A monoclonal antibody recognizing the FAD-binding site of 4-aminobenzoate hydroxylase from Agaricus bisporus

A monoclonal antibody against 4-aminobenzoate hydroxylase (EC 1.14.13.27) from Agaricus bisporus, a common edible mushroom, has been produced by the fusion of BALB/c mouse spleen cells immunized with the denatured enzyme and P3x63Ag8U1 myeloma cells in order to locate and characterize the catalytic site of the enzyme. The monoclonal antibody immunoblotted the enzyme and immunoprecipitated its apoenzyme. The immunoprecipitation was inhibited in the presence of FAD, and the monoclonal antibody competitively inhibited the binding of FAD to the apoenzyme. The monoclonal antibody, therefore, recognizes the FAD-binding site of 4-aminobenzoate hydroxylase. Interestingly, it was shown that the monoclonal antibody was cross-reactive with FAD-dependent enzymes such as salicylate hydroxylase (EC 1.14.13.1) and D-amino acid oxidase (EC 1.4.3.3), and that it was specific for the FAD-binding sites of these enzymes. This fact suggests that these FAD-dependent enzymes have immunologically similar structures on their FAD-binding sites.     

FAD-dependent malate dehydrogenase from Mycobacterium sp. strain Takeo: A possible role of phospholipid

Using a homogeneously purified FAD-dependent malate dehydrogenase, a phospholipid-requiring enzyme from the particulate fraction of $\text{Mycobacterium}$ sp. strain Takeo, specific and near stoichiometric binding of FAD to the apoenzymecardiolipin complex, but not to the free apoenzyme, was demonstrated by sucrose density gradient centrifugation in the presence of FAD. The result strongly indicates that the FAD-binding site of the apoenzyme is formed by the protein-phospholipid interaction. This is also supported by the fact that the protective effect of FAD on the stability and on pCMB inhibition of the apoenzyme was observed only in the presence of cardiolipin.     

Characterization of fumarate reductase from baker's yeast: essential sulfhydryl group for binding of FAD

Fumarate reductase apoenzyme having the ability to reconstitute active enzyme was obtained by dialyzing the holoenzyme against 1 M KBr. The dissociation constant of the FAD-apoenzyme complex was 2.3 X 10(-8) M. The denatured holoenzyme and apoenzyme possessed seven sulfhydryl (SH) groups as determined with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB). In the native apoenzyme, five SH-groups reacted with DTNB, and four of them were completely protected by the addition of FAD, while in the native holoenzyme, one was modified without inactivation. These results indicate that one SH-group is located on the surface of the enzyme molecule, four at or near the FAD-binding site, and two deeply embedded in the molecule. The modification of the apoenzyme caused inhibition of binding of FAD, resulting in loss of the ability to reconstitute enzymatic activity. Analyses of the data by statistical and kinetic methods suggested that a reactive SH-group is involved among the four SH-groups in the binding of FAD to the apoenzyme.     

Oxygen reactivity of p-hydroxybenzoate hydroxylase containing 1-deaza-FAD

The flavin prosthetic group (FAD) of p-hydroxybenzoate hydroxylase (EC 1.14.13.2) was replaced by 1-deaza-FAD (carbon substituted for nitrogen at position 1). An improved method for production of apoenzyme by precipitation with acidic ammonium sulfate was developed. The modified enzyme, in the presence of p-hydroxybenzoate, catalyzed the oxidation of NADPH by oxygen, yielding NADP+ and H2O2, but the ability to hydroxylate p-hydroxybenzoate and other substrates was lost. An analysis of the mechanism of NADPH-oxidase catalysis showed a close analogy between the reaction pathways for native and modified enzymes. In the presence of p-hydroxybenzoate, the rate of NADPH consumption catalyzed by the 1-deaza-FAD form was about 11% that of the native enzyme. Both formed a stabilized flavin-C (4a)-OOH intermediate upon reaction of reduced enzyme with oxygen, but the 1-deaza-FAD enzyme could not utilize this peroxide to hydroxylate substrates, and the peroxide decomposed to oxidized enzyme and H2O2.     

