Implications of the three-dimensional structure of alpha 1-antitrypsin for structure and function of serpins

Heparin-induced conformational change and activation of mucus proteinase inhibitor

Low molecular mass heparin (5.1 kDa) forms a tight complex with mucus proteinase inhibitor, the physiologic neutrophil elastase inhibitor of the upper respiratory tract. This binding strongly enhances the intrinsic fluorescence of the inhibitor and the rate of neutrophil elastase inhibitor association. One mole of this heparin fragment binds 1 mol of inhibitor with a Kd of 50 nM. From the variation of Kd with ionic strength, it is inferred that (i) 85% of the heparin--inhibitor binding energy i due to electrostatic interactions, (ii) about seven ionic interactions are involved in heparin--inhibitor binding. strength, it is inferred that (i) 85% of the heparin--inhibitor binding energy is due to electrostatic interactions, (ii) about seven ionic interactions are involved in heparin--inhibitor binding. and (iii), about one-third of low quantum yield of Trp30, the single tryptophan residue of the inhibitor, blue-shifts its maximum emission wavelength by 6 nm, decreases the acrylamide quenching rate constant by a factor of 4, and increases the mean intensity weighted lifetime by a factor of 2.5. These important spectroscopic changes evidence a heparin--induced conformational change of the inhibitor which buries Trp30 in a very hydrophobic environment. Heparin accelerates the inhibition of elastase in a concentration-dependent manner. When both enzyme and inhibitor are saturated by the polymer, the second-order association rate constant is 7.7 x 10(7) M-1 s-1, a value that is 27-fold higher than that measured with the free partners. This finding may have important physiologic and therapeutic bearing.

Purification and characterization of an antithrombin III inactivating enzyme from the venom of the African night adder (Causus rhombeatus)

A serine proteinase was isolated from the venom of the night adder (Causus rhombeatus) by fast protein liquid chromatography (anion-exchange, gel filtration and hydrophobic interaction). The protein (termed CR-serpinase) had an estimated mol. wt of 45,500 as determined by SDS-PAGE, pI of 4.7 and a carbohydrate content of 18.9%. Incubation of CR-serpinase with purified human antithrombin III at a molar ratio of 1:66 resulted in a loss of more than 90% of the initial AT III activity within 10 min. The reaction was dependent on heparin. In SDS-PAGE inactivation of human antithrombin III was correlated with the occurrence of two cleavage products. The cleavage site in the antithrombin III molecule was determined to be Arg 393-Ser 394 by amino-terminal sequencing. CR-Serpinase had no thrombin-like activity since no fibrinogen conversion was induced and had no procoagulant activity. CR-Serpinase activity was not inhibited by antithrombin III-heparin and was not decreased by a 10-min preincubation in normal human plasma. Inactivation of antithrombin III by CR-serpinase appeared to be very specific.

Evidence for the extent of insertion of the active site loop of intact alpha 1 proteinase inhibitor in beta-sheet A

The extent of insertion of beta-strand s4A into sheet A in intact serpin alpha 1-proteinase inhibitor (alpha 1PI has been probed by peptide annealing experiments [Schulze et al. (1990) Eur. J. Biochem. 194, 51-56]. Twelve synthetic peptides of systematically varied length corresponding in sequence to the unprimed (N-terminal) side of the active site loop were complexed with alpha 1PI. The complexes were then characterized by circular dichroism spectroscopy and tested for inhibitory activity. Four peptides formed complexes which retained inhibitory activity, one of which was nearly as effective as the native protein. Comparison with the three dimensional structures of cleaved alpha 1PI [Löbermann et al. (1984) J. Mol. Biol. 177, 531-556] and plakalbumin [Wright et al. (1990) J. Mol. Biol. 213, 513-528] supports a model in which alpha 1PI requires the insertion of a single residue, Thr345, into sheet A for activity.

Crystal structure of cleaved equine leucocyte elastase inhibitor determined at 1.95 A resolution

The crystal structure of active-site cleaved equine leucocyte elastase inhibitor, a member of the serpin superfamily, has been solved and refined to a crystallographic R-factor of 17.6% at 1.95 A resolution. Despite being an intracellular inhibitor with rather low sequence homology of 30% to human alpha 1-antichymotrypsin and alpha 1-proteinase inhibitor, the three-dimensional structures are very similar, with deviations only at the sites of insertions and few mobile secondary structure elements. The better resolution in comparison with the structures of other cleaved serpins allows a more precise description of the so-called R-state of the serpins.

