of thrombin, factor Xa and urokinase. D-Val-Leu-Lys-pNA however was a very poor substrate but D-Val-Leu-Lys-CH2Cl a very efficient inhibitor for thrombin. The variability in susceptibility of the substrates towards the enzymes was due to differences in their Michaelis constant, in their deacylation rate constant or both. the variable efficiency of the inhibitors was mostly due to differences in their dissociation constant and much less to differences in their alkylation rate constant. Only a poor correlation (r = 0.25) was found between the efficiency of the p-nitroanilides as substrate and their homologous chloromethyl ketones as inhibitor. The most notable discrepancy was observed with the D-Val-Leu-Lys derivatives towards thrombin.     

Labeled proteinase inhibitors: versatile tools for the characterization of serine proteinases in solid-phase assays.

Peptidyl chloromethyl ketones were used for the specific labeling of proteinases by attaching a biotin group to the N-terminal end of the peptide. Such labeled peptide inhibitors allowed the detection and quantitation of proteolytic enzymes immobilized on the plastic surface of a microtiter plate, as well as on nitrocellulose. The validity of these solid-phase assays was demonstrated using subtilisin Carlsberg as a model enzyme and biotinyl-epsilon-aminocaproyl-L-alanyl-L-alanyl-L-propyl-L-phenylal++ + anyl- chloromethyl ketone as a specific reagent. In addition to being usable for the screening of a particular proteinase in a large number of samples, these assays can be adapted for the analysis of specific proteolytic enzyme present in complex mixtures.     

The properties of peptidyl diazoethanes and chloroethanes as protease inactivators

Earlier work has demonstrated the irreversible inactivation of serine and cysteine proteinases by peptides with a C-terminal chloromethyl ketone group. With a C-terminal diazomethyl ketone, on the other hand, peptides become reagents specific for cysteine proteinases. We have now synthesized and examined the properties of reagents with an additional methyl side chain near the reactive grouping with the goal of diminishing side reactions in a cellular environment. Derivatives of neutral amino acids as well as of lysine and arginine have been prepared. The chloroethyl ketones are about 60% less reactive to chemical nucleophiles than the chloromethyl ketones. However, the susceptibilities of the proteases examined varied remarkably. Cathepsins B and L of the papain family of cysteine proteinases were much less susceptible (about 2 orders of magnitude less) to both peptidyl diazoethyl and chloroethyl ketones. In marked contrast, clostripain, a cysteine proteinase of a separate family was decisively more susceptible to chloroethyl ketones. The serine proteinases showed a drop in susceptibility to the chloroethyl ketones generally, and this was similar to the drop in chemical reactivity in proceeding from the chloromethyl to the chloroethyl ketone.     

Intermolecular interaction of avidin and PEGylated biotin

The equilibrium binding constants and stoichiometries between PEGylated biotins and avidin have been studied for a range of PEGylated biotin molecular weights. These studies show that as the molecular weight of PEG (polyethylene glycol) increases over the range 588, 3400, and 5000 g/mol, the equilibrium dissociation constants of PEGylated biotins with avidin increase to approximately 10 (-8) M compared with 10 (-15) M for the biotin-avidin complex. The stoichiometries of PEGylated biotins with avidin are 4:1 for 588 and 3400 g/mol PEG and 1:1 for 5000 g/mol PEG. The data demonstrate that the equilibrium binding constant and the stoichiometry of the avidin-biotin-PEG complex system can be adjusted by the length of PEG chains. This approach may be used with PEGylated biotin analogues for pretargeting in drug delivery, such as a biotin-PEGylated enzyme for converting an inactive prodrug into a cytotoxin. When a PEG chain is chosen as an appropriate spacer, the length of the PEG chain must be considered because PEG can block the binding sites on avidin.     