Characteristics of monoclonal antibody binding with the C domain of human angiotensin converting enzyme

Binding of a panel of eight monoclonal antibodies (mAbs) with the C domain of angiotensin converting enzyme (ACE) to human testicular ACE (tACE) (corresponding to the C domain of the somatic enzyme) was studied and the inhibition of the enzyme by the mAb 4E3 was found. The dissociation constants of complexes of two mAbs, IB8 and 2H9, with tACE were 2.3 +/- 0.4 and 2.5 +/- 0.4 nM, respectively, for recombinant tACE and 1.6 +/- 0.3 nM for spermatozoid tACE. Competition parameters of mAb binding with tACE were obtained and analyzed. As a result, the eight mAbs were divided into three groups, whose binding epitopes did not overlap: (1) 1E10, 2B11, 2H9, 3F11, and 4E3; (2) 1B8 and 3F10; and (3) IB3. A diagram demonstrating mAb competitive binding with tACE was proposed. Comparative analysis of mAb binding to human and chimpanzee ACE was carried out, which resulted in revealing of two amino acid residues, Lys677 and Pro730, responsible for binding of three antibodies, 1E10, 1B8, and 3F10. It was found by mutation of Asp616 located close to Lys677 that the mAb binding epitope 1E10 contains Asp616 and Lys677, whereas mAbs 1B8 and 3F10 contain Pro730.     

Purification and properties of 4-aminobenzoate hydroxylase, a new monooxygenase from Agaricus bisporus

A new FAD-dependent monooxygenase, 4-aminobenzoate hydroxylase that catalyzes the decarboxylative hydroxylation of 4-aminobenzoate and forms 4-hydroxyaniline in the presence of NAD(P)H and O2 has been purified to homogeneity by ammonium sulfate fractionation, affinity chromatography, chromatofocusing, and Sephadex G-100 chromatography from Agaricus bisporus, a common edible mushroom. The molecular weight of the enzyme, which consists of a single polypeptide, is 49,000. The enzyme contains 0.91 mol of FAD/mol of enzyme. Stoichiometric studies show that 1 mol of 4-aminobenzoate is converted to an equimolecular amount of 4-hydroxyaniline and CO2 with the consumption of 1 mol each of NADH and molecular oxygen. Results obtained isotopically with 18O2 show that one atom of molecular oxygen is incorporated into 4-hydroxyaniline formed from 4-aminobenzoate. The enzyme is most active between pH 6.5 and 8.0 in the oxidation of NADH and between pH 6.0 and 7.5 in the case of NADPH. The Km values for 4-aminobenzoate, NADH, and O2 are 20.4, 13.6, and 200 microM, respectively, and that for NADPH is 133 microM. Other substituted benzoates with free amino and carboxyl groups in the ortho or para position (e.g. 4-aminosalicylate and anthranilate) serve as substrates for hydroxylation, but, in these cases, H2O2 is formed simultaneously with the hydroxylation. The enzyme is insensitive to the chelators of iron and copper, sodium arsenite, and KCN. Heavy metal ions and p-chloromercuribenzoate severely inhibit the enzyme enzyme     

Redox properties of the isolated flavin mononucleotide- and flavin adenine dinucleotide-binding domains of neuronal nitric oxide synthase

Electron transfer through neuronal nitric oxide synthase (nNOS) is regulated by the reversible binding of calmodulin (CaM) to the reductase domain of the enzyme, the conformation of which has been shown to be dependent on the presence of substrate, NADPH. Here we report the preparation of the isolated flavin mononucleotide (FMN)-binding domain of nNOS with bound CaM and the electrochemical analysis of this and the isolated flavin adenine dinucleotide (FAD)-binding domain in the presence and absence of NADP(+) and ADP (an inhibitor). The FMN-binding domain was found to be stable only in the presence of bound CaM/Ca(2+), removal of which resulted in precipitation of the protein. The FMN formed a kinetically stabilized blue semiquinone with an oxidized/semiquinone reduction potential of -179 mV. This is 80 mV more negative than the potential of the FMN in the isolated reductase domain, that is, in the presence of the FAD-binding domain. The FMN semiquinone/hydroquinone redox couple was found to be similar in both constructs. The isolated FAD-binding domain, generated by controlled proteolysis of the reductase domain, was found to have similar FAD reduction potentials to the isolated reductase domain. Both formed a FAD-hydroquinone/NADP(+) charge-transfer complex with a long-wavelength absorption band centered at 780 nm. Formation of this complex resulted in thermodynamic destabilization of the FAD semiquinone relative to the hydroquinone and a 30 mV increase in the FAD semiquinone/hydroquinone reduction potential. Binding of ADP, however, had little effect. The possible role of the nicotinamide/FADH(2) stacking interaction in controlling electron transfer and its likely dependence on protein conformation are discussed.     