A Leu----His substitution at residue 93 in human corticosteroid binding globulin results in reduced affinity for cortisol

A steroid binding capacity assay and a radioimmunoassay were both used to measure corticosteroid binding globulin (CBG) in serum samples from 22 patients with sepsis. An approximately 50% discordancy between the two values in one patient suggested the presence of a CBG variant with reduced affinity for cortisol, and this was confirmed by Scatchard analysis. We therefore used the polymerase chain reaction to amplify exons that encode for human CBG from the genomic DNA of this patient. This revealed two mutations within the coding sequences: one of which results in a Leu----His substitution at residue 93 and another which encodes a Ser----Ala substitution at residue 224 of the human CBG polypeptide. To assess the impact of each substitution on the steroid binding affinity of CBG, each mutation was introduced separately into a normal human CBG cDNA, and the normal and mutated cDNAs were expressed in Chinese hamster ovary cells. Scatchard analysis of the CBG produced in culture indicated that the His93 mutation (Kd = 2.24 +/- 1.75 nM) reduced the cortisol binding affinity of CBG (mean +/- SD) significantly (P less than 0.024) when compared to normal CBG (Kd = 0.64 +/- 0.31 nM), while the Ala224 mutation (Kd = 0.63 +/- 0.33 nM) did not influence cortisol binding affinity. We therefore conclude that residue 93 may play an important role in determining the structure of the CBG steroid binding site.

Serpins: Implications of a mobile reactive centre

The serpins are unique among the families of serine proteinase inhibitors in having a reactive centre that is situated on a mobile loop. The structures of three alternative conformations are now known, and it can be deduced that the active form involves the partial insertion of the loop into the A sheet of the molecule. The ability of the loop to move in and out of this sheet has been adapted by evolution to allow the modulation of inhibitory activity. Manipulation of the structure of the loop and of other functional domains in the serpin superfamily enables the production of serpins with tailor-made activities. The ability of the loop to lock in latent conformations or to take part in intermolecular polymerization has implications for the production and stabilization of recombinant serpins. This review has been adapted from Current Opinion in Structural Biology 1992, 2:438-446.

Crystallization, activity assay and preliminary X-ray diffraction analysis of the uncleaved form of the serpin antichymotrypsin

Crystals of recombinant wild-type antichymotrypsin have been prepared by the method of vapor diffusion with polyethylene glycol 4000 as a precipitant at pH 5.7. Two crystal forms are observed. One form belongs to tetragonal space group P4(3)2(1)2 (or P4(1)2(1)2) and has unit cell dimensions a = b = 126 A, c = 243 A, with two molecules in the asymmetric unit. The other crystal form belongs to orthorhombic space group P2(1)2(1)2(1) and has unit cell parameters of a = 73 A, b = 78 A and c = 80 A, with one molecular in the asymmetric unit. Diffraction intensity measurements have been made on the tetragonal crystal form to a limiting resolution of 4.1 A, and reflections have been observed on X-ray still photographs to a limiting resolution of 2.5 A for the orthorhombic form. An activity assay of redissolved tetragonal form crystals indicates that the uncleaved, functional serpin has been crystallized.

Methionine oxidation and inactivation of alpha 1-proteinase inhibitor by Cu2+ and glucose

The effect of glucose/Cu2+ incubation on (a) pure methionine oxidation, (b) the oxidation of active-site methionine in alpha 1-proteinase inhibitor (alpha 1PI) and (c) the resulting activity and structural changes of this inhibitor was investigated. While no methionine was oxidized during a 24 day, 37 degrees C incubation with 0.01 M EDTA and 100 mM glucose, 64.2% oxidation occurred in 6 days when 0.01 mM Cu2+ was added to the 100 mM glucose. The first-order rate constant for oxidation in 10 mM glucose, 0.01 mM Cu2+ was 0.0218 day-1. Oxidation was inhibited by catalase, but accelerated by ascorbate ion. The active-site methionyl residue of alpha 1PI was oxidized 71.3% after a 4 day incubation in 100 mM glucose, 0.01 mM Cu2+ (pH 7.45), 0.1 M phosphate buffer. The elastase and trypsin inhibiting activities were lowered to 3.1 and 1.5% of control samples during this incubation. The inclusion of 1 mM DETAPAC, a transition metal chelator, resulted in a 98 + % retention of activity. Intrinsic fluorescence (350 nm excitation, 415 nm emission) of alpha 1PI increased 576% over control for the sample incubated in 100 mM glucose, 0.01 mM Cu2+ and SDS-PAGE revealed protein fragment molecular weights of 44.4 and 39.8 kDa. These studies suggest that both methionine oxidation and free radical induced fragmentation contribute to loss of alpha 1PI activity during glucose/Cu2+ incubations.

