A furin-like convertase mediates propeptide cleavage of BACE, the Alzheimer's beta -secretase

The novel transmembrane aspartic protease BACE (for Beta-site APP Cleaving Enzyme) is the beta-secretase that cleaves amyloid precursor protein to initiate beta-amyloid formation. As such, BACE is a prime therapeutic target for the treatment of Alzheimer's disease. BACE, like other aspartic proteases, has a propeptide domain that is removed to form the mature enzyme. BACE propeptide cleavage occurs at the sequence RLPR downward arrowE, a potential furin recognition motif. Here, we explore the role of furin in BACE propeptide domain processing. BACE propeptide cleavage in cells does not appear to be autocatalytic, since an inactive D93A mutant of BACE is still cleaved appropriately. BACE and furin co-localize within the Golgi apparatus, and propeptide cleavage is inhibited by brefeldin A and monensin, drugs that disrupt trafficking through the Golgi. Treatment of cells with the calcium ionophore, leading to inhibition of calcium-dependent proteases including furin, or transfection with the alpha(1)-antitrypsin variant alpha(1)-PDX, a potent furin inhibitor, dramatically reduces cleavage of the BACE propeptide. Moreover, the BACE propeptide is not processed in the furin-deficient LoVo cell line; however, processing is restored upon furin transfection. Finally, in vitro digestion of recombinant soluble BACE with recombinant furin results in complete cleavage only at the established E46 site. Taken together, our results strongly suggest that furin, or a furin-like proprotein convertase, is responsible for cleaving the BACE propeptide domain to form the mature enzyme.

Characterization of Alzheimer's beta -secretase protein BACE. A pepsin family member with unusual properties

The cerebral deposition of amyloid beta-peptide is an early and critical feature of Alzheimer's disease. Amyloid beta-peptide is released from the amyloid precursor protein by the sequential action of two proteases, beta-secretase and gamma-secretase, and these proteases are prime targets for therapeutic intervention. We have recently cloned a novel aspartic protease, BACE, with all the known properties of beta-secretase. Here we demonstrate that BACE is an N-glycosylated integral membrane protein that undergoes constitutive N-terminal processing in the Golgi apparatus. We have used a secreted Fc fusion-form of BACE (BACE-IgG) that contains the entire ectodomain for a detailed analysis of posttranslational modifications. This molecule starts at Glu(46) and contains four N-glycosylation sites (Asn(153), Asn(172), Asn(223), and Asn(354)). The six Cys residues in the ectodomain form three intramolecular disulfide linkages (Cys(216)-Cys(420), Cys(278)-Cys(443), and Cys(330)-Cys(380)). Despite the conservation of the active site residues and the 30-37% amino acid homology with known aspartic proteases, the disulfide motif is fundamentally different from that of other aspartic proteases. This difference may affect the substrate specificity of the enzyme. Taken together, both the presence of a transmembrane domain and the unusual disulfide bond structure lead us to conclude that BACE is an atypical pepsin family member.

A comparison of folding techniques in the chemical synthesis of the epidermal growth factor-like domain in neu differentiation factor alpha/beta

The 52-residue alpha/beta chimera of the epidermal growth factor-like domain in neu differentiation factor (NDFealpha/beta) has been synthesized and folded to form a three disulfide bridge (Cys182-Cys196, Cys190-Cys210, Cys212-Cys221) containing peptide. We investigated two general strategies for the formation of the intramolecular disulfide bridges including, the single-step approach, which used fully deprotected and reduced peptide, and a sequential approach that relied on orthogonal cysteine protection in which specific pairs are excluded from the first oxidation step. Because there are 15 possible disulfide bridge arrangements in a peptide with six cysteines, the one-step approach may not always provide the desired disulfide pairing. Here, we compare the single-step approach with a systematic evaluation of the sequential approach. We employed the acetamidomethyl group to protect each pair of cysteines involved in disulfide bridges, i.e. Cys182 to Cys196, Cys190 to Cys210 and Cys212 to Cys221. This reduced the number of possible disulfide patterns from 15 to three in the first folding step. We compared the efficiencies of folding for each protected pair using RP-HPLC, mapped the disulfide connectivity of the predominant product and then formed the final disulfide from the partially folded intermediate via 12 oxidation. Only the peptide having the Cys182-Cys196 pair blocked with acetamidomethyl forms the desired disulfide isomer (Cys190-Cys210/Cys212-Cys221) as a single homogeneous product. By optimizing both approaches, as well as other steps in the synthesis, we can now rapidly provide large-scale syntheses of NDFealpha/beta and other novel EGF-like peptides.

Disulfide assignment of the C-terminal cysteine knot of agouti-related protein (AGRP) by direct sequencing analysis

We have assigned the disulfide structure of Md-65 agouti-related protein (Md65-AGRP) using differential reduction and alkylation followed by direct sequencing analysis. The mature human AGRP is a single polypeptide chain of 112 amino acid residues, consisting of an N-terminal acidic region and a unique C-terminal cysteine-rich domain. The C-terminal domain, a 48 amino acid peptide named Md65-AGRP, was expressed in Escherichia coil cells and refolded under different conditions from the mature recombinant protein. The disulfide bonds in the cystine knot structure of Md65-AGRP were partially reduced using tris(2-carboxyethyl) phosphine (TCEP) under acidic conditions, followed by alkylation with N-ethylmaleimide (NEM). The procedure generated several isoforms with varying degrees of NEM alkylation. The multiple forms of Md65-AGRP generated by partial reduction and NEM modification were then completely reduced and carboxymethylated to identify unreactive disulfide bonds. Differentially labeled Md65-AGRP were directly sequenced and analyzed by MALDI mass spectrometry. The results confirmed that Md65-AGRP contained the same disulfide structure as that of Md5-AGRP reported previously [Bures, E. J., Hui, J. O., Young, Y. et al. (1998) Biochemistry 37, 12172-12177].

The intermolecular disulfide bridge of human glial cell line-derived neurotrophic factor: its selective reduction and biological activity of the modified protein

Recombinant human glial cell line-derived neurotrophic factor has been implicated to have therapeutic potential in the treatment of neurodegenerative diseases. The mature protein is a single polypeptide of 134 amino acid residues and functions as a disulfide-linked dimer. Reduction of the protein with dithiothreitol at pH 7.0 and in the absence of denaturant showed that the single intermolecular cystine bridge was reduced preferentially. Direct alkylation of the generated free sulfhydryl group using iodoacetamide or iodoacetate without denaturant was incomplete. Unfolding the protein in 6 M guanidine hydrochloride prior to the modification showed rapid disulfide scrambling. However, the sulfhydryl-modifying reagent N-ethylmaleimide was able to label quantitatively the free cysteinyl residue in the absence of any added chaotropic agent. By a combination of peptide mapping, Edman degradation, and mass spectrometric analysis, the labeled residue was identified to be Cys101, hence verifying the location of the intermolecular disulfide bond. The modified protein behaved as a noncovalent dimer when chromatographed through a Superdex 75 column under nondenaturing conditions and was comparable in biological activity to an unmodified control sample. The results therefore indicate that the intermolecular disulfide bridge of the protein is not essential for its biological function.

Human leptin receptor. Determination of disulfide structure and N-glycosylation sites of the extracellular domain

The leptin receptor (OB-R) is a member of the class I cytokine receptor family and mediates the weight regulatory effects of its ligand through interaction with cytoplasmic kinases. The extracellular domain of this receptor is comprised of two immunoglobulin-like and cytokine-receptor homology domains each and type III fibronectin domains. The extracellular domain of human leptin receptor was expressed in and purified from Chinese hamster ovary cells and was found to contain extensive N-glycosylation (approximately 36% of the total protein). The purified protein had a molecular weight of approximately 145,000 and exhibited ligand binding ability as evidenced by formation of ligand-receptor complex, followed by chemical cross-linking. The determined disulfide motif of the soluble leptin receptor contained several distinct cystine knots as well as 10 free cysteines. The N-glycosylation analysis revealed that Asn624 of the WSXWS motif (residues 622-626) within the C-terminal cytokine receptor homology domain was glycosylated, indicating that this region is solvent-exposed. On the other hand, the N-terminal WSXWS motif was not glycosylated.