Active site-independent recognition of substrates and product by bovine prothrombinase: a fluorescence resonance energy transfer study

The conversion of prothrombin to thrombin is catalyzed by prothrombinase, an enzyme complex composed of the serine proteinase factor Xa and a cofactor protein, factor Va, assembled on membranes. Kinetic studies indicate that interactions with extended macromolecular recognition sites (exosites) rather than the active site of prothrombinase are the principal determinants of binding affinity for substrate or product. We now provide a model-independent evaluation of such ideas by physical studies of the interaction of substrate derivatives and product with prothrombinase. The enzyme complex was assembled using Xa modified with a fluorescent peptidyl chloromethyl ketone to irreversibly occlude the active site. Binding was inferred by prethrombin 2-dependent perturbations in the fluorescence of Oregon Green(488) at the active site of prothrombinase. Active site-independent binding was also unequivocally established by fluorescence resonance energy transfer between 2,6-dansyl tethered to the active site of Xa and eosin tethered to the active sites of either thrombin or meizothrombin des fragment 1. Comparable interprobe distances obtained from these measurements suggest that substrate and product interact equivalently with the enzyme. Competition established the ability of a range of substrate or product derivatives to bind in a mutually exclusive fashion to prothrombinase. Equilibrium dissociation constants obtained for the active site-independent binding of prothrombin, prethrombin 2, meizothrombin des fragment 1 and thrombin to prothrombinase were comparable with their affinities inferred from kinetic studies using active enzyme. Our findings directly establish that binding affinity is principally determined by the exosite-mediated interaction of either the substrate, both possible intermediates, or product with prothrombinase. A single type of exosite binding interaction evidently drives affinity and binding specificity through the stepwise reactions necessary for the two cleavage reactions of prothrombin activation and product release.     

Comparative study of the active site caging of serine proteases: thrombin and factor Xa

Bovine thrombin and human factor Xa were acylated at their active site selectively with inhibitors derived from the parent compound 4-guanidinophenyl (E)-4-diethylamino-2-hydroxy-alpha-methylcinnamate hydrochloride, 1b. Peptidyl side chains were attached to the phenol ring via amide connection, which served as a recognition motif in inhibiting different serine proteases. Upon irradiation with 366 nm light, the trans-cinnamate attached to the active-site serine isomerizes to the cis isomer which then rapidly lactonizes to release the free enzyme. The peptidyl side chain sequences specific for each serine protease were revealed via constructing and screening a library of homologous compounds. This methodology may be applied to other proteases. One application based on enzyme-specific, photoactivatable inhibitors is to isolate a designated active protease from a mixture of several proteases. Thus, a cinnamate inhibitor with a biotin moiety, 1d, was synthesized. A solution of enzyme-specific, biotinylated inhibitor was added into a mixture of proteases containing a target enzyme. The target enzyme was acylated at the active site and subsequently bore a biotin tail. An avidin column was used to separate the biotinylated enzyme from the unmodified ones, by a strong binding between biotin and avidin. After a brief irradiation on the avidin column, the retained enzymes were released from the biotin tag and eluted off the column. To demonstrate the idea, thrombin and factor Xa have been separated from each other by this strategy.     

Inhibition of neutrophil sulfhydryl groups by choloromethyl ketones. A mechanism for their inhibition of superoxide production

The inhibition of O2- production by serine protease inhibitors such as chloromethyl ketone derivatives, has been used as evidence to indicate that protease activity is essential for the production of O2- by neutrophils. However, chloromethyl ketones are potent inhibitors of sulfhydryl groups. This study demonstrates that chloromethyl ketones inhibited non-protein sulfhydryl groups as well as O2- production by human neutrophils stimulated with phorbol myristate acetate (PMA). Their inhibition of O2- production could be prevented by reduced glutathione. The results suggest that inhibition of O2- production by chloromethyl ketones is largely due to their inhibition of sulfhydryl groups.     

Kinetic intermediates in prothrombin activation. Bovine prethrombin 1 conversion to thrombin by factor X