The mechanism of Z alpha 1-antitrypsin accumulation in the liver

Most northern Europeans have only the normal M form of the plasma protease inhibitor alpha 1-antitrypsin, but some 4% are heterozygotes for the Z deficiency variant. For reasons that have not been well-understood, the Z mutation results in a blockage in the final stage of processing of antitrypsin in the liver such that in the Z homozygote only 15% of the protein is secreted into the plasma. The 85% of the alpha 1-antitrypsin that is not secreted accumulates in the endoplasmic reticulum of the hepatocyte; much of it is degraded but the remainder aggregates to form insoluble intracellular inclusions. These inclusions are associated with hepatocellular damage, and 10% of newborn Z homozygotes develop liver disease which often leads to a fatal childhood cirrhosis. Here we demonstrate the molecular pathology underlying this accumulation and describe how the Z mutation in antitrypsin results in a unique molecular interaction between the reactive centre loop of one molecule and the gap in the A-sheet of another. This loop-sheet polymerization of Z antitrypsin occurs spontaneously at 37 degrees C and is completely blocked by the insertion of a specific peptide into the A-sheet of the antitrypsin molecule. Z antitrypsin polymerized in vitro has identical properties and ultrastructure to the inclusions isolated from hepatocytes of a Z homozygote. The concentration and temperature dependence of this loop-sheet polymerization has implications for the management of the liver disease of the newborn Z homozygote.

Sequence and molecular characterization of human monocyte/neutrophil elastase inhibitor

cDNA encoding human monocyte/neutrophil elastase inhibitor (EI), a M(r) approximately 42,000 protein with serpin-like functional properties, has been sequenced. The 1316-base-pair sequence was obtained from overlapping clones and amplified DNA from libraries of monocyte-like and neutrophil-like cells. Hybridization with EI cDNA identified three EI mRNA species of 1.5, 1.9, and 2.6 kilobases in U937 monocyte-like cells and no hybridizing mRNA in lymphoblastoid cells lacking detectable EI. The cDNA open reading frame encodes a 379-amino acid protein, of which 167 residues were confirmed by tryptic peptides. Although EI may function extracellularly as well as intracellularly, its deduced sequence lacks a typical cleavable N-terminal signal sequence. Sequence analysis established that EI is a member of the serpin superfamily. EI has greatest homology (50.1% identity of amino acids) with plasminogen activator inhibitor 2, also a monocyte protein, and ovalbumin and gene Y, which were previously grouped as an ancient branch of the serpin superfamily. The extent of EI identity with the functionally related serpin alpha 1 antitrypsin is only 30.1%. Sequence alignment indicates that the reactive center P1 residue is Cys-344, consistent with abrogation of elastase inhibitory activity by iodoacetamide and making EI a naturally occurring Cys-serpin. The cleavable bond, Cys-Met, suggests an oxidation-sensitive molecule capable of inhibiting more than one serine protease. Oxidation sensitivity would limit the place of action of EI to the immediate vicinity of carrier cells. The molecular structure will help clarify the likely role of EI in regulating protease action and preventing tissue damage by phagocytic cells.