Crystal structure of cholesterol oxidase from Brevibacterium sterolicum refined at 1.8 A resolution

Cholesterol oxidase (3 beta-hydroxysteroid oxidase, EC 1.1.3.6) is an FAD-dependent enzyme that carries out the oxidation and isomerization of steroids with a trans A : B ring junction. The crystal structure of the enzyme from Brevibacterium sterolicum has been determined using the method of isomorphous replacement and refined to 1.8 A resolution. The refined model includes 492 amino acid residues, the FAD prosthetic group and 453 solvent molecules. The crystallographic R-factor is 15.3% for all reflections between 10.0 A and 1.8 A resolution. The structure is made up of two domains: an FAD-binding domain and a steroid-binding domain. The FAD-binding domain consists of three non-continuous segments of sequence, including both the N terminus and the C terminus, and is made up of a six-stranded beta-sheet sandwiched between a four-stranded beta-sheet and three alpha-helices. The overall topology of this domain is very similar to other FAD-binding proteins. The steroid-binding domain consists of two non-continuous segments of sequence and contains a six-stranded antiparallel beta-sheet forming the "roof" of the active-site cavity. This large beta-sheet structure and the connections between the strands are topologically similar to the substrate-binding domain of the FAD-binding protein para-hydroxybenzoate hydroxylase. The active site lies at the interface of the two domains, in a large cavity filled with a well-ordered lattice of 13 solvent molecules. The flavin ring system of FAD lies on the "floor" of the cavity with N-5 of the ring system exposed. The ring system is twisted from a planar conformation by an angle of approximately 17 degrees, allowing hydrogen-bond interactions between the protein and the pyrimidine ring of FAD. The amino acid residues that line the active site are predominantly hydrophobic along the side of the cavity nearest the benzene ring of the flavin ring system, and are more hydrophilic on the opposite side near the pyrimidine ring. The cavity is buried inside the protein molecule, but three hydrophobic loops at the surface of the molecule show relatively high temperature factors, suggesting a flexible region that may form a possible path by which the substrate could enter the cavity. The active-site cavity contains one charged residue, Glu361, for which the side-chain electron density suggests a high degree of mobility for the side-chain. This residue is appropriately positioned to act as the proton acceptor in the proposed mechanism for the isomerization step.     

Cadherin conformations associated with dimerization and adhesion

To investigate conformations of C-cadherin associated with functional activity and physiological regulation, we generated monoclonal antibodies (mAbs) that bind differentially to monomeric or dimeric forms. These mAbs recognize conformational epitopes at multiple sites along the C-cadherin ectodomain aside from the well known Trp-2-mediated dimer interface in the N-terminal EC1 domain. Group 1 mAbs, which bind monomer better than dimer and the Trp-2-mutated protein (W2A) better than wild type, recognize epitopes in EC4 or EC5. Dimerization of the W2A mutant protein via a C-terminal immunoglobulin Fc domain restored the dimeric mAb-binding properties to EC4-5 and partial homophilic binding activity but did not restore full cell adhesion activity. Group 2 and Group 3 mAbs, which bind dimer better than monomer and wild type better than W2A, recognize epitopes in EC1 and the interface between EC1 and EC2, respectively. None of the mAbs could distinguish between different physiological states of C-cadherin at the cell surface of either Xenopus embryonic cells or Colo 205 cultured cells, demonstrating that changes in dimerization do not underlie regulation of adhesion activity. On the cell surface the EC3-EC5 domains are much less accessible to mAb binding than EC1-EC2, suggesting that they are masked by the state of cadherin organization or by other molecules. Thus, the EC2-EC5 domains either reflect, or are involved in, cadherin dimerization and organization at the cell surface.     