Two pathways are possible during the proteolytic formation of alpha-thrombin (alpha-IIa) from prothrombin (II) or prethrombin 1 (P1). One of the pathways, with prethrombin 2 or prethrombin 2 associated with fragment 2 (P2F2) as intermediates, has long been known to exist when activation is catalyzed by Factor Xa (Xa) alone. The second pathway, with meizothrombin or meizothrombin (des fragment 1) (MzIIa(-F1)) as intermediate, has been shown to exist when Factor Va and phospholipids are present with Xa. Until now, MzIIa(-F1) has not been detected in reactions catalyzed by Xa alone. In this study, we demonstrate that P1 activation by Xa alone occurs via both pathways, and we provide rate constants and kinetic equations for calculating the relative contributions of each of the pathways to the formation of alpha-IIa by Xa. Investigation of the initial rates of proteolytic cleavage of P2F2 and P1 by Xa alone indicated first-order dependence on substrate concentration with no evidence of saturation of Xa with either substrate at concentrations as high as 200 microM. Apparent second-order rate constants (kc/Km) of 113 +/- 9 M-1 s-1 for the formation of thrombin from P2F2 and 1,410 +/- 19 M-1 s-1 for the disappearance of P1 were determined at pH 7.5, 25 degrees C, 10 mM CaCl2, 0.15 M ionic strength. A two-step sequential first-order pathway employing these rate constants for thrombin activity production from P1 via P2F2 could not, however, account for the quantity of thrombin that was produced during the early stages of P1 activation. Addition of a parallel first-order reaction to produce thrombin activity from P1 independently of P2F2, tentatively identified as the formation of MzIIa(-F1), yielded progress curves in quantitative agreement with the experimental data. kc/Km for the parallel reaction was estimated to be 98 +/- 10 M-1 s-1. Independent determination of the second-order rate constant for the cleavage of isolated MzIIa (-F1), 15,000 +/- 420 M-1 s-1, indicated that MzIIa(-F1) could meet the kinetic requirements for an intermediate in the parallel activation pathway. The transient formation of MzIIa (-F1), as well as the generation of alpha-IIa, was directly demonstrated during activation of P1 by active site-affinity labeling of the reaction products with a biotin derivative of D-Phe-Pro-Arg chloromethyl ketone and visualization by semiquantitative Western blotting.(ABSTRACT TRUNCATED AT 400 WORDS)     

Formation of meizothrombin as intermediate in factor Xa-catalyzed prothrombin activation

The conversion of prothrombin into thrombin by Factor Xa requires the cleavage of two peptide bonds in prothrombin. Dependent on the order of cleavage, prethrombin 2 or meizothrombin occurs as intermediate. Since prethrombin 2 has as yet been the only observed intermediate, prothrombin activation is generally considered to proceed via prethrombin 2. In this paper we present new methods that allow differentiation between meizothrombin and thrombin formed during the initial phase of prothrombin activation. These methods, which make use of the different reactivities of meizothrombin and thrombin toward fibrinogen and antithrombin III plus heparin, enabled us to show the generation of considerable amounts of meizothrombin during Factor Xa-catalyzed prothrombin activation. Both meizothrombin and thrombin incorporated the active site-directed fluorescent chloromethyl ketone 5-dimethylaminonaphthalene-1-sulfonyl-Glu-Gly-Arg-CH2Cl. Gel electrophoretic analysis of chloromethyl ketone-treated aliquots of prothrombin activation mixtures confirmed meizothrombin formation. These observations demonstrate that prothrombin may also be converted into thrombin via meizothrombin.     

Active site selective labeling of serine proteases with spectroscopic probes using thioester peptide chloromethyl ketones: demonstration of thrombin labeling using N alpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-CH2Cl

The feasibility of a new approach to incorporation of spectroscopic probes into the active sites of certain serine proteases has been demonstrated. The method is based on inactivation of a serine protease with a thioester derivative of a peptide chloromethyl ketone. The thiol group generated by reaction of the covalent enzyme-inhibitor complex with NH2OH provides a unique site for subsequent labeling with thiol-reactive probes. To evaluate the method, N alpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-CH2Cl was synthesized by reaction of the thrombin-specific tripeptide chloromethyl ketone with succinimidyl (acetylthio)acetate and purified by sulfopropyl-Sephadex and Sephadex G-10 chromatography. Reverse-phase high-performance liquid chromatography indicated that the product was 90 +/- 3% pure. The compound was quantitated by using 5,5'-dithiobis(2-nitrobenzoic acid) to measure the concentration of thiol produced in the presence of NH2OH. On this basis, titrations of the irreversible loss of human alpha-thrombin activity had end points of 1.1 +/- 0.1 mol of inhibitor/mol of active sites, indicating a 1:1 stoichiometry for inactivation. Incubation of N alpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-thrombin with 5-(iodoacetamido)fluorescein in the presence of NH2OH resulted in incorporation of 0.96 mol of the fluorescence probe/mol of active sites and the appearance of fluorescein fluorescence associated with the active site containing B-chain on sodium dodecyl sulfate-polyacrylamide gels. Fluorescence labeling of thrombin required reaction of the inhibitor at the active site as well as subsequent generation of the thiol group with NH2OH. It is concluded that active site selective labeling can be achieved by using this approach, which is likely to be applicable to other proteases, peptide chloromethyl ketones, and a wide variety of probes.     