Design and synthesis of a helix heparin-binding peptide

Elaboration of heparin-protein-binding interactions is necessary to understand how heparin modulates protein function. The heparin-binding domain of some proteins is postulated to be a helix structure which presents a surface of high positive charge density. Thus, a synthetic 19-residue peptide designed to be alpha-helical in character was synthesized, and its interaction with heparin was studied. The peptide was shown to be 75% helix by circular dichroism (CD) spectrometry in neutral pH buffer (at 2 degrees C); helicity increased to nearly 85% under high ionic strength conditions or to nearly 100% in 75% ethanol. Increasing the temperature of the solution caused a change in the spectral envelope consistent with a coil transition of the peptide. The midpoint of the transition (i.e., the temperature at which the helix content was determined to be 50%) was 25 degrees C, and the determined van't Hoff enthalpy change (delta HvH) was 3.2 kcal/mol of peptide. By CD, heparin increases the helix content of the peptide to 100% and increases the apparent thermal stability of the peptide by about 1 kcal/mol. The melting point for the helix/coil transition of the heparin-peptide complex was 50 degrees C. The thermal coefficient of the transition (approximately 300 deg.cm2.dmol-1.degree C-1) was essentially the same for the peptide alone or the peptide-heparin complex. Dissociation of the complex under high ionic strength conditions was also observed in the CD experiment. Biological assays showed less heparin-binding activity than expected (micromolar KD values), but this was attributed to the absence of critical lysyl residues in the peptide.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning of a mouse 47-kDa heat-shock protein (HSP47), a collagen-binding stress protein, and its expression during the differentiation of F9 teratocarcinoma cells

A 47-kDa heat-shock protein (HSP47) is a major collagen-binding stress protein residing in the endoplasmic reticulum, and is assumed to be a molecular chaperone specific to collagen. Two-dimensional gel electrophoresis and immunoprecipitation studies showed that the expression of HSP47 was significantly induced during the differentiation of mouse teratocarcinoma F9 cells by treatment with retinoic acid alone or with retinoic acid and dibutyryladenosine 3',5'-phosphate. The induction of type-IV collagen was also observed during F9-cell differentiation. For further analysis, we cloned cDNA encoding mouse HSP47 from a cDNA library of BALB/c 3T3 cells and performed Northern-blot analysis. The cDNA contained a signal sequence at the N-terminus and an endoplasmic-reticulum-retention signal, RDEL, at the C-terminus. An homology search revealed that mouse HSP47, as well as chick HSP47, belonged to the serine protease inhibitor superfamily. While chick HSP47 mRNA was 4.5 kb with a long (2-kb) 3' untranslated region, mouse and human HSP47 mRNA were 2.5 kb, with a 0.8-kb 3' untranslated region. Northern-blot analysis revealed that the concurrent induction of HSP47 and type-IV collagen during F9-cell differentiation, and the transient induction of HSP47 after heat shock was regulated at the level of mRNA accumulation. These results suggested that HSP47 was closely related to collagens in terms of its expression as well as in its functional relevance.

Viral inhibition of inflammation: cowpox virus encodes an inhibitor of the interleukin-1 beta converting enzyme

Cowpox virus effectively inhibits inflammatory responses against viral infection in the chick embryo. This study demonstrates that one of the viral genes necessary for this inhibition, the crmA gene (a cytokine response modifier gene), encodes a serpin that is a specific inhibitor of the interleukin-1 beta converting enzyme. This serpin can prevent the proteolytic activation of interleukin-1 beta, thereby suppressing an interleukin-1 beta response to infection. However, the modification of this single cytokine response is not sufficient to inhibit inflammatory responses. This suggests that cowpox virus encodes several cytokine response modifiers that act together to inhibit the release of pro-inflammatory cytokines in response to infection. These viral countermeasures to host defenses against infection may contribute significantly to the pathology associated with poxvirus infections.

Antithrombin Budapest 3. An antithrombin variant with reduced heparin affinity resulting from the substitution L99F

The molecular basis and functional properties of a variant antithrombin (AT) protein. AT Budapest 3, were studied. A single base substitution was identified in codon 99, CTC----TTC, altering the normal leucine to phenylalanine. The proband presented with a history of venous thrombotic disease and was found to be homozygous for the mutation. The variant protein demonstrated reduced heparin affinity and reduced antiproteinase activity in the presence of either unfractionated heparin or the AT-binding heparin pentasaccharide, when compared to normal AT. A small change in the isoelectric point was also identified. The substituted amino acid residue of AT Budapest 3 is located near to the proposed AT heparin binding site, and it is suggested that reduced heparin affinity of the variant protein may result from substitution-induced distortion of positive charge geometry in the binding site and/or changes in its position relative to the rest of the inhibitor molecule.