Identification of Asp95 as the site of succinimide formation in recombinant human glial cell line-derived neurotrophic factor

Human glial cell line-derived neurotrophic factor is a single polypeptide of 134 amino acids and functions as a disulfide-linked dimer. Incubation of the protein in pH 5.0 and at 37 degreesC for 1 week showed that 5% of the material was converted to a form that eluted after the major protein peak on a cation-exchange column. The modified component gave an average molecular mass of 30367.0 u (theoretical = 30384.8 u). Within measurement error, this 17.8-u decrease in mass indicated the loss of a water molecule. This observation, together with the protein's behavior on cation-exchange chromatography and the mode of incubation used to generate the modification, was consistent with cyclic imide (succinimide) formation at an aspartyl residue. Hence, only a monomer of the dimeric protein was modified. The modified monomer was purified and subjected to peptic degradation. By a combination of N-terminal analysis and mass spectrometry, the region containing Asp95-Lys96 was identified to be modified. This was further confirmed by carboxypeptidase Y digestion of the modified peptide where the modified region was found to be resistant to further enzymatic degradation. Furthermore, incubation of the modified monomer in pH 8. 5 for 2 h yielded two peaks, in agreement with the succinimide model where the cyclic imide was hydrolyzed into a mixture of isoaspartate and aspartate. Tryptic mapping of the isoaspartyl-containing protein showed that Asp95 was refractory to Edman degradation, confirming it was in the isoaspartate form. Hence, the modification observed was due to succinimide formation at Asp95. This is the first report of succinimide formation at an Asp-Lys linkage.

Coexpression of G-CSF with an unglycosylated G-CSF receptor mutant results in secretion of a stable complex

Previously, we have shown that the entire extracellular domain of the granulocyte-colony stimulating factor receptor (sG-CSFr) produced in Chinese hamster ovary (CHO) cells forms a stable complex with its ligand G-CSF, at a stoichiometry of 2:2. A truncated receptor molecule consisting of the cytokine receptor homology domain and N-terminus Ig-like domain (Ig CRH) behaves quite similarly. Both of these forms of the receptor are highly glycosylated. To address the importance of glycosylation toward receptor activity and stability, and possibly obtain nonglycosylated receptor for crystallization, mutations were made to replace four Asn residues which are N-glycosylated in the truncated receptor. Virtually no receptor was recovered from conditioned media of CHO cells transfected with this mutant construct, although a high-level of mRNA coding for receptor was detected; this mRNA was translated as determined by Western blots of cell lysates. These results indicate that the translated product is apparently not secreted from these cells. Cells transfected with mutant receptor cDNA were cotransfected with a cDNA construct expressing G-CSF in which the single O-glycosylation site was eliminated by mutation. Upon fermentation of the cotransfectants, we observed a large amount of receptor-ligand complex in the conditioned media. The purified unglycosylated complex appeared to be of the same binding stoichiometry and approximate binding affinity as that of complex formed by addition of purified ligand and unmutated receptor. These results show that while glycosylation of sG-CSFr is not necessary for ligand binding, it appears to be crucial in folding and export from the cell.

Determination of disulfide structure in agouti-related protein (AGRP) by stepwise reduction and alkylation

The agouti-related protein gene (Agrp) plays an important role in body weight regulation. The mature human protein is a single polypeptide chain of 112 amino acid residues, consisting of an N-terminal acidic region and a unique C-terminal cysteine-rich domain. The disulfide structure of recombinant human AGRP was determined by chemical methods using partial reduction with tris(2-carboxyethyl)phosphine under acidic conditions, followed by direct alkylation with N-ethylmaleimide or fluorescein-5-maleimide. Partial reduction and alkylation provided several forms of AGRP that were modified in a stepwise fashion. The resulting proteins were characterized by peptide mapping, sequence analysis, and mass spectrometry, showing that AGRP contained a highly reducible disulfide bond, C85-C109, followed by less reactive ones, C90-C97, C74-C88, C67-C82, and C81-C99, respectively. The chemically defined disulfide connectivity of the recombinant human AGRP was homologous to that of omega-agatoxin IVB except for an additional disulfide bond, C85-C109.

Interactions between brain-derived neurotrophic factor and the TRKB receptor. Identification of two ligand binding domains in soluble TRKB by affinity separation and chemical cross-linking

The extracellular domain of the human neurotrophin TRKB receptor expressed in Chinese hamster ovary cells is a highly glycosylated protein, possessing binding ability for brain-derived neurotrophic factor (BDNF). Two distinct ligand binding domains of TRKB were isolated from proteolytic digests of the receptor by affinity separation on immobilized BDNF. One of these domains consists of amino acid residues 103-181 and contains both the third leucine-rich motif and the second cysteine cluster domain. The second domain is close to the second immunoglobulin-like domain (amino acid residues 342-394). Each of these two domains can bind BDNF independently. Disulfide linkages present in the first domain are necessary for BDNF binding, probably because of preservation of the native conformation. To study the second domain in greater detail, a truncated form of TRKB containing the second immunoglobulin-like domain (residues 248-398) was expressed in Escherichia coli. This domain was cross-linked to BDNF through a 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide coupling reaction. Several synthetic peptides corresponding to amino acid residues 343-379 were able to bind immobilized BDNF. Amino acid substitution and cross-linking analysis indicated that amino acids Phe347, Asp354, and Tyr361 are intimately involved in BDNF binding. These results, obtained from a variety of experimental techniques, highlight the importance of two distinct regions of the extracellular domain of the TRKB receptor in binding BDNF.

Glial cell line-derived neurotrophic factor: selective reduction of the intermolecular disulfide linkage and characterization of its disulfide structure

Glial cell line-derived neurotrophic factor is a protein known to enhance the survival of dopaminergic neurons against several neurotoxins. It has been shown to have therapeutic potential in the treatment of Parkinson's disease and other neurodegenerative diseases. We have determined the inter- and intramolecular disulfide linkages of the dimeric molecule by a combination of direct peptide analysis and peptide analysis after either partial reduction or partial oxidation of the protein. Under an acidic condition, the interchain disulfide bond was selectively cleaved with tris(2-carboxyethyl)phosphine, revealing that Cys101 was involved in the intermolecular disulfide linkage. Three other disulfides, Cys68-Cys131, Cys72-Cys133, and Cys41-Cys102, were identified as intramolecular linkages. The determined disulfide structure is highly homologous to that of transforming growth factor beta 2. Since one intramolecular disulfide points through a ring consisting of eight amino acid residues based on the similarity with transforming growth factor beta 2, the disulfide-linked peptides were not purified by conventional methods. Only the peptides from an N-terminal region (residues -1 to 37) were liberated by proteolytic treatment with trypsin or endoproteinase Lys-C, resulting in a stable cystine-knot protein.