Serine proteinases of lower vertebrates

Recent data on the effect of serine proteinases of lower vertebrates are generalized. Hydrolysis specificity and kinetics of different synthetic substrates, dependence of the activity of enzymes on pH, their irreversible inhibition by chloromethyl ketones of amino acids and peptides as well as high-molecular proteinase inhibitors are considered in detail. The data testify to the fact that chymotrypsins and trypsins of higher vertebrates and serine proteinases of lower vertebrates act as an acid-base catalysis. Enzymes in the pyloric cacca of fishes are in the state of proenzymes and are transformed into an active form with the aid of their own proteolytic factors. The esterase and proteolytic activity of fish proteinases is concentrated in the same active site and reaches the highest values at pH 7,8. New data are presented on particularities of the lower vertebrate proteinases, on the similarity and differences in their specificity. A distinct difference is shown in the nature of the binding site of the active centre in a number of serine proteinases of fishes as compared to chymotrypsin and trypsin of higher vertebrates.     

Molecular probes for muscarinic receptors: derivatives of the M1-antagonist telenzepine.

Functionalized congeners of the M1-selective muscarinic antagonist telenzepine (4,9-dihydro-3-methyl-4-[(4-methyl-1-piperazinyl)acetyl]-10H- thieno[3,4-b][1,5]benzodiazepin-10-one) were developed and found to bind to the receptor with affinities (Ki values) in approximately the nanomolar range. The derivatives contain a 10-aminodecyl group, which provides a nucleophilic functionality for further derivatization. The attachment of a spacer chain to the distal piperazinyl nitrogen was based on previous findings of enhanced affinity at muscarinic receptors in an analogous series of alkylamino derivatives of pirenzepine [J. Med. Chem. (1991) 34, 2133-2145]. The telenzepine derivatives contain prosthetic groups for radioiodination, protein cross-linking, photoaffinity labeling, and fluorescent labeling and biotin for avidin complexation. The affinity for muscarinic receptors in rat forebrain (mainly m1 subtype) was determined in competitive binding assays vs [3H]-N-methylscopolamine. A (p-aminophenyl)-acetyl derivative for photoaffinity labeling had a Ki value of 0.29 nM at forebrain muscarinic receptors (16-fold higher affinity than telenzepine). A biotin conjugate displayed a Ki value of 0.60 nM at m2-receptors and a 5-fold selectivity versus forebrain. The high affinity of these derivatives makes them suitable for the characterization of muscarinic receptors in pharmacological and spectroscopic studies, for peptide mapping, and for histochemical studies.     

Digestive proteinases in Lasioderma serricorne (Coleoptera: Anobiidae)

The cigarette beetle, Lasioderma serricorne (Fabricius), is a common pest of stored foods. A study of digestive proteinases in L. serricorne was performed to identify potential targets for proteinaceous biopesticides, such as proteinase inhibitors. Optimal casein hydrolysis by luminal proteinases of L. serricorne was in pH 8.5-9.0 buffers, although the pH of luminal contents was slightly acidic. Results from substrate and inhibitor analyses indicated that the primary digestive proteinases were serine proteinases. The most effective inhibitors of caseinolytic hydrolysis were from soybean (both Bowman Birk and Kunitz), with some inhibition by chymostatin, N-tosyl-L-phenylalanine chloromethyl ketone, and leupeptin. Casein zymogram analysis identified at least eight proteolytic activities. Activity blot analyses indicated one major proteinase activity that hydrolysed the trypsin substrate N-alpha-benzoyl-L-arginine rho-nitroanilide, and three major proteinase activities that hydrolysed the chymotrypsin substrate N-succinyl ala-ala-pro-phe rho-nitroanilide. The absence of cysteine, aspartic, and metallo proteinases in L. serricorne digestion was evidenced by the lack of activation by thiol reagents, alkaline pH optima, and the results from class-specific proteinase inhibitors. The data suggest that protein digestion in L. serricorne is primarily dependent on trypsin- and chymotrypsin-like proteinases.     