The refined 1.9-A X-ray crystal structure of D-Phe-Pro-Arg chloromethylketone-inhibited human alpha-thrombin: structure analysis, overall structure, electrostatic properties, detailed active-site geometry, and structure-function relationships

Thrombin is a multifunctional serine proteinase that plays a key role in coagulation while exhibiting several other key cellular bioregulatory functions. The X-ray crystal structure of human alpha-thrombin was determined in its complex with the specific thrombin inhibitor D-Phe-Pro-Arg chloromethylketone (PPACK) using Patterson search methods and a search model derived from trypsinlike proteinases of known spatial structure (Bode, W., Mayr, I., Baumann, U., Huber, R., Stone, S.R., & Hofsteenge, J., 1989, EMBO J. 8, 3467-3475). The crystallographic refinement of the PPACK-thrombin model has now been completed at an R value of 0.156 (8 to 1.92 A); in particular, the amino- and the carboxy-termini of the thrombin A-chain are now defined and all side-chain atoms localized; only proline 37 was found to be in a cis-peptidyl conformation. The thrombin B-chain exhibits the characteristic polypeptide fold of trypsinlike serine proteinases; 195 residues occupy topologically equivalent positions with residues in bovine trypsin and 190 with those in bovine chymotrypsin with a root-mean-square (r.m.s.) deviation of 0.8 A for their alpha-carbon atoms. Most of the inserted residues constitute novel surface loops. A chymotrypsinogen numbering is suggested for thrombin based on the topological equivalences. The thrombin A-chain is arranged in a boomeranglike shape against the B-chain globule opposite to the active site; it resembles somewhat the propeptide of chymotrypsin(ogen) and is similarly not involved in substrate and inhibitor binding. Thrombin possesses an exceptionally large proportion of charged residues. The negatively and positively charged residues are not distributed uniformly over the whole molecule, but are clustered to form a sandwichlike electrostatic potential; in particular, two extended patches of mainly positively charged residues occur close to the carboxy-terminal B-chain helix (forming the presumed heparin-binding site) and on the surface of loop segment 70-80 (the fibrin[ogen] secondary binding exosite), respectively; the negatively charged residues are more clustered in the ringlike region between both poles, particularly around the active site. Several of the charged residues are involved in salt bridges; most are on the surface, but 10 charged protein groups form completely buried salt bridges and clusters. These electrostatic interactions play a particularly important role in the intrachain stabilization of the A-chain, in the coherence between the A- and the B-chain, and in the surface structure of the fibrin(ogen) secondary binding exosite (loop segment 67-80).(ABSTRACT TRUNCATED AT 400 WORDS)

Reliable phenotyping of alpha-1-antitrypsin by hybrid isoelectric focusing in an ultranarrow immobilized pH gradient

Genetically determined phenotypes of the highly polymorphic human alpha 1-antitrypsin were examined by hybrid isoelectric focusing in a narrow immobilized pH gradient. The chosen pH range from 4.45 to 4.75 was useful for identification and classification of the common PI M subtypes and a number of PI variants in the microheterogeneous regions of m6, m7, and m8. A high degree of resolution and an improved sharpness of PI bands was achieved with this excellent technique. It allowed the distinction of a new PI M variant, which has been designated M8, or Mingolstadt, according to the PI nomenclature. The pI difference of this mutant to the slightly cathodically located subtype M3 is approximately 0.001 pH unit. In addition, some common as well as rare phenotypes are presented.

The nucleotide and deduced amino acid structures of sheep and pig fetuin. Common structural features of the mammalian fetuin family

