Titration of Active Centers in Thrombin Solutions. Standardization of the Enzyme*

Fractions from Commercial Collagenase Preparations: Use in Enzymic Isolation of the Islets of Langerhans from Porcine Pancreas

Transplantation of isolated islets of Langerhans is an intriguing possibility for the treatment of diabetes mellitus. The isolation of islets from pancreata requires the specific dissociation of the tissue. Commercial collagenases from Clostridium histolyticum are widely used for this purpose. Unfortunately, the effectiveness of these commercial enzymes is not predictable and differs considerably between suppliers and even from lot to lot. This is due mainly to differences in their specific collagenase activity and to the presence of other lytic enzymes, as well as to other contaminants. Free flow zone electrophoresis (FFZE) was used to separate the effective protein components from undesired compounds and to prepare a digestive enzyme mixture with controlled composition of lytic activities. Fractionation of crude collagenases by FFZE resulted in partially purified protein fractions that were enriched for collagenase and tryptic activities, and contained only trace amounts of neutral protease. These preparations proved to be highly effective in an in vitro assay for the liberation of viable islets from porcine pancreas. To scale up the production of these collagenases with defined enzyme composition, we fractionated two different lots of a commercial collagenase from C. histolyticum (one lot effective in islet isolation, the other not) by using fast protein liquid chromatography (FPLC) on hydroxyapatite. Again, high efficacy of islet release from pancreatic tissue was correlated to high specific tryptic and collagenase activities and low levels of neutral protease. The chromatographic protocol developed in this study converted a non-effective collagenase lot into a preparation that allowed successful islet isolation.

Thrombin

Thrombin is a Na+-activated, allosteric serine protease that plays opposing functional roles in blood coagulation. Binding of Na+ is the major driving force behind the procoagulant, prothrombotic and signaling functions of the enzyme, but is dispensable for cleavage of the anticoagulant protein C. The anticoagulant function of thrombin is under the allosteric control of the cofactor thrombomodulin. Much has been learned on the mechanism of Na+ binding and recognition of natural substrates by thrombin. Recent structural advances have shed light on the remarkable molecular plasticity of this enzyme and the molecular underpinnings of thrombin allostery mediated by binding to exosite I and the Na+ site. This review summarizes our current understanding of the molecular basis of thrombin function and allosteric regulation. The basic information emerging from recent structural, mutagenesis and kinetic investigation of this important enzyme is that thrombin exists in three forms, E*, E and E:Na+, that interconvert under the influence of ligand binding to distinct domains. The transition between the Na+ -free slow from E and the Na+ -bound fast form E:Na+ involves the structure of the enzyme as a whole, and so does the interconversion between the two Na+ -free forms E* and E. E* is most likely an inactive form of thrombin, unable to interact with Na + and substrate. The complexity of thrombin function and regulation has gained this enzyme pre-eminence as the prototypic allosteric serine protease. Thrombin is now looked upon as a model system for the quantitative analysis of biologically important enzymes.

Trypsin immobilization on three monolithic disks for on-line protein digestion

The preparation and characterization of three trypsin-based monolithic immobilized enzyme reactors (IMERs) developed to perform rapid on-line protein digestion and peptide mass fingerprinting (PMF) are described. Trypsin (EC 3.4.21.4) was covalently immobilized on epoxy, carbonyldiimidazole (CDI) and ethylenediamine (EDA) Convective Interaction Media (CIM) monolithic disks. The amount of immobilized enzyme, determined by spectrophotometric measurements at 280nm, was comprised between 0.9 and 1.5mg per disk. Apparent kinetic parameters Km* and Vmax*, as well as apparent immobilized trypsin BAEE-units, were estimated in flow-through conditions using N-alpha-benzoyl-L-arginine ethyl ester (BAEE) as a low molecular mass substrate. The on-line digestion of five proteins (cytochrome c, myoglobin, alpha1-acid glycoprotein, ovalbumin and albumin) was evaluated by inserting the IMERs into a liquid chromatography system coupled to an electrospray ionization ion-trap mass spectrometer (LC-ESI-MS/MS) through a switching valve. Results were compared to the in-solution digestion in terms of obtained scores, number of matched queries and sequence coverages. The most efficient IMER was obtained by immobilizing trypsin on a CIM EDA disk previously derivatized with glutaraldehyde, as a spacer moiety. The proteins were recognized by the database with satisfactory sequence coverage using a digestion time of only 5min. The repeatability of the digestion (R.S.D. of 5.4% on consecutive injections of myoglobin 12microM) and the long-term stability of this IMER were satisfactory since no loss of activity was observed after 250 injections.

Thrombin allostery

Thrombin is a Na(+)-activated, allosteric serine protease that plays multiple functional roles in blood pathophysiology. Binding of Na(+) is the major driving force behind the procoagulant, prothrombotic and signaling functions of the enzyme. This review summarizes our current understanding of the molecular basis of thrombin allostery with special emphasis on the kinetic aspects of Na(+) activation. The molecular mechanism of thrombin allostery is a remarkable example of long-range communication that offers a paradigm for many other biological systems.