Characteristics of monoclonal antibody binding with the <Emphasis Type="Italic">C</Emphasis> domain of human angiotensin converting enzyme

Binding of a panel of eight monoclonal antibodies (mAbs) with the C domain of angiotensin converting enzyme (ACE) to human testicular ACE (tACE) (corresponding to the C domain of the somatic enzyme) was studied, and the inhibition of the enzyme by the mAb 4A3 was found. The dissociation constants of complexes of two mAbs, 1B8 and 2H9, with tACE were 2.3 ± 0.4 and 2.5 ± 0.4 nM, respectively, for recombinant tACE and 4.7 ± 0.5 and 1.6 ± 0.3 nM for spermatozoid tACE. Competition parameters of mAb binding with tACE were obtained and analyzed. As a result, the eight mAbs were divided into three groups, whose binding epitopes did not overlap: (1) 1E10, 2B11, 2H9, 3F11, and 4E3; (2) 1B8 and 3F10; and (3) 1B3. A diagram demonstrating mAb competitive binding with tACE was proposed. Comparative analysis of mAb binding to human and chimpanzee ACE was carried out, which resulted in revealing of two amino acid residues, Lys677 and Pro730, responsible for binding of three antibodies, 1E10, 1B8, and 3F10. It was found by mutation of Asp616 located close to Lys677 for Leu that the mAb binding epitope 1E10 contains Asp616 and Lys677, whereas mAbs 1B8 and 3F10 contain Pro730.     

Mapping two functional domains of clathrin light chains with monoclonal antibodies

The two forms of clathrin light chains (LCA and LCB) or clathrin-associated proteins (CAP1 and CAP2) have presented an immunochemical paradox. Biochemically similar, both possess two known functional parameters: binding the clathrin heavy chain and mediating the action of an uncoating ATPase. All previously reported anti-CAP mAbs, however, react specifically with only CAP1 (Brodsky, F. M., 1985, J. Cell Biol., 101:2047-2054; Kirchhausen, T., S. C. Harrison, P. Parham, and F. M. Brodsky, 1983, Proc. Natl. Acad. Sci. USA, 80:2481-2485). Four new anti-CAP mAbs are reported here: two, C-7H12 and C-6C1, react with both forms; two others, C-10B2 and C-4E5, react only with the lower form. Sandwich ELISAs indicated that C-10B2, C-4E5, C-6C1, and C-7H12 react with distinct epitopes. Monoclonal antibodies C-10B2 and C-4E5 immunoprecipitate clathrin-coated vesicles (CCVs) and react with CAP2 epitopes accessible to chymotrypsin on the vesicle. These mAbs inhibit phosphorylation of CAP2 by endogenous CCV casein kinase II. In contrast, C-6C1 and C-7H12 react with epitopes that are relatively insensitive to chymotrypsin. CAP peptide fragments containing these epitopes remain bound to reassembled cages or CCVs after digestion. Immunoprecipitation and ELISAs demonstrate that C-7H12 and C-6C1 react with unbound CAPs but not with CAPs bound to triskelions or CCVs. The data indicate that the CAPs consist of at least two discernible structural domains: a nonconserved, accessible domain that is relevant to the phosphorylation of CAP2 and a conserved, inaccessible domain that mediates the binding of CAPs to CCVs.     