This study was initiated to gain further insight into the structural features of the mammalian fetuin family. The cDNA structures of sheep and pig fetuin were determined. The cDNA insert encoding sheep (pig) fetuin comprised 1550 (1470) nucleotides, including 54 (46) nucleotides encoding a signal peptide of 18 (15) residues and 1038 (1041) nucleotides encoding the 346 (347) amino acids of the mature plasma protein. The predicted amino-terminal sequence of the mature pig fetuin was confirmed by the amino-terminal sequence of the purified protein. However, two alternative sheep amino-terminal sequences were found in fetuin purified from the plasma of a single sheep fetus; the minor product was the one predicted by comparison with other fetuin sequences while the major product was two amino acids longer. Comparison of the deduced amino acid sequences of sheep and pig fetuin showed an extensive sequence identity between them (75%) and with other proteins of the mammalian fetuin family, i.e. human alpha 2-HS glycoprotein, and bovine and rat fetuins. Twelve cysteine residues were found at invariant positions in all fetuin sequences, suggesting strongly that the arrangement of disulphide bridges identified in human alpha 2-HS glycoprotein is common to the members of the family. Further sequence comparisons revealed that the structures of mammalian fetuins are organised in three domains: two cystatin-like domains (D1 and D2) and a complex carboxyl-terminal domain (D3). The proposed three-domain structure of the protein is reflected in the organisation of the rat fetuin structural gene which has recently been published.

Complete cDNA sequence of bovine alpha 1-antitrypsin

A cDNA clone coding for the entire bovine alpha 1-antitrypsin molecule has been isolated from a lambda gt11 bovine liver cDNA library using a human alpha 1-antitrypsin cDNA as a probe. The bovine cDNA was sequenced by the dideoxynucleotide chain termination method. Comparison of the translated amino acid sequence of the bovine alpha 1-antitrypsin with those of the human, baboon, sheep, rat and mouse demonstrates the preservation of most of the critical structural determinants. The bovine and the sheep molecules have a sequence homology of 94% and both the molecules contain four cysteine residues; there is only one cysteine in the others.

Identification of hydrophobic fragments of alpha 1-antitrypsin and C1 protease inhibitor in human bile, plasma and spleen

Hydrophobic peptides were isolated from the phospholipid fraction of human bile, plasma and spleen by exclusion chromatography in organic solvents. From plasma, the activation peptide of C1 protease inhibitor was recovered, from spleen the activation peptide of alpha 1-antitrypsin, and from bile, both these peptides, as well as a fragment generated by proteolytic cleavage of alpha 1-antitrypsin six residues N-terminal of the P1-P1' peptide bond. Cleavages in this region inactivate antiproteases but have previously not been reported to occur in vivo. These peptides in human bile may reflect physiological actions in regulation of antiproteolytic activity or bile secretion processes, and/or be of importance for the physicochemical state of cholesterol, phospholipids and bile acids in bile.

A model for the determination of the 3D-spatial distribution of the functions of the hormone-binding domain of receptors that bind 3-keto-4-ene steroids

A method of comparing the hydrophobic clusters of proteins (hydrophobic cluster analysis, HCA) has revealed that the 3D-folding pattern of the hormone-binding domain (HBD) of steroid hormone receptors (SHRs) may have an unexpectedly high degree of analogy with the known 3D-crystal structures of proteins belonging to the serine proteinase inhibitor (SERPIN) superfamily, e.g. alpha 1-antitrypsin and ovalbumin. The present paper briefly reviews some of the biochemical evidence that supports the structural validity of the SERPIN model and shows how the model can be used to establish hypothetical 3D-locations for functions attributed to different amino-acids or peptide sequences of the HBD: i.e. heat-shock protein binding, transcription activation, phosphorylation, steroid binding, but also ATP-binding. Indeed, the model has enabled the identification of a Rossmann-fold in SHRs that might bind ATP. Visualization of all these functions should help to interpret the chain of concerted events induced by steroid binding.

The N-terminal domain of antithrombin-III is essential for heparin binding and complex-formation with, but not cleavage by, alpha-thrombin