Optimization of a trypsin-bioreactor coupled with high-performance liquid chromatography–electrospray ionization tandem mass spectrometry for quality control of biotechnological drugs

The optimization of a silica-based trypsin bioreactor and its use in the quality control of biotechnological drugs like peptides and proteins is described. Five bioreactors based on monolithic material have been prepared, with different amount of bound trypsin. The performances of these bioreactors were compared to the proteolytic activity of a bioreactor based on silica material. The trypsin-based chromatographic columns were coupled on-line with an LC/ESI/MS/MS system for digestion and identification of proteins. First, human serum albumin has been used as test protein to compare the ability of the bioreactors to hydrolyse high-molecular-weight proteins. The best chromatographic material (epoxy monolithic silica) and the optimum amount of enzyme bound (7.13 mg) have been identified to obtain the highest protein recovery and an analytical reproducibility of the whole digestion, separation and identification process. The optimized enzyme-reactor has been used for the on-line digestion of some biotechnological drugs such as somatotropin. Somatotropin for parentheral use has been analyzed, without sample pre-treatment, with both an on-line procedure and the traditional off-line procedure described in the European Pharmacopoeia. It was found that the cleavage efficiency (aminoacidic recovery, %AA) achieved within minutes by the developed protocol is at least comparable or even better than the conventional 4h consuming method.

Murine Thrombin Lacks Na+ Activation but Retains High Catalytic Activity

Human thrombin utilizes Na+ as a driving force for the cleavage of substrates mediating its procoagulant, prothrombotic, and signaling functions. Murine thrombin has Asp-222 in the Na+ binding site of the human enzyme replaced by Lys. The charge reversal substitution abrogates Na+ activation, which is partially restored with the K222D mutation, and ensures high activity even in the absence of Na+. This property makes the murine enzyme more resistant to the effect of mutations that destabilize Na+ binding and shift thrombin to its anticoagulant slow form. Compared with the human enzyme, murine thrombin cleaves fibrinogen and protein C with similar k(cat)/K(m) values but activates PAR1 and PAR4 with k(cat)/K(m) values 4- and 26-fold higher, respectively. The significantly higher specificity constant toward PAR4 accounts for the dominant role of this receptor in platelet activation in the mouse. Murine thrombin can also cleave substrates carrying Phe at P1, which potentially broadens the repertoire of molecular targets available to the enzyme in vivo.

Stabilization of proteases by entrapment in a new composite hydrogel

A new one-step procedure for entrapping proteases into a polymeric composite calcium alginate-poly(N-vinyl caprolactam) hydrogel was developed that provided 75-90% retention of the activity of entrapped enzymes compared to soluble ones. Properties of entrapped carboxypeptidase B, trypsin, and thrombin were investigated. The immobilized enzymes were active within a wide pH range. The temperature optima of entrapped trypsin and carboxypeptidase B were approx 25 degrees C higher than that of the soluble enzymes, and the resistance to heating was also increased. The effects of various polar and nonpolar organic solvents on the entrapped proteases were investigated. The immobilized enzymes retained their activity within a wide concentration range (up to 90%) of organic solvents. Gel-entrapped trypsin and carboxypeptidase (CPB) were successfully used for obtaining human insulin from recombinant proinsulin. The developed stabilization method can be used to catalyze various reactions proceeding within wide pH and temperature ranges.

Conformational integrity of human alpha-thrombin

It is known that storage at pH 6 stabilizes thrombin against inactivation. In order to determine whether structural changes accompany this stabilization, the conformation of human alpha-thrombin at pH 6.0 and 7.5 was investigated by chemical modification, difference spectroscopy, circular dichrosim, and thermal stability. It was shown that the CD spectra at the 230-200 nm peptide transition were indistinguishable at the two pH values, indicating no differences in the secondary structure as also indicated by the thermal stability of the enzyme at pH 6.0, 7.4 and 8.3. However, differences were observed in the 300-250 nm aromatic transition suggesting some changes in the microenvironment of the aromatic chromophores. Solvent perturbation in 20% ethylene glycol and 20% dimethylsulfoxide showed that at pH 7.5, 4.3 +/- 0.3 tryptophan and 8.6 +/- 0.4 tyrosine residues were exposed and accessible to the solvent whereas at pH 6.0 these values were 3.6 +/- 0.1 tryptophan and 7.8 +/- 0.4 tyrosine residues. At pH 7.5, 6.0 +/- 0.5 tryptophan residues were found reactive toward dimethyl-(2-hydroxy-5-nitrobenzyl)sulfonium bromide while 2.5 +/- 0.3 were found reactive at pH 6.0. Accompanying these structural changes were ultraviolet absorption and CD spectral changes with transition midpoints at pH 6.45 characteristic of histidine ionization. These spectral changes were lost when alpha-thrombin was modified by diethylpyrocarbonate but not by N-alpha-tosyl-L-Lysinechloro-methyl ketone. It is concluded that a second histidine residue, not the active site His-43, is associated with the pH dependent conformational changes at pH 6.0. The ionization of this histidine residue and the accompanying conformational changes could explain the reduced catalytic efficiency and stability of alpha-thrombin at pH 6.