Disulfide structure and N-glycosylation sites of an extracellular domain of granulocyte-colony stimulating factor receptor

An extracellular domain containing 603 amino acid residues of human granulocyte-colony stimulating factor receptor was expressed in Chinese hamster ovary cells. The affinity-purified material has previously been shown to dimerize when combined with the ligand. In this paper we have characterized the primary structure of this active receptor. Laser desorption mass spectrometry of the purified receptor showed a broad peak at a molecular weight of 84,000, ranging from 77,000 to 91,000. The molecular weight heterogeneity is due to glycosylation. Since the molecular weight based on the amino acid sequence is 67,322, by subtraction the carbohydrate content is approximately 17,000. Disulfide structure of the receptor was determined by peptide mapping in the absence and presence of reducing agent. Sequence and mass spectral analyses of these peptides showed the receptor to contain eight disulfide bonds and three free cysteines. These disulfide bonds are consistent with the known domain motifs of the receptor in that no interdomain disulfides were present. One of the three free cysteines is reactive with alkylating agents, while the others are less reactive, probably being buried in the interior of the molecule. Blocking the free cysteines did not affect the ligand binding. Carbohydrate moieties are somewhat evenly spaced throughout the molecule, at eight different N-glycosylation sites, some of which show heterogeneity in their compositions. Glycosylation seems necessary for stabilizing the molecule against disulfide-linked oligomerization of the receptor, indicating that the free cysteine residues become reactive for oxidation and disulfide exchange upon deglycosylation.

Human neurotrophin-3: a one-step peptide mapping method and complete disulfide characterization of the recombinant protein

Human neurotrophin-3 (NT-3) is a member of the nerve growth factor (NGF) family of neurotrophic factors, and the recombinant protein is being developed as a therapeutic for neurodegenerative diseases. The final product purity and lot-to-lot variation are monitored routinely by peptide mapping. However, only the N-terminal region of NT-3 was susceptible to proteolysis under native conditions. Complete digestion required that the protein be chemically modified by reduction and S-alkylation prior to proteolysis. Complete proteolytic degradation of the protein was achieved simply by an initial denaturation of NT-3 in 6 M guanidinium chloride (pH6) for 2 hr at 37 degrees C, followed by a tenfold dilution with the digestion buffer (0.1 M Tris-HCl, 1 mM CaCl2 at pH 7.0) and immediate addition of chymotrypsin at 1% by weight. Direct comparison of the peptide map with an identical aliquot that had been reduced and alkylated also allowed the establishment of the cystine linkages present in NT-3: Cys14 to Cys79, Cys57 to Cys108, and Cys67 to Cys110. This disulfide structure is homologous to the NGF family of neurotrophic factors.

Extracellular domain of granulocyte-colony stimulating factor receptor. Interaction with its ligand and identification of a domain in close proximity of ligand-binding region

An extracellular domain of human granulocyte-colony stimulating factor (G-CSF) receptor was expressed in and purified from Chinese hamster ovary cells. Complex formation between G-CSF and the receptor was studied by size exclusion chromatography, followed by chemical cross-linking. The receptor-ligand complex contained an equimolar ratio of each protein. Crosslinking experiments using disucciniimide suberate revealed that the native complex contained at least two types of cross-linked complexes; one form contained one or two G-CSF molecules per receptor molecule, whereas another form contained one or two G-CSF per two receptor molecules. The tryptic peptide map of the cross-linked complex provided a unique peptide peak which was not found in a peptide map of the original protein. Sequence analysis and mass spectrometry of the peptide indicated that two peptides were covalently linked by cross-linker, one peptide from G-CSF and the other from the receptor. In the cross-linked peptide, Lys-242 of the receptor cross-linked the amino terminal Met of G-CSF through the cross-linker. It was also shown that the N-terminal Met of G-CSF was readily acetylated in the receptor-ligand complex, indicating that it was not directly involved in receptor binding. The results show that the N-terminal Met of G-CSF is located at a distance of approximately 11 A from a reactive Lys-242 of the receptor in the ligand-receptor complex.

Extracellular domain of neurotrophin receptor trkB: disulfide structure, N-glycosylation sites, and ligand binding

An extracellular domain of a human neurotrophin receptor trkB was expressed in Chinese hamster ovary cells and isolated as a glycoprotein possessing binding activity for brain-derived neurotrophic factor. The extracellular domain contains 398 amino acids and has a molecular weight of 60.6 kDa according to laser desorption mass spectrometry, indicating that the extracellular domain of trkB contains 33.3% carbohydrate moieties. Six disulfide linkages were determined to be Cys1-Cys7, Cys5-Cys14, Cys121-Cys145, Cys123-Cys163, Cys187-Cys235, and Cys271-Cys314, respectively. Cys300 was detected as a free sulfhydryl residue. Cysteine clusters 1 and 2 located in the N-terminal domain possess a similar type of disulfide structure and two other disulfide bonds in the C-terminal region are homologous to that of the Ig-like C2 domain. Among 12 potential N-linked glycosylation sites proposed in the soluble domain of trkB, 10 sites are actually glycosylated.

Purification and identification of brain-derived neurotrophic factor from human serum

Brain-derived neurotrophic factor (BDNF), a 27-kDa noncovalently linked homodimer with subunits of approximately 13.5 kDa as viewed by SDS-PAGE, is thought to be primarily produced in the central nervous system. We report here the isolation of BDNF from pooled normal human sera, using a two-step purification process followed by SDS-PAGE, transfer to a polyvinylidene difluoride membrane, and subsequent identification of the protein by sequence analysis of the appropriate band(s) from the membrane. The level of BDNF in pooled human sera was estimated to be approximately 15 ng/ml as determined by an enzyme-linked immunosorbant assay. The average for six individuals was 18.9 +/- 5.7 ng/ml. There is an approximately 200-fold increase in the levels of BDNF in serum relative to plasma. Results from experiments using differential centrifugation suggest that the source of this increase is due to release from platelets. The presence of high levels of BDNF in serum suggests a role for this neurotrophin either in nerve repair at sites of injured tissue or in nonneuronal functions.

Analysis of translational termination of recombinant human methionyl-neurotrophin 3 in Escherichia coli

A highly efficient UGA stop codon readthrough event during the synthesis of human neurotrophin 3 in E. coli is described. The incorporation of a Trp residue at the UGA stop codon is confirmed combining both the chemical analyses and the molecular and genetic data in this report. The 3' adjacent nuleotide to the UGA stop codon plays a crucial role in determining the readthrough efficiency in the order of A > G > C > U. The replacement of UGA with UAA or UAG totally abolished this readthrough phenomenon and the use of StpR host cells also prevented the occurrence of UGA readthrough. Gene dosage (or plasmid copy number) effect was not indicated in this event; however, the titration of RF-2 by mRNA transcripts under over-expression conditions might explain why tRNAtrp competes so well with RF-2 for UGA. Another apparently less produced readthrough product resulting from a transcript with no stop codon is also recorded, and the addition of a second in-frame stop codon increased the amount of the observed readthrough product.

Formation of heterodimers from three neurotrophins, nerve growth factor, neurotrophin-3, and brain-derived neurotrophic factor

Three neurotrophic factors, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and nerve growth factor (NGF) form noncovalent homodimers in solution. Since they are highly homologous proteins, it seemed probable that two monomers of these proteins might associate together to form a heterodimer. This was tested by denaturing the two different proteins together in 6 M guanidine HCl and refolding them in phosphate-buffered saline. When the refolded mixture of BDNF and NT-3 was subjected to Mono S cation exchange chromatography, a new peak was observed eluting between NT-3 and BDNF, which accounted for about 30% of the protein used. This new protein species migrated as a single band upon native gel electrophoresis with mobility between that of the NT-3 homodimer and the BDNF homodimer, indicating that a complex had been formed. Sedimentation equilibrium data show that the dissociation constant of this heterodimer is < 3 x 10(-10) M. The heterodimer was stable upon incubation at 37 degrees C in phosphate-buffered saline over 11 days. Having determined that the heterodimer is highly stable, it was subjected to various biological assays. Autophosphorylation assay using TrkB receptor showed that the heterodimer is indistinguishable from the BDNF or NT-3 homodimer in the ability to induce phosphorylation of the receptor. It was also indistinguishable from the homodimers in the neurotrophic activity using chick dorsal root ganglion explant. In the sympathetic neuron survival assay, the heterodimer behaved more similarly to NT-3, whereas in the dopamine uptake assay, it was intermediate between the two homodimers. In addition, the heterodimer was shown to be retrogradely transported in the dorsal root ganglion neurons. A heterodimer between NGF and BDNF is formed but much less effectively than the NT-3.BDNF heterodimer, and it is not stable even at 4 degrees C. These results indicate that BDNF and NT-3 have an intersubunit contact surface for dimerization resembling each other's but different from the contact surface of NGF.