Normal and mutant forms of human antithrombin-III (AT-III) were synthesized in a cell-free system in order to identify putative functional domains required for heparin binding and complex-formation with alpha-thrombin. Heparin-Sepharose chromatography resulted in the elution of approx. 70% of cell-free-derived normal AT-III-(1-432)-polypeptide as a peak between 0.2 M- and 0.7 M-NaCl. The cell-free-derived normal AT-III also reacted with alpha-thrombin. Approx. 15% of this AT-III formed covalent complexes with alpha-thrombin in 2 min. Unfractionated heparin accelerated the rate of formation of such complexes. Two truncated forms of AT-III (amino acid residues 219-432 and 251-432), containing only the putative thrombin-binding domain, were synthesized independently in this cell-free system. These truncated AT-III polypeptides did not bind heparin and were unable to form stable covalent complexes with alpha-thrombin. However, both of these AT-III polypeptides were cleaved by alpha-thrombin, presumably at the reactive centre Arg-393-Ser-394. The formation of the disulphide bond between Cys-247 and Cys-430 in AT-III-(219-432)-polypeptide had no effect on the results obtained. Mutations in full-length AT-III at Cys-430 had no effect on the ability of AT-III to bind heparin. There was, however, a slight decrease in the formation of stable inhibitory complexes with alpha-thrombin. A cell-free-derived AT-III mutant, devoid of amino acid residues 41-49, which comprise heparin-binding region 1 of AT-III, had slightly decreased heparin binding compared with cell-free-derived normal AT-III-(1-432)-polypeptide. This mutant AT-III polypeptide was unable, however, to form a stable complex with alpha-thrombin. We conclude therefore that the N-terminal domain of AT-III is essential for both heparin binding and complex-formation with alpha-thrombin, but not for the cleavage of AT-III at its reactive centre by alpha-thrombin.

Natural protein proteinase inhibitors and their interaction with proteinases

The substrate-like 'canonical' inhibition by the 'small' serine proteinase inhibitors and the product-like inhibition by the carboxypeptidase inhibitor have provided the only atomic models of protein inhibitor--proteinase interactions for about 15 years. The recently published structures of cystatin/stefin--papain complexes and of hirudin--thrombin complexes reveal novel non-substrate-like interactions. In addition, the structure of pro-carboxypeptidase shows a model of inactivation which bears resemblance to proteinase/protein inhibitor systems. Considerable progress in understanding the transition between native and cleaved states of the serpins has also been made by several recent structural studies.

Expression of human α1 antitrypsin in transgenic sheep

We have recently described the production of large amounts (< or = 65 grams per litre) of enzymatically active human alpha 1 antitrypsin in the milk of transgenic sheep (Wright et al., 1991). Here, we describe in more detail the expression of the human protein in the milk of these animals throughout the lactation period. Human alpha 1 antitrypsin is also found at much lower levels in the plasma of transgenic ewes before, during and after lactation. It is also detected in male plasma at very low levels. We have previously shown human alpha 1 antitrypsin purified from transgenic sheep milk to be indistinguishable from commercially available human plasma derived alpha 1 antitrypsin in terms of gross sugar content and in vitro activity. Here we extend this comparison to more detailed analyses of glycosylation state, amino-terminal sequence, pI value, and molecular weight determination by mass spectrometry.

Structural basis of latency in plasminogen activator inhibitor-1

Human plasminogen activator inhibitor-1 (PAI-1) is the fast-acting inhibitor of tissue plasminogen activator and urokinase and is a member of the serpin family of protease inhibitors. Serpins normally form complexes with their target proteases that dissociate very slowly as cleaved species and then fold into a highly stable inactive state in which the residues that flank the scissile bond (P1 and P1';) are separated by about 70 A. PAI-1 also spontaneously folds into a stable inactive state without cleavage; this state is termed 'latent' because inhibitory activity can be restored through denaturation and renaturation. Here we report the structure of intact latent PAI-1 determined by single-crystal X-ray diffraction to 2.6 A resolution. The three-dimensional structure reveals that residues on the N-terminal side of the primary recognition site are inserted as a central strand of the largest beta sheet, in positions similar to the corresponding residues in the cleaved form of the serpin alpha 1-proteinase inhibitor (alpha 1-PI). Residues C-terminal to the recognition site occupy positions on the surface of the molecule distinct from those of the corresponding residues in cleaved serpins or in the intact inactive serpin homologue, ovalbumin, and its cleavage product, plakalbumin. The structure of latent PAI-1 is similar to one formed after cleavage in other serpins, and the stability of both latent PAI-1 and cleaved serpins may be derived from the same structural features.