Synergistic effects of lectins in the interaction of thrombin with human platelets

Several lectins have been studied for their effects on the interaction of thrombin with human platelets. Wheat germ agglutinin, concanavalin A and Ricinus communis lectin increased the number of high affinity sites for diisopropylphosphothrombin on washed platelets from 3000 to about 12 000 but the binding affinities were unchanged (Kd approx 4 nM). Two other lectins, Lens culinaris and Bandieria simplicifolia, were without effect. (2) Using formalinized platelets to avoid possible complications of the platelet release reaction, wheat germ agglutinin showed a marked increase (5-fold) in the binding of active thrombin, peanut agglutinin had no effect while Ricinus communis and :Bandieria simplicifolia showed marginal increases (2-fold). Thrombin binding was decreased to about one quarter with Lens culinaris, Phaseolus vulgaris and concanavalin A. (3) Wheat germ agglutinin caused a synergistic increase of platelet aggregation at low concentrations of thrombin (12.5 mU/ml) and ADP (1 microM), both in the absence and presence of added fibrinogen, but had no effect on ristocetin-induced aggregation.

[Proteolytic enzyme activity in the cerebrospinal fluid after head injury--a preliminary report (author's transl)]

Report on seven cases of severe head injury, where proteolytic enzyme activity in the ventricular fluid was measured within a period of eight days. The result was positive in four cases. This finding is discussed in relation to clinical features and morphology. Earlier reports and these results are considered.

Characterization of kallikrein from human saliva isolated by use of affinity chromatography

Kallikrein was isolated from paraffin stimulated saliva by use of two steps affinity chromatography. A610-fold purification of the enzyme was achieved by use of Sepharose 4B conjugated with soy bean trypsin inhibitor and pancreatic trypsin inhibitor. The isoelectric point of kallikrein was found to be pH 4.3 and the molecular weight was calculated to be about 38.000 by gel filtration on a AcA44 Ultrogel column. The enzyme had a pH optimum at pH 8.6 using BAEE and TAME as substrates. The michaelis constant for kallikrein on these two substrates was 4.0-10(-4) M and 5.5-10(-4)M. respectively.

Characterization of trypsin-like enzymes from human saliva isolated by use of affinity chromatography

Paraffin-stimulated whole mixed saliva was collected from 6 individuals and pooled. Three trypsin-like enzymes from saliva supernatant and two from the saliva sediment were isolated by use of affinity chromatography on a soybean trypsin inhibitor Sepharose 4 B column. The isoelectric points for these enzymes were pI 8.0, 6.8 and 6.4 from the supernatant and 6.4 and 6.2 from the sediment. The enzymes were characterized with respect to pH-optimum, pH-stability, temperature stability, substrate specificity and influence of metal ions and inhibitors. The Michaelis constants were determined for these enzymes on the substrates BAEE and TAME.

Purification of protease II from Escherichia coli by affinity chromatography and separation of two enzyme species from cells harvested at late log phase

N-Acetyl-D-arginine linked to an agarose matrix has been used to purify protease II from Escherichia coli by affinity chromatography. The specific adsorption of protease II to this absorbent was achieved in 220 mM potassium phosphate buffer pH 7.6, and the enzyme was eluted with L-arginine. Enzyme preparations from cells harvested at late log phase have been resolved into two molecular species which differ in specific activity, kinetic constants and carbohydrate content. Both species appeared homogeneous by electrophoresis in conventional buffers and also in the presence of sodium dodecyl sulfate. Only one enzyme species was obtained by the same procedure using bacteria harvested at the middle of exponential growth.

High affinity binding of human and bovine thrombins to p-chlorobenzylamido-epsilon-aminocaproyl agarose

Human thrombin (EC 3.4.21.5) binds tightly to p-chlorobenzylamido-epsilon-aminocaproyl agarose, and is not eluted by 2 M NaCl at pH 8. Its zymogen, human prothrombin, does not bind to the same absorbent. 2 M NaCl partially elutes DFP-treated thrombin. For native human and bovine thrombins, protein and activity are quantitatively eluted by 25% dioxane, and upon rechromatography the active human enzyme exhibits the same binding properties. Equally tight binding of human thrombin occurs with derivatives of the m- and p-chlorobenzylamines. With the o-chloro derivative or benzylamine itself insolubilized to epsilon-aminocaproyl agarose, thrombin is eluted by high ionic strength. Bovine trypsin and bovine factor Xa bind less tightly than thrombin to p-chlorobenzylamido-epsilon-aminocaproyl agarose, being eluted by high ionic strength. It is proposed that the specific thrombin adsorption is related to a secondary binding site of high affinity and with hydrophobic properties. This site is not available in the zymogen. Furthermore, the less specific protease, trypsin, and the more specific protease, factor Xa, lack this binding site.