Characterization of disulfide linkages in platelet-derived growth factor AA

Intermolecular and intramolecular disulfide linkages of recombinant human platelet-derived growth factor A chain dimer were determined by chemical methods including selective reduction-alkylation, peptide isolation, or detection of diphenylthiohydantoin derivative of cystine from Edman reactions. Cys-37 and Cys-46 were selectively reduced with reducing agents under native conditions and revealed to be involved in intermolecular bridges. Other disulfide linkages including Cys-10-Cys-54, Cys-43-Cys-91, and Cys-47-Cys-93 form intramolecular bridges. The disulfide structure is homologous to that of platelet-derived growth factor B chain dimer.

Formation of mitogenically active PDGF-B dimer does not require interchain disulfide bonds

Platelet-derived growth factor (PDGF), a major mitogen for mesenchymal cells, is a disulfide-bonded dimer of two subunit polypeptides named A and B. All of the three possible dimeric forms, i.e. AA, BB, and AB, exist in nature. The dimeric structure has been presumed to be necessary for biological activity, since reduction of the dimer results in loss of activity and simultaneous conversion to monomeric form as determined by SDS-gel electrophoresis. However, reduction of the native molecule destroys intrachain, as well as interchain, disulfide bonds, and it is possible that the former rather than the latter are critical for proper conformation of the active protein. We show here that PDGF-B polypeptides in which all 8 cysteines or the 2nd, 4th, 5th, and 8th cysteines have been mutated to serines fail to form covalent dimers and possess dramatically less mitogenic activity than native PDGF-BB. Another mutant, PDGF-B(C2,4S), in which just the 2 cysteines involved in interchain disulfides were converted to serine, ran as a monomer on SDS-polyacrylamide gels as expected. Somewhat unexpectedly, however, the mitogenic activity of the PDGF-B(C2,4S) analog was similar to the activity of wild-type PDGF-BB disulfide-bonded dimer under physiological conditions. The activity of the analog was more sensitive to the effect of low pH than was the activity of wild-type PDGF-BB. Molecular weight analysis utilizing light scattering and sedimentation equilibrium demonstrated that the PDGF-B(C2,4S) analog exists as a noncovalent dimer at pH 4-7 but dissociates to a monomer at pH 2.5. Disulfide analysis of the mutant protein demonstrated that the intrachain disulfide bonds are the same as those formed in wild-type PDGF-BB homodimers. We conclude that proper formation of intrachain disulfide bonds is critical to maintaining the correct conformation of PDGF monomers, but that appropriately folded monomers can associate into active noncovalent dimers in the absence of interchain disulfide bonds. Interchain disulfide bonds thus appear to increase the stability of the PDGF dimer rather than being crucial to its existence.

Direct assignment of disulfide bonds by Edman degradation of selected peptide fragments

Disulfide linkages in peptides or proteins were analyzed by automated gas-phase Edman sequencing performed in minimized reducing agents. If cystine linkage was regulated at the same position in two peptides during peptide preparation, the diphenylthiohydantoin derivative of cystine was significantly recovered by Edman reaction. In contrast, when the crosslinked half cystines were present at different positions in the peptides, the derivative could be poorly detected. Upon direct sequence analysis of intact bovine insulin, the PTH derivatives of cystine from both Cys-A7 and Cys-B7 were significantly released after Edman cycle 7 and gave approximately 20% recovery of common PTH amino acids. However, Cys-A11 linked to Cys-A6 was poorly detectable after Edman cycle 11. For general use of this method, proteins need to be subjected to several sets of proteolytic or chemical cleavages in the hope that at least one of the fragments will have cystine linkage at the same position. This method was applied to several fragments of platelet-derived growth factor B chain and brain-derived neurotrophic factor.

Sites of iodination in recombinant human brain-derived neurotrophic factor and its effect on neurotrophic activity

Recombinant human brain-derived neurotrophic factor (BDNF) is now under extensive investigation because of its potential clinical applications. Radioactively labeled proteins are usually required to study receptor binding and pharmacokinetic properties of proteins. This study was undertaken to see if iodination affects the biological and conformational properties of a recombinant BDNF. BDNF was iodinated using a stoichiometric amount of nonradioactive cold NaI to minimize multiple iodinations. Of the four tyrosines present in BDNF--Tyr-52, Tyr-54, Tyr-63, and Tyr-86--only Tyr-63 and Tyr-86 were iodinated under the experimental conditions used. Iodination of Tyr-63 resulted in modification without alteration of the biological activity, whereas iodination of Tyr-86 resulted in a molecule with highly compromised biological activity. Similar inactivation was observed if both Tyr-63 and Tyr-86 were iodinated. These modified proteins exhibited conformation and dimerization apparently identical to those of the native protein, as demonstrated by analytical ultracentrifugation, gel filtration, light scattering, and circular dichroism. From these results, we concluded that Tyr-52 and Tyr-54 are not accessible to the reagent and are probably buried in the hydrophobic core, whereas Tyr-63 and Tyr-86 are exposed on the surface of the molecule; of the two exposed residues, only Tyr-86 contributes to the biological activity.

Recombinant human erythropoietin (rHuEPO): cross-linking with disuccinimidyl esters and identification of the interfacing domains in EPO

Several amino groups of recombinant human erythropoietin are selectively cross-linked by specific cross-linkers including disuccinimidyl suberate or dithiobis(succinimidyl propionate). Intramolecular cross-linkings are obtained without significant change of the protein conformation using appropriate concentrations (0.2 mM) of the cross-linkers, which possess an 11-12-A length of a spacer between two reacting groups. Intramolecularly cross-linked peptides obtained suggest that several amino groups in erythropoietin (EPO) are positioned at a distance of near 12 A in the solution state. These interfacing amino groups include Lys 20-Lys 154, Lys 45-Lys 140, Lys 52-Lys 154, Lys 52-Lys 140, and Ala 1-Lys 116. A comparison of the cross-linking results between nonglycosylated EPO and glycosylated EPO suggests that both proteins retain high similarity regarding protein conformation. These results fit a structural model similar to that of human growth hormone, in which four alpha-helical bundles and a long stretch of beta-sheet structure are involved in the active protein.

Recombinant human brain-derived neurotrophic factor (rHuBDNF). Disulfide structure and characterization of BDNF expressed in CHO cells

Three disulfide linkages of recombinant human brain-derived neurotrophic factor (BDNF) were determined by peptide sequence analysis and characterized by mass spectrometry. The three disulfide bonds for BDNF expressed in Chinese hamster ovary cells include Cys-13-Cys-80, Cys-58-Cys-109 and Cys-68-Cys-111, and the disulfide structure was homologous to that of nerve growth factor.