Inhibitory activity and conformational transition of alpha 1-proteinase inhibitor variants

Several variants of alpha 1-proteinase inhibitor (alpha 1-PI) were investigated by spectroscopic methods and characterized according to their inhibitory activity. Replacement of Thr345 (P14) with Arg in alpha 1-PI containing an Arg residue in position 358 (yielding [Thr345----Arg, Met358----Arg]alpha 1-PI) results in complete loss of its inhibitory activity against human alpha-thrombin; whereas an exchange of residue Met351 (P8) by Glu [( Met351----Glu, Met358----Arg]alpha 1-PI) does not alter activity. [Thr345----Arg, Met358----Arg]alpha 1-PI is rapidly cleaved by thrombin, while [Met358----Arg]alpha 1-PI and [Met351----Glu, Met358----Arg]alpha 1-PI form stable proteinase-inhibitor complexes. The stability of [Thr345----Arg, Met358----Arg]alpha 1-PI against guanidinium chloride denaturation is significantly enhanced compared to wild-type alpha 1-PI, and does not change after cleavage, resembling ovalbumin, a serpin with no inhibitory activity, from which the Thr345----Arg amino acid exchange had been derived. [Met351----Glu, Met358----Arg]alpha 1-PI and [Met358----Arg]alpha 1-PI resemble the wild-type protein in this respect. The CD spectra of intact and cleaved alpha 1-PI variants do not compare well with the wild-type protein, probably reflecting local structural differences. Insertion of a synthetic peptide, which corresponds to residues Thr345----Met358 of human alpha 1-PI, leads to the formation of binary complexes with all variants having the characteristic features of the binary complex between peptide and wild-type protein.

Type-1 inhibitor of plasminogen activators. Distinction between latent, activated and reactive centre-cleaved forms with thermal stability and monoclonal antibodies

Type-1 inhibitor of plasminogen activators (PAI-1) occurs in purified preparations in a latent form that can be activated with denaturants; in vivo, latency is prevented by binding to vitronectin. We have compared latent, denaturant-activated and reactive centre-cleaved human PAI-1 with respect to thermal stability and affinity to monoclonal antibodies. By both criteria, latent and cleaved PAI-1 are very similar or indistinguishable, and clearly different from active PAI-1. Our findings suggest that the conformations of latent and reactive centre-cleaved PAI-1 are similar and resemble the so-called relaxed (R) serpin conformation, while that of active PAI-1 is different and resembles the stressed (S) serpin conformation.

Thrombin neutralizes plasminogen activator inhibitor 1 (PAI-1) that is complexed with vitronectin in the endothelial cell matrix

Vitronectin endows plasminogen activator inhibitor 1 (PAI-1), the fast-acting inhibitor of both tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), with additional thrombin inhibitory properties. In view of the apparent association between PAI-1 and vitronectin in the endothelial cell matrix (ECM), we analyzed the interaction between PAI-1 and thrombin in this environment. Upon incubating 125I-labeled alpha-thrombin with endothelial cell matrix (ECM), the protease formed SDS-stable complexes exclusively with PAI-1, with subsequent release of these complexes into the supernatant. Vitronectin was required as a cofactor for the association between PAI-1 and thrombin in ECM. Metabolic labeling of endothelial cell proteins, followed by incubation of ECM with t-PA, u-PA, or thrombin, indicated that all three proteases depleted PAI-1 from ECM by complex formation and proteolytic cleavage. Proteolytically inactive thrombin as well as anticoagulant thrombin, i.e., thrombin in complex with its endothelial cell surface receptor thrombomodulin, did not neutralize PAI-1, emphasizing that the procoagulant moiety of thrombin is required for a functional interaction with PAI-1. A physiological implication of our findings may be related to the mutual neutralization of both PAI-1 and thrombin, providing a new link between plasminogen activation and the coagulation system. Evidence is provided that in ECM, procoagulant thrombin may promote plasminogen activator activity by inactivating PAI-1.

Mobile reactive centre of serpins and the control of thrombosis

Two protease inhibitors in human plasma play a key part in the control of thrombosis: antithrombin inhibits coagulation and the plasminogen activator inhibitor PAI-1 inhibits fibrinolysis, the dissolving of clots. Both inhibitors are members of the serpin family and both exist in the plasma in latent or inactive forms. We show here that the reactive centre of the serpins can adopt varying conformations and that mobility of the reactive centre is necessary for the function of antithrombin and its binding and activation by heparin; the identification of a new locked conformation explains the latent inactive state of PAI-1. This ability to vary conformation not only allows the modulation of inhibitory activity but also protects the circulating inhibitor against proteolytic attack. Together these findings explain the retention by the serpins of a large and unconstrained reactive centre as compared to the small fixed peptide loop of other families of serine protease inhibitors.