Syntheses of functionalized biotin N-1' derivatives: new tools for the control of gene expression with small molecules

The exceptionally high affinity of biotin toward avidin and streptavidin is at the basis of (strept)avidin-biotin biotechnology, which has numerous applications in life sciences. Recent biotin developments for in vivo and in vitro acylation of selective targeted protein and intein-mediated site specific protein biotinylation require the free biotin carboxyl function to covalently bind with the targeted protein. However, recently this carboxylic function has been used to substitute biotin with numerous ligands and flags. In the present work, we propose the N-1' labeling possibilities of biotin, keeping the valeric chain free. We describe liquid and solid-phase syntheses of functionalized biotin N-1' derivatives. Although the N-1' modification involves a two-log decrease in affinity, in vitro these molecules kept their high avidin affinity (around 10(-12) M) and the in vivo acylation ability of new biotin derivatives.     

Use of avidin-biotin-peroxidase complex (ABC) in immunoperoxidase techniques: a comparison between ABC and unlabeled antibody (PAP) procedures

The use of avidin-biotin interaction in immunoenzymatic techniques provides a simple and sensitive method to localize antigens in formalin-fixed tissues. Among the several staining procedures available, the ABC method, which involves an application of biotin-labeled secondary antibody followed by the addition of avidin-biotin-peroxidase complex, gives a superior result when compared to the unlabeled antibody method. The availability of biotin-binding sites in the complex is created by the incubation of a relative excess of avidin with biotin-labeled peroxidase. During formation of the complex, avidin acts as a bridge between biotin-labeled peroxidase molecules; and biotin-labeled peroxidase molecules, which contains several biotin moieties, serve as a link between the avidin molecules. Consequently, a "lattice" complex containing several peroxidase molecules is likely formed. Binding of this complex to the biotin moieties associated with secondary antibody results in a high staining intensity.     

Influence of proteinase inhibitors and substrates on 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-binding capacity of the rat hepatic Ah receptor

These studies investigated the effects of various serine proteinase inhibitors and substrates on the TCDD-binding capacity of the rat hepatic Ah receptor. TCDD binding to the Ah receptor was inhibited by serine proteinase inhibitors phenylmethylsulfonyl fluoride (PMSF), tosyl-lysine chloromethyl ketone (TosLysCH2Cl), tosylamide-phenylethyl chloromethyl ketone (TosPheCH2Cl) and substrates tosyl-L-arginine methyl ester (TosArgOMe) and D-tryptophan methyl ester (TrpOMe). The order of potency was TosPheCH2Cl greater than TosLysCH2Cl much greater than PMSF approximately equal to TosArgOMe approximately equal to TrpOMe. Reactivity of the chloromethyl ketones with sulfhydryl groups was suggested by their steep inhibition curves above the concentration of nonprotein sulfhydryl groups, and the partial mitigation of inhibition by 1 mM dithiothreitol. Inhibition by these reagents was irreversible, while that by TosArgOMe and TrpOMe was completely reversible by gel filtration. The mechanism of inhibition by TosArgOMe and TrpOMe was formally competitive, with inhibition constants similar to those reported in steroid hormone receptor systems. Neither inhibitors nor substrates displaced previously bound TCDD.     

The factor V activation paradox

The prothrombinase complex consists of the protease factor Xa, Ca2+, and factor Va assembled on an anionic membrane. Factor Va functions both as a receptor for factor Xa and a positive effector of factor Xa catalytic efficiency and thus is key to efficient conversion of prothrombin to thrombin. The activation of the procofactor, factor V, to factor Va is an essential reaction that occurs early in the process of tissue factor-initiated blood coagulation; however, the catalytic sequence leading to formation of factor Va is a subject of disagreement. We have used biophysical and biochemical approaches to establish the second order rate constants and reaction pathways for the activation of phospholipid-bound human factor V by native and recombinant thrombin and meizothrombin, by mixtures of prothrombin activation products, and by factor Xa. We have also reassessed the activation of phospholipid-bound human prothrombin by factor Xa. Numerical simulations were performed incorporating the various pathways of factor V activation including the presence or absence of the pathway of factor V-independent prothrombin activation by factor Xa. Reaction pathways for factor V activation are similar for all thrombin forms. Empirical rate constants and the simulations are consistent with the following mechanism for factor Va formation. alpha-Thrombin, derived from factor Xa cleavage of phospholipid-bound prothrombin via the prethrombin 2 pathway, catalyzes the initial activation of factor V; generation of factor Va in a milieu already containing factor Xa enables prothrombinase formation with consequent meizothrombin formation; and meizothrombin functions as an amplifier of the process of factor V activation and thus has an important procoagulant role. Direct activation of factor V by factor Xa at physiologically relevant concentrations does not appear to be a significant contributor to factor Va formation.