Disulfide bonds in recombinant human platelet-derived growth factor BB dimer: characterization of intermolecular and intramolecular disulfide linkages

Interchain cystines of PDGF-BB dimer were characterized by Edman reaction and by SDS-PAGE analysis on the protein which was chemically cleaved at Trp-40. It was found that Cys-43 has a key role in dimer formation, asymmetrically cross-linked to a cysteine residue of another identical subunit. The remaining cystines participate in the intramolecular disulfide linkages. Pepsin digestion of PDGF-BB dimer generated several small peptides and one ubiquitous Cys-containing peptide. Sequence analyses of several Cys-containing peptides indicated the existence of three intramolecular disulfide linkages including Cys-16--Cys-60, Cys-49--Cys-97, and Cys-53--Cys-99. Two interchain disulfide bonds of Cys-43--Cys-52 between two subunits were deduced from the partial reduction and alkylation of PDGF-BB. This study provides chemically determined disulfide linkages of PDGF-BB.

Detection of intratumoral aromatase in breast carcinomas. An immunohistochemical study with clinicopathologic correlation

The expression of aromatase was evaluated in 38 breast carcinomas by an immunohistochemical method (ABC) using an specific polyclonal antibody against human placental aromatase. Fifteen tumors (40%) showed significant immunoreactivity, as defined by cytoplasmic positivity of moderate intensity present in at least 15% of the cells. The results were correlated with the estrogen and progesterone hormone receptor status and several clinicopathologic parameters such as age, tumor size, lymph node status, and stage of the disease. There was a significant, but inverse, correlation between the aromatase activity and the estrogen receptor status (P = 0.04), indicating the likelihood of negative estrogen if substantial aromatase activity was present. No statistically significant correlation was found between the presence of intratumoral aromatase and the rest of the parameters studied (P greater than 0.7). Nor was there a correlation between the aromatase content of the tumors and the menopausal status. The degree of intratumoral heterogeneity of the aromatase content was minimal in six cases where multiple samples from each tumor were analyzed. This is the first study reporting the detection of aromatase in archival material from breast carcinomas using immunohistochemical techniques. The lack of biologic significance of its presence in breast cancer reported here and by others using biochemical assays should be validated in larger series with longer follow-up. The method described can be readily used for that objective.

A novel species of cytochrome P-450 (P-450ib) specific for the small intestine of rabbits. cDNA cloning and its expression in COS cells

We have isolated a cDNA clone for a P-450, designated P-450ib (Ichihara, K., Kusunose, E., Kaku, M., Yamamoto, S., and Kusunose, M. (1985) Biochim. Biophys. Acta 831, 99-105), from a cDNA library of rabbit small intestine mucosa by using synthetic DNA fragment by the polymerase chain reaction, as a hybridization probe. The cDNA with a 1,829-base pair insert encodes a polypeptide of 501 amino acids. The deduced amino acid sequence contains all of the sequences of the NH2-terminal and 14 tryptic fragments from purified P-450ib. As the NH2-terminal methionine was not found in the sequence from the purified protein, the apoprotein of P-450ib is composed of 500 amino acids with a molecular weight of 57,193. P-450ib shows 35-41% sequence similarity with several members of 8 subfamilies in the P-450 II family, whereas it has a less than 30% sequence similarity with other P-450 families, suggesting that this P-450 is the first member of a novel subfamily within the P-450 II family. RNA blot analysis shows that mRNA hybridized to the cDNA is expressed in the small intestine, but not significantly in other tissues including liver, colon, kidney, lung, spleen, brain, stomach, and cecum, indicating that P-450ib is a P-450 specific to the small intestine. The protein expressed in COS-7 cells using the cDNA in an expression vector, pKCRH2, shows benzphetamine N-demethylase activity and gives a band identical with that of P-450ib in its mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Structural and functional analysis of NADPH-cytochrome P-450 reductase from human liver: complete sequence of human enzyme and NADPH-binding sites

The complete amino acid sequence of human liver NADPH-cytochrome P-450 reductase has been determined by microsequence analysis and mass spectrometry. The total sequence consists of 676 amino acids initiated by an amino-terminal acetyl group. There is no evidence for posttranslational modifications, including Asn-linked glycosylation. The human enzyme exhibits sequence homology in the range of 92-95% with other mammalian enzymes. Sequence differences were mainly confined to several hydrophilic regions in the NH2-terminal and COOH-terminal domains. Since the human enzyme is immunochemically distinct from the rabbit enzyme despite similar enzymatic properties, it is likely that these variable hydrophilic regions are potential antigenic determinants. The NADPH-depleted enzyme is inactivated by either fluorescein isothiocyanate, a lysine-specific reagent, or 5-(iodoacetamido)fluorescein, a cysteine-specific reagent. In both cases, protection by NADP(H) prevents enzyme inactivation by the reagents. Isolation of fluorescent peptide from 5-(iodoacetamido)fluorescein-inactivated enzyme identified Cys 565 as the specifically NADPH-protected residue.

Reaction of rat liver DT-diaphorase (NAD(P)H:quinone acceptor reductase) with 5'-[p-(fluorosulfonyl)benzoyl]-adenosine

Rat liver DT-diaphorase is inactivated by 5'-[p-(fluorosulfonyl)benzoyl]adenosine (5'-FSBA), following pseudo-first-order kinetics. A double-reciprocal plot of 1/kobs versus 1/[5'-FSBA] yields a straight line with a positive y intercept, indicative of reversible binding of the inhibitor before an irreversible incorporation. The dissociation constant (Kd) for the initial reversible enzyme-inhibitor complex is estimated at 2.86 mM with k2 = 0.22 min-1 (at pH 7.5 and 25 degrees). A stoichiometry of 2 mol of 5'-FSBA bound/mol of enzyme (i.e., 1 mol of the inhibitor bound/mol of subunit), at 100% inactivation, was determined from inactivation kinetics and from incorporation studies using 5'-[p-(fluorosulfonyl)benzoyl]-[14C]-adenosine. The irreversible inactivation as well as the covalent incorporation could be completely prevented by the presence of NAD(P)H during the incubation. These results indicate that 5'-FSBA inactivated DT-diaphorase by occupying its NAD(P)H binding site. Reverse phase high pressure liquid chromatography of tryptic digests of [14C]5'-FSBA-labeled DT-diaphorase revealed one radioactive peak containing two comigrating peptides. They are 146I-T-T-G-G-S-G-S-M-Y155 and 262S-I-P-A-D-N-Q-I-K270. By comparison of these sequences to those of the nucleotide binding sites of several kinases and dehydrogenases, it is suggested that the peptide I-T-T-G-G-S-G-S-M-Y is the one modified by 5'-FSBA and would be predicted to be the region where the pyrophosphate group of NAD(P)H binds.

Structure-function relationship of NAD(P)H:quinone reductase: characterization of NH2-terminal blocking group and essential tyrosine and lysine residues

The amino terminal blocked peptide of rat liver NAD(P)H:quinone reductase (DT-diaphorase) was determined by amino acid sequence analysis and by mass spectrometry. The mature protein is composed of 273 amino acids and contains an acetylated amino terminus, which was not identified by previous cDNA analysis. The enzyme was inactivated by p-nitrobenzenesulfonyl fluoride (NBSF) or 2,4,6-trinitrobenzenesulfonate (TNBS) with pseudo-first-order kinetics. These studies suggest that essential tyrosine and lysine may be present in the active site of this enzyme. The NBSF inhibition was protected by 1-naphthol and 1-naphthylamine, but not by NAD+. However, TNBS inhibition was not prevented by the naphthalene derivatives or NAD+. Specific peptides labeled with NBSF or TNBS were isolated by high-performance liquid chromatography and were sequenced. These analyses revealed that the NBSF-labeled tyrosine resides in a predominantly hydrophobic region and TNBS-labeled lysine in a predominantly hydrophilic region.