Fluorescence correlation spectroscopy as a sensitive and useful tool for revealing potential overlaps between the epitopes of monoclonal antibodies on viral particles

Although the enzyme-linked immunosorbent assay (ELISA) is well established for quantitating epitopes on inactivated virions used as vaccines, it is less suited for detecting potential overlaps between the epitopes recognized by different antibodies raised against the virions. We used fluorescent correlation spectroscopy (FCS) to detect the potential overlaps between 3 monoclonal antibodies (mAbs 4B7-1H8-2E10, 1E3-3G4, 4H8-3A12-2D3) selected for their ability to specifically recognize poliovirus type 3. Competition of the Alexa488-labeled mAbs with non-labeled mAbs revealed that mAbs 4B7-1H8-2E10 and 4H8-3A12-2D3 compete strongly for their binding sites on the virions, suggesting an important overlap of their epitopes. This was confirmed by the cryo-electron microscopy (cryo EM) structure of the poliovirus type 3 complexed with the corresponding antigen-binding fragments (Fabs) of the mAbs, which revealed that Fabs 4B7-1H8-2E10 and 4H8-3A12-2D3 epitopes share common amino acids. In contrast, a less efficient competition between mAb 1E3-3G4 and mAb 4H8-3A12-2D3 was observed by FCS, and there was no competition between mAbs 1E3-3G4 and 4B7-1H8-2E10. The Fab 1E3-3G4 epitope was found by cryoEM to be close to but distinct from the epitopes of both Fabs 4H8-3A12-2D3 and 4B7-1H8-2E10. Therefore, the FCS data additionally suggest that mAbs 4H8-3A12-2D3 and 4B7-1H8-2E10 bind in a different orientation to their epitopes, so that only the former sterically clashes with the mAb 1E3-3G4 bound to its epitope. Our results demonstrate that FCS can be a highly sensitive and useful tool for assessing the potential overlap of mAbs on viral particles.     

Steroid-transforming enzymes from microorganisms. IX. Affinity chromatographic preparation and studies of the apoenzyme from a 4-en-3-oxosteroid: (acceptor)-1-en-oxidoreductase from Nocardia opaca]

From the flavoenzyme, 4-en-3-oxosteroid: (acceptor)-1-en-oxidoreductase of Nocardia opaca, prosthetic group and apoenzyme were separated quantitatively by means of affinity chromatography in the presence of 2 M (NH4)2 at pH 3.0. Subsequently the apoenzyme was eluted from affinity matrix by 0.01 M phosphate buffer, pH 8.0, whereas under these conditions the intact enzyme could not be eluted. The whole enzyme activity applied could be restored by incubation of the eluted apoenzyme with FAD. The binding strength of the apoenzyme to the immobilized steroid ligand is highly decreased in comparison to the native enzyme and can be interpreted by the action of rest hydrophobicity. That indicates the essential character of FAD for both ligand binding and transformation.     

Monoclonal antibody mapping of structural and functional plectin epitopes

To map structural and functional epitopes of the cytomatrix protein plectin, a set of mAbs was prepared by immunization of mice. Using immunoblot analysis of plectin fragments obtained after limited digestion with various proteases, two groups of mAbs were distinguished. The epitopes of one group (1) were located on a 130-kD terminal segment of the plectin 300-kD polypeptide chain, whereas those of the other group (2) bound within a 40kD segment confined to a central domain of the polypeptide chain. Domains containing the epitopes of group 2 mAbs were shown to include in vitro phosphorylation sites for kinase A, whereas kinase C phosphorylation sites were found on the same terminal segment that contained group 1 mAb epitopes. Rotary shadowing EM of mAb (Fab fragment) -decorated plectin molecules at various states of aggregation, ranging from characteristic dumbbell-shaped single molecules to highly complex multimeric structures, revealed that the epitopes of group 1 as well as those of group 2 mAbs were located on plectin's roughly 200-nm long rod domain interlinking its two globular end domains. Epitopes of group 1 mAbs were localized within a region near the center of the rod, those of group 2 in more peripheral sections near the globular end domains. Solid-phase binding assays carried out in the presence of Fab fragments of mAbs demonstrated an interference of certain group 1 mAbs in the interactions of plectin with vimentin and lamin B. On the other hand, plectin's self-interaction was inhibited mainly by Fab fragments with epitopes in the peripheral rod domain (group 2 mAbs). Together, these results suggested that the molecular binding sites of plectin for vimentin and lamin B, as well as the phosphorylation sites for kinase C, were confined to a defined central section of plectin's rod domain. In addition, they suggest an involvement of peripheral rod sections in plectin self-association.     