Microsequence analysis of peptides and proteins. IX. Manual gas-phase microsequencing of multiple samples

A novel apparatus for performing manual gas-phase Edman chemistry on protein and peptide samples is described. Edman chemistry is performed in 6 to 10 Teflon continuous flow reactors (CFR), previously described by J.E. Shively et al. (1987) Anal. Biochem. 163, 517-529). The CFRs are packed with 10-15 mg of Polybrene-coated spherical silica (Porasil B, Waters Associates). The gas-phase coupling reagent and cleavage reagent are 5% aqueous triethylamine and anhydrous trifluoroacetic acid, respectively, delivered by a stream of argon gas. The delivery of the gas-phase reagents is manually controlled with Hamilton 3-way valves and 2-way valves, and that of the solvents, ethyl acetate and butyl chloride, by syringe pipetting. The average cycle time is 15-20 min for 6 to 10 samples run simultaneously. Conversion of the anilinothiazolinone to phenylthiohydantoin (PTH) amino acid derivatives is accomplished manually with 25% aqueous trifluoroacetic acid. The PTH amino acids are analyzed by reversed-phase HPLC using an autosampler for handling multiple samples. Excellent results were obtained in the 100-200 pmol range. Protein samples can be sequenced from 15-20 cycles, and peptide samples usually to the COOH terminus. Initial yields ranged from 30 to 60% and repetitive yields ranged from 90 to 96%. The sample washout and size of background peaks are significantly reduced, compared to older methods of manual sequence analysis. The yields and background signal to noise are comparable to automated gas-phase Edman chemistry. The improved manual Edman described represents a low cost alternative to automated sequence analysis, and has the advantage being able to process multiple samples simultaneously.

Isolation of a cDNA for adrenodoxin reductase (ferredoxin-NADP+ reductase). Implications for mitochondrial cytochrome P-450 systems

Using specific antibodies against adrenodoxin reductase (AR), we screened lambda gt11 cDNA expression libraries constructed from bovine adrenal cortex mRNA, and isolated several putative clones coding for this enzyme. Concurrently we determined the amino acid sequences of fragments from it. A deoxyinosine-containing oligonucleotide probe, generated for one of the sequences, reacted specifically with one of the cloned cDNAs of about 1600 base pairs. The codon sequence of this cDNA matched the peptide sequences, further confirming its identity as a copy of AR mRNA. RNA blot analysis indicates that in the adrenal cortex and corpus luteum there is only one major mRNA (approximately 2000 bp) for AR. The levels of this mRNA are at least 40-fold lower in the liver and kidney which are also known to contain in homologue of AR. As compared to adrenodoxin and cytochrome P-450scc mRNAs, AR mRNA levels in the adrenal cortex appear to be about 10-fold lower. Southern blot analysis of bovine and human genomic DNAs reveals that in both of these species there is only one gene for AR. These results indicate that only a single reductase serves the different mitochondrial P-450 systems in steroidogenic tissues.

cDNA cloning and inducible expression during pregnancy of the mRNA for rabbit pulmonary prostaglandin omega-hydroxylase (cytochrome P-450p-2)

We have isolated cDNA clones of the mRNA for prostaglandin omega-hydroxylase (cytochrome P-450p-2) (Yamamoto, S., Kusunose, E., Ogita, K., Kaku, M., Ichihara, K., and Kusunose, M. (1984) J. Biochem. (Tokyo) 96, 593-603) in rabbit lung by using synthetic oligonucleotides as probes. The cDNA sequence contains an open reading frame of 1,470 nucleotides, the first 9 amino acids of which correspond to the residues 17-25 of cytochrome P-450p-2 determined from protein analysis. The predicted primary structure contains amino acid sequences of 23 tryptic fragments of cytochrome P-450p-2 and the deduced amino acid composition is in agreement with that determined from the purified protein. The complete polypeptide, including residues 1-16, contains 506 amino acids with a calculated molecular weight of 58,515. Cytochrome P-450p-2 shared 74% amino acid similarity with rat hepatic lauric acid omega-hydroxylase (cytochrome P-450LA omega) (Hardwick, J.P., Song, B.-J., Huberman, E., and Gonzalez, F. J. (1987) J. Biol. Chem. 262, 801-810), whereas it showed less than 25% similarity to other forms of cytochrome P-450, indicating that the two cytochrome P-450s constitute a unique cytochrome P-450 gene family. DNA blot analysis of the total genomic DNA of rabbits suggest the presence of several genes or gene-like DNA sequences which cross-hybridized with the cloned cDNA. RNA blot analysis showed that progesterone treatment increased the amount of mRNA hybridizable to the cDNA by about 100-fold in the lung of rabbits as compared with the basal level without the treatment. This high level of the mRNA was also observed in the lung of pregnant rabbits.

Complete amino acid sequence of 21-hydroxylase cytochrome P-450 from porcine adrenal microsomes

Adrenal microsomal 21-hydroxylase is essential for biosynthesis or metabolism of various steroid hormones. The complete amino acid sequence of this cytochrome P-450 from pig adrenal was determined by sequence analysis on several sets of proteolytic fragments. The mature protein consists of 492 amino acid residues, corresponding to a molecular weight of 55,484. Seven out of nine total cysteine residues are localized in the amino terminal half of the molecule. The carboxyl terminal half contains only two cysteines, one of which is located at the highly conserved heme-binding region proposed in all cytochromes P-450. A structural comparison between 21-hydroxylase and 17 alpha-hydroxylase reveals that there is a preponderance of sequence homology at the carboxyl terminal region. These studies indicate that a single gene product is expressed for steroid 21-hydroxylase in porcine adrenal glands.

Affinity alkylation of the active site of C21 steroid side-chain cleavage cytochrome P-450 from neonatal porcine testis: a unique cysteine residue alkylated by 17-(bromoacetoxy)progesterone

The affinity alkylating progesterone analogue 17-(bromoacetoxy)progesterone has been used to label the active site of a microsomal cytochrome P-450 enzyme from neonatal pig testis. The enzyme causes removal of the C20 and C21 side chains from the substrates progesterone and pregnenolone by catalyzing both 17-hydroxylase and C17,20-lyase reactions, which produce the corresponding C19 steroidal precursors of testosterone. The progesterone analogue causes simultaneous inactivation of the two catalytic activities of the enzyme by a first-order kinetic process that obeys saturation kinetics. Progesterone and 17-hydroxyprogesterone each protect the enzyme against inactivation. The progesterone and analogue is a competitive inhibitor of the enzyme with Ki values of 8.4 microM and 7.8 microM for progesterone and 17-hydroxyprogesterone, respectively. The enzyme inactivation and kinetic data are consistent with a theory proposing that the analogue and the two substrates compete for the same active site. The radioactive analogue 17-[( 14C]bromoacetoxy)progesterone causes inactivation of the enzyme with incorporation of 1.5-2.2 mol of the analogue per mole of inactivated enzyme. When this experiment is carried out in the presence of a substrate, then 0.9-1.2 mol of radioactive analogue is incorporated per mole of inactivated enzyme. The data suggest that the analogue can bind to two different sites, one of which is related to the catalytic site. Radiolabeled enzyme samples, from reactions of the 14C-labeled analogue with the enzyme alone or with enzyme in the presence of a substrate, were subjected to amino acid analysis and also to tryptic digestion and peptide mapping.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytochrome P450c17 (steroid 17 alpha-hydroxylase/17,20 lyase): cloning of human adrenal and testis cDNAs indicates the same gene is expressed in both tissues

P450c17 is the single enzyme mediating both 17 alpha-hydroxylase (steroid 17 alpha-monooxygenase, EC 1.14.99.9) and 17,20 lyase activities in the synthesis of steroid hormones. It has been suggested that different P450c17 isozymes mediate these activities in the adrenal gland and testis. We sequenced 423 of the 509 amino acids (83%) of the porcine adrenal enzyme; based on this partial sequence, a 128-fold degenerate 17-mer was synthesized and used to screen a porcine adrenal cDNA library. This yielded a 380-base cloned cDNA, which in turn was used to isolate several human adrenal cDNAs. The longest of these, lambda hac17-2, is 1754 base pairs long and includes the full-length coding region, the complete 3'-untranslated region, and 41 bases of the 5'-untranslated region. This cDNA encodes a protein of 508 amino acids having a predicted molecular weight of 57,379.82. High-stringency screening of a human testicular cDNA library yielded a partial clone containing 1303 identical bases. RNA gel blots and nuclease S1-protection experiments confirm that the adrenal and testicular P450c17 mRNAs are indistinguishable. These data indicate that the testis possesses a P450c17 identical to that in the adrenal. The human amino acid sequence is 66.7% homologous to the corresponding regions of the porcine sequence, and the human cDNA and amino acid sequences are 80.1 and 70.3% homologous, respectively, to bovine adrenal P450c17 cDNA. Both comparisons indicate that a central region comprising amino acid residues 160-268 is hypervariable among these species of P450c17. Comparison of the amino acid sequence of P450c17 with two other human steroidogenic cytochromes P450 show much greater homology with P450c21 (28.9%), another microsomal enzyme, than with P450scc (12.3%), a mitochondrial enzyme.

Complete amino acid sequence of NADPH-cytochrome P-450 reductase from porcine hepatic microsomes

The complete amino acid sequence of porcine hepatic microsomal NADPH-cytochrome P-450 reductase has been determined by microsequence analysis on several sets of proteolytic fragments. Sequence studies were performed initially on a 20-kilodalton (kDa) fragment and then on 80-kDa fragment. The amino-terminal end of the mature protein was blocked with an acetyl group, followed by 676 amino acid residues. It has been revealed that the COOH-terminal 20-kDa fragment has been derived from original enzyme by cleavage at the Asn-Gly (residues 502-503) linkage by an unknown mechanism. An NADPH-protected cysteine residue is located at residue 565, near a region exhibiting high sequence homology with ferredoxin-NADP+ reductase. The FMN and FAD binding regions are possibly located in the amino-terminal region and the middle part of the protein molecule, respectively, as suggested by Porter and Kasper [Porter, T. D., & Kasper, C. B. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 973-977]. When this sequence is compared with that of rat enzyme, 60 amino acid residues are substituted, probably due to species differences. However, total sequence homology between these enzymes is 90%. Hydropathy plot analysis reveals that two regions from residues 27-43 and from residues 523-544 exhibit a high degree of hydrophobicity, suggesting membrane binding or interaction with cytochrome P-450.

Cloning and characterization of cDNA encoding 3-methylcholanthrene inducible rat mRNA for UDP-glucuronosyltransferase

We have isolated cDNA clones of the mRNA for rat UDP-glucuronosyltransferase that catalyzes the glucuronidation of 4-nitrophenol, by using synthetic oligonucleotides as hybridization probes. The complete nucleotide sequence of the 1,927-base pairs cDNA insert has been determined. With untranslated sequences of 124 and 216 base pairs in the 5'- and 3'-terminal regions, respectively, the cDNA insert contained 1,587 base pairs that encode a complete primary structure of a putative precursor form of 4-nitrophenol UDP-glucuronosyltransferase with a calculated molecular weight of 60,114. The cDNA sequence also indicates the presence of 25 amino acids preceding the sequence determined by microsequence of the isolated protein. This extrapeptide, for the most part, consists of hydrophobic amino acids which are characteristic of the signal peptides as found for secretory proteins and most transmembrane proteins. Furthermore, the deduced amino acid sequence contains a putative halt transfer signal of a hydrophobic segment (residues 487-510), which is flanked on both sides by the peptide segments of highly charged amino acid residues (residues 463-486 and 511-529). These features are consistent with the properties of transmembrane proteins. Specific cDNA probes were used to analyze the induction of the enzyme in rat tissues by treatment with 3-methylcholanthrene. RNA blot analysis showed that 3-methylcholanthrene increased 10- to 15-fold the amount of hybridizable mRNA in liver. The livers and kidneys from 3-methylcholanthrene-treated rats were found to contain almost the same amount of hybridizable mRNA, although the basal level in the kidney was much higher than that of the liver, and the amounts in the lung were much lower than that of the liver and kidney.

Characterization of ethanol-inducible human liver N-nitrosodimethylamine demethylase

Through the use of monospecific antibodies directed against hepatic cytochrome P-450j, an enzyme induced in rats treated with ethanol or isoniazid, we have purified from human liver the related cytochrome P-450 termed HLj. HLj resembles rat P-450j and P-450 LM3a, the homologous cytochrome in rabbit liver, in its NH2-terminal amino acid sequence, in being in highest concentration in liver microsome samples prepared from two patients intoxicated by ethanol and one patient given isoniazid, and in catalyzing the metabolic activation of the procarcinogen N-nitrosodimethylamine. Furthermore, each of nine human liver RNA samples contained a species of mRNA hybridizable to a cloned HLj cDNA. We conclude that HLj is related by structure, function, and some regulatory characteristics to rat P-450j and rabbit P-450 LM3a, cytochromes critical for metabolism of several clinically relevant cytotoxic and carcinogenic agents.

Chemical modification and inactivation of rat liver microsomal cytochrome P-450c by 2-bromo-4'-nitroacetophenone

The alkylating agent 2-bromo-4'-nitroacetophenone (BrNAP) binds covalently to each of 10 isozymes of purified rat liver microsomal cytochrome P-450 (P-450a-P-450j) but substantially inhibits the catalytic activity of only cytochrome P-450c. Regardless of pH, incubation time, presence of detergents, or concentration of BrNAP, treatment of cytochrome P-450c with BrNAP resulted in no more than 90% inhibition of catalytic activity. Alkylation with BrNAP did not cause the release of heme from the holoenzyme or alter the spectral properties of cytochrome P-450c, data that exclude the putative heme-binding cysteine, Cys-460, as the major site of alkylation. Two residues in cytochrome P-450c reacted rapidly with BrNAP, for which reason maximal loss of catalytic activity was invariably associated with the incorporation of approximately 1.5 mol of BrNAP/mol of cytochrome P-450c. Two major radio-labeled peptides were isolated from a tryptic digest of [14C]BrNAP-treated cytochrome P-450c by reverse-phase high performance liquid chromatography. The amino acid sequence of each peptide was determined by microsequence analysis, but the identification of the residues alkylated by BrNAP was complicated by the tendency of the adducts to decompose when subjected to automated Edman degradation. However, results of competitive binding experiments with the sulfhydryl reagent 4,4'-dithiodipyridine identified Cys-292 as the major site of alkylation and Cys-160 as the minor site of alkylation by BrNAP in cytochrome P-450c.

Identification of a human liver cytochrome P-450 homologous to the major isosafrole-inducible cytochrome P-450 in the rat

The rat 3-methylcholanthrene-inducible family of liver cytochromes P-450 contains two proteins (P-450c and P-450d) that are immunochemically related, possess 68% total sequence homology, and are induced by a number of toxic or carcinogenic compounds. To determine whether equivalent isozymes of hepatic cytochrome P-450 are expressed in humans, as they are in several mammalian species, we performed immunoblot analyses on microsomes prepared from 14 human liver specimens and found that each one contained a 52.5-kDa protein (termed HLd) that reacted with antibodies specific for rat P-450d. In addition, one specimen contained a 54-kDa protein (termed HLc) that reacted with antibodies specific for rat P-450c. HLd was purified through the use of immunoaffinity chromatography and was found to be 56% homologous to rat P-450d and 61% homologous to the equivalent isozyme in the rabbit (P-450 LM4) through their first 18 NH2-terminal amino acids. Finally, levels of immunoreactive HLd varied more than 10-fold among these patients but were unrelated to the patients' drug treatments, smoking habits, or amount of immunoreactive HLp, a human liver cytochrome P-450 related to the glucocorticoid-inducible family of rat cytochromes P-450. We conclude that, in man, there is a cytochrome P-450 family composed of two isozymes (HLc and HLd) that are immunochemically and structurally related to the 3-methylcholanthrene-inducible family observed in several other species.