Escherichia coli glyoxalate carboligase. Properties and reconstitution with 5-deazaFAD and 1,5-dihydrodeazaFADH2.

Glyoxalate carboligase (EC 4.1.1.47) has been purified to electrophoretic homogeneity from Escherichia coli. The enzyme was found to be a dimer of subunits of identical molecular weight of 68,000. Resolution of the holoenzyme into apoenzyme and FAD led to a dissociation of the dimer into monomers. The apoenzyme could be reconsitituted to full catalytic activity with FAD or the flavin coenzyme analogue 5-deazaFAD. Reconstitution of the apoenzyme with the reduced flavin analogue 1,5-dihydro-5-deazaFADH2 led to the recovery of 50% of enzymatic activity. The reconstitution of apoglyoxalate carboligase with all three coenzymes followed Michaelis-Menten kinetics with Km values of 0.25, 0.74, and 0.72 muM for FAD deazaFAD, and deazaFADH2, respectively.     

Modulation of cooperativity in Mycobacterium tuberculosis NADPH-ferredoxin reductase: cation-and pH-induced alterations in native conformation and destabilization of the NADP+-binding domain

FprA, a Mycobacterium tuberculosis NADPH-ferredoxin reductase, consists of two structural domains, a FAD-binding and a NADP-binding domain, respectively. For the first time, we demonstrated that native FprA, on thermal treatment underwent partial denaturation with unfolding of only the FAD-binding domain and release of the protein-bound flavin. The NADP-binding domain of this protein is highly resistant to denaturation under these conditions. However, the presence of either 150 mM NaCl or KCl or 10 muM MgCl(2) or CaCl(2) or slightly acidic pH of 6.0 resulted in a highly cooperative and complete thermal unfolding of the protein. Physicochemical investigations showed that the monovalent cations or low concentrations of divalent cations induced compaction of the protein conformation. However, divalent cations at higher concentrations resulted in FAD release leading to stabilization of an enzymatically inactive apoenzyme. Detailed thermal denaturation studies on the native protein and the isolated NADP-binding domain showed that cations and pH 6.0 destabilized only the heat-stable NADP-binding domain. The experimental studies demonstrate that modulation of intramolecular ionic interactions induce significant conformational changes in the NADP-binding domain of FprA, resulting in a substantial increase in the structural cooperativity of the whole molecule. The results presented in this paper are of importance as they demonstrate alterations in the native three-dimensional structure of FprA and cooperativity in protein molecule on slight alteration of pH or modification of ionic interactions in protein.     

Interaction of steroids with D-amino acid oxidase.

1. Progesterone inhibited D-amino acid oxidase (D-amino acid : O2 oxidoreductase (deaminating), EC 1.4.3.3) in competition with its substrate, D-alanine. Binding of progesterone brought about the increase in both fluorescence intensity and fluorescence polarization of FAD, which indicates that the environment surrounding FAD chromophore is modified due to a conformational change in the apoenzyme. 2. Ethinyl estradiol, testosterone, testosterone propionate, corticosterone and aldosterone also inhibited the enzyme slightly in the same manner. Their binding also produced a slight increase in FAD fluorescence without decreasing the fluorescence polarization. 3. Cholesterol did not inhibit the enzyme, though it increased the fluorescence polarization of FAD. This indicates the binding of cholesterol with the enzyme at a site other than the substrate binding site.     

Crystalline green 5-hydroxyvaleryl-CoA dehydratase from Clostridium aminovalericum