The synthesis of aldosterone by the adrenal cortex. Two zones (fasciculata and glomerulosa) possess one enzyme for 11 beta-, 18-hydroxylation, and aldehyde synthesis

In order to establish the nature of the aldosterone synthetase activity in the adrenal cortex, we have used porcine adrenal, bovine adrenal cortex, highly purified bovine and porcine 11 beta-/18-hydroxylase, and antibodies raised against the latter enzyme. Mitochondria from two zones (glomerulosa and fasciculata) of the bovine cortex synthesize aldosterone, but those from glomerulosa are much more active than those from fasciculata. Partially purified (cholate-extracted plus ammonium sulfate-precipitated) extracts of mitochondria from the two zones are equally active in catalyzing all three steps in the conversion of 11-deoxycorticosterone to aldosterone. 18-Hydroxylase and aldehyde synthetase activities (18-hydroxycorticosterone----aldosterone) were completely precipitated from cholate extracts of mitochondria from bovine adrenal by antibodies to the pure porcine enzyme. No activity corresponding to any of the three steps in the conversion of 11-deoxycorticosterone to aldosterone was found in extramitochondrial fractions of the bovine cortex. Synthesis of aldosterone by the pure porcine enzyme was inhibited by antibodies to this enzyme and by metyrapone (an inhibitor of 11 beta-/18-hydroxylase). When fractions of porcine adrenal, resulting from purification of the enzyme from mitochondria, were exhaustively tested for any of the enzyme activities required for the synthesis of aldosterone, activity was found only in those fractions containing the 11 beta-/18-hydroxylase, i.e. no additional enzyme was discarded during the purification procedure. It is concluded that the only adrenocortical enzyme capable of synthesizing aldosterone in bovine and porcine adrenal is the well known 11 beta-hydroxylase, that the conversion of 18-hydroxycorticosterone to aldosterone is catalyzed by this cytochrome P-450, and that this step (aldehyde synthetase) requires the heme of the P-450 as demonstrated by the photochemical action spectrum.

The primary structure of cytochrome P-450d purified from rat liver microsomes: prediction of helical regions and domain analysis

The complete amino acid sequence of rat liver microsomal cytochrome P-450d (induced by isosafrole) was deduced by microsequence analysis of the tryptic peptides after separation by reverse-phase HPLC and alignment by comparison to the cDNA sequence reported by K. Kawajiri, O. Gotoh, K. Sogawa, Y. Tagashira, M. Muramatsu, and Y. Fujii-Kuriyama (1984, Proc. Natl. Acad. Sci. USA 81, 1649-1653). Results from the two approaches are in complete agreement with the exception of two residues, Lys-30 (Arg in cDNA) and Phe-261 (Ser in cDNA). As previously reported by us (L. H. Botelho, D. E. Ryan, P.-M. Yuan, R. Kutney, J. E. Shively, and W. Levin (1982, Biochemistry 21, 1152-1155) the NH2-terminal sequence of the mature protein lacks the NH2-terminal Met residue. Comparison of the rat cytochrome P-450d sequence with the mouse cytochrome P3-450 cDNA sequence reported by S. Kimura, F.J. Gonzalez, and D.W. Nebert (1984, Nucl. Acids Res. 12, 2917-2928) reveals a high sequence homology with a total of 32 amino acid differences including six conferring charge changes. Prediction of the secondary structure of cytochrome P-450d yields a maximum of 17 helices, two of which may be poly(Pro)-like helices adjacent to potential membrane-spanning alpha-helices. Four of the alpha-helices are sufficiently hydrophobic to traverse the endoplasmic reticulum. The remaining helices are largely amphiphilic. Analysis of the helices in reference to predicted membrane topology suggests that cytochrome P-450d either has one large and one small globular domain separated by a transmembrane domain and anchored by NH2-terminal and COOH-terminal transmembrane domains, or has one large globular domain anchored at both ends by transmembrane domains.

Characterization of a major form of rat hepatic microsomal cytochrome P-450 induced by isoniazid

Cytochrome P-450j has been purified to electrophoretic homogeneity from isoniazid-treated adult male rats; and this enzyme appears to be a major protein induced in hepatic microsomes after administration of isoniazid, as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The hemoprotein has a minimum molecular weight of approximately 51,500, and the ferrous-carbonyl complex of cytochrome P-450j has a Soret maximum at 451-452 nm. The oxidized heme iron appears to be predominately in the high spin state as deduced from the Soret maximum at 395 nm. Ethylisocyanide binds to ferrous cytochrome P-450j to yield spectral maxima at approximately 458 and 430 nm with a resultant 458/430 ratio of 0.7 at pH 7.4. Cytochrome P-450j has no measurable catalytic activity for the metabolism of benzo[a]pyrene (3- and 9-hydroxylation), hexobarbital, testosterone, and 5 alpha-androstane-3 alpha,17 beta-diol-3,17-disulfate. Low, but detectable, catalytic activity is obtained for the metabolism of 7-ethoxycoumarin, benzphetamine, p-nitroanisole, zoxazolamine, and 2-hydroxylation of 17 beta-estradiol. In contrast, cytochrome P-450j effectively catalyzes p-hydroxylation of aniline with a turnover of 12.7 nmol/min/nmol cytochrome P-450j. Hydroxyl radical scavengers, Fe-EDTA, superoxide dismutase, and catalase have no effect on aniline p-hydroxylation catalyzed by cytochrome P-450j. Cytochrome P-450j is distinct from nine other rat hepatic microsomal cytochromes P-450 (P-450a-P-450i) previously purified in this laboratory, as well as different isozymes described by other investigators, based on several parameters including minimum molecular weight, spectral properties, and catalytic activity. In Ouchterlony double diffusion plates, antibodies against cytochromes P-450a-P-450f show no cross-reaction with cytochrome P-450j. Structural differences among cytochromes P-450a-P-450j are apparent from the NH2-terminal sequence of cytochrome P-450j, as well as the electrophoretic profiles of proteolytic digests of the hemoproteins.

Amino acid sequence of COOH-terminal 20K Da fragment from pig liver microsomal NADPH-cytochrome P-450 reductase

We have determined the complete amino acid sequence of a 20K Da COOH-terminal fragment of porcine NADPH-cytochrome P-450 reductase. The 20K Da fragment is probably produced by a proteolytic cleavage of the intact protein in porcine liver microsomes, and since the cleavage does not affect enzymatic activity, the fragment has been studied as a distinct domain. The sequence comprises 175 amino acids including three cysteine residues, one of which has been previously identified as protected by NADPH from S-carboxymethylation. The NADPH-protected cysteine lies in a stretch of 12 residues with partial homology to glutathione reductase, and is adjacent to a hydrophobic region containing a glycine-rich stretch homologous to other FAD-containing proteins. The predicted secondary structure over this entire region is beta-sheet/beta-turn/beta-sheet/alpha-helix/beta-sheet/beta-turn/alpha-h elix corresponding to hydrophobic residues 21-28/glycine-rich residues 29-33/residues 34-38/residues 39-54/residues 56-61/NADPH-protected cysteine residues 62-78/residues 71-82. It is possible that the 20K Da domain provided a significant portion of the sequence responsible for binding FAD and NADPH in the intact enzyme. This data provides a basis for further active site studies.