A green enzyme from Clostridium aminovalericum with valeryl-CoA dehydrogenase activity was purified to homogeneity (169 +/- 3 kDa) and crystallized. By SDS/PAGE, one type of subunit (42 kDa) was detected indicating a homotetrameric structure. The unusual ultraviolet/visible spectrum of the green enzyme (maxima at 394 nm, 438 nm and 715 nm) was converted to a normal flavoprotein spectrum either by reduction with dithionite and reoxidation under air, or by removal of the prosthetic group at pH 2 and reconstitution with FAD (not FMN). Besides FAD (4 mol/169 kDa), the enzyme contained 4 mol of a CoA ester which was similar but not identical to 5-hydroxy-2-pentenoyl-CoA. The reconstituted holoenzyme as well as the native green enzyme, but not the apoenzyme, catalysed the reversible dehydration of 5-hydroxyvaleryl-CoA to 4-pentenoyl-CoA in the absence of an external electron acceptor. In its presence (preferentially ferricenium ion), the green or yellow enzyme catalysed the formation of (E)-5-hydroxy-2-pentenoyl-CoA and 2,4-pentadienoyl-CoA either from 4-pentenoyl-CoA or from 5-hydroxyvaleryl-CoA. The reversible hydration of 2,4-pentadienoyl-CoA to (E)-5-hydroxy-2-pentenoyl-CoA was mediated by both enzymes as well as by the apoenzyme in the absence of FAD. Hydration of 4-pentenoate in 2H2O yielded optically active 5-hydroxy[2,4-2H2]valerate by the combined action of 5-hydroxyvalerate CoA-transferase, the green dehydratase and catalytical amounts of acetyl-CoA. The data show that the reversible hydration of the isolated double bond of 4-pentenoyl-CoA to 5-hydroxyvaleryl-CoA. which apparently violates the Markovnikov rule, is preceded by oxidation to 2,4-pentadienoyl-CoA. The latter compound, a vinyl analogue of 2-enoyl-CoA, is then easily hydrated to (E)-5-hydroxy-2-pentenoyl-CoA and finally reduced to 5-hydroxyvaleryl-CoA.     

Steroid-transforming enzymes from microorganisms. X. Enrichment of a 4-en-3-oxosteroid-5 alpha-reductase from Mycobacterium smegmatis as well as separation and enrichment of the apoenzyme by means of affinity chromatography

The 4-en-3-oxosteroid-5 alpha-reductase from Mycobacterium smegmatis was bound biospecifically on the affinant containing an immobilized testosterone ligand. The enzyme obtained by elution with ethylene glycol and urea in a 32 fold purity has a S. A. of 8.73 X 10(-3) microM androstenedione min-1 mg-1. The coenzyme (FAD) could be separated from the immobilized enzyme substrate complex on the affinity matrix, in the presence of (NH4)2SO4 at pH 3.0. After elution of the apoenzyme 97% of the initial enzyme activity was obtained by incubation with FAD. The reactivated enzyme results in a 40-fold enrichment.     

Mapping of conformational mAb epitopes to the C domain of human angiotensin I-converting enzyme

Angiotensin I-converting enzyme (ACE, CD143) has two homologous domains, each having a functional active site. Fine epitope mapping of 8 mAbs to the C-terminal domain of human ACE was carried out using plate precipitation assays, mAbs' cross-reactivity with ACE from different species, site-directed mutagenesis, and antigen- and cell-based ELISAs. Almost all epitopes contained potential glycosylation sites. Therefore, these mAbs could be used to distinguish different glycoforms of ACE expressed in different tissues or cell lines. mAbs 1B8 and 3F10 were especially sensitive to the composition of the N-glycan attached to Asn 731; mAbs 2H9 and 3F11 detected the glycosylation status of the glycan attached to Asn 685 and perhaps Asn1162; and mAb 1E10 and 4E3 recognized the glycan on Asn 666. The epitope of mAb 1E10 is located at the N-terminal end of the C domain, close to the unique 36 amino acid residues of testicular ACE (tACE). Moreover, it binds preferentially to tACE on the surface of human spermatozoa and thus may find application as an immunocontraceptive drug. mAb 4E3 was the best mAb for quantification of ACE-expressing somatic cells by flow cytometry. In contrast to the other mAbs, binding of mAb 2B11 was not markedly influenced by ACE glycosylation or by the cell culture conditions or cell types, making this mAb a suitable reference antibody. Epitope mapping of these C-domain mAbs, particularly those that compete with N-domain mAbs, enabled us to propose a model of the two-domain somatic ACE that might explain the interdomain cooperativity. Our findings demonstrated that mAbs directed to conformational epitopes on the C-terminal domain of human ACE are very useful for the detection of testicular and somatic ACE, quantification using flow cytometry and ELISA assays, and for the study of different aspects of ACE biology.