Isolation and functional expression of pituitary peptidylglycine alpha-amidating enzyme mRNA. A variant lacking the transmembrane domain

Biosynthesis and Function of VIP and Oxytocin: Mechanisms of C-terminal Amidation, Oxytocin Secretion and Transport

In this review, we provide the status of research on vasoactive intestinal peptide (VIP) and oxytocin, typical C-terminal α-amidated peptide hormones, including their precursor protein structures, processing and C-terminal α-amidation, and the recently identified mechanisms of regulation of oxytocin secretion and its transportation through the blood brain barrier. More than half of neural and endocrine peptides, such as VIP and oxytocin, have the α-amide structure at their C-terminus, which is essential for biological activities. We have studied the synthesis and function of C-terminal α-amidated peptides, including VIP and oxytocin, since the 1980s. Human VIP mRNA encoded not only VIP but also another related C-terminal α-amidated peptide, PHM-27 (peptide having amino-terminal histidine, carboxy-terminal methionine amide, and 27 amino acid residues). The human VIP/PHM-27 gene is composed of 7 exons and regulated synergistically by cyclic AMP and protein kinase C pathways. VIP has an essential role in glycemic control using transgenic mouse technology. The peptide C-terminal α-amidation proceeded through a 2-step mechanism catalyzed by 2 different enzymes encoded in a single mRNA. In the oxytocin secretion from the hypothalamus/the posterior pituitary, the CD38-cyclic ADP-ribose signal system, which was first established in the insulin secretion from pancreatic β cells of the islets of Langerhans, was found to be essential. A possible mechanism involving RAGE (receptor for advanced glycation end-products) of the oxytocin transportation from the blood stream into the brain through the blood-brain barrier has also been suggested.

Neuropeptide amidation: cloning of a bifunctional alpha-amidating enzyme from Aplysia

One of the most common mechanisms of posttranslational modifications to generate biologically active (neuro)peptides is the process of peptide alpha-amidation. The only enzyme known to catalyze this important modification is peptidylglycine alpha-amidating monooxygenase (PAM): a (bifunctional) zymogen, giving rise to a monooxygenase (PHM) and a lyase (PAL). The highly peptidergic central nervous system and endocrine system of the marine mollusk Aplysia has homologs of various mammalian peptide processing enzymes, including furin, Afurin2, prohormone convertase 1 (PC1), PC2, carboxypeptidase E (CPE) and CPD. Previously, it has been shown that the abdominal ganglion of Aplysia, which contains approximately 800 peptidergic bag cell neurons, contains the highest specific alpha-amidating activity. We have identified and cloned multiple overlapping central nervous system and bag cell cDNAs that encode a predicted 748-residue protein that is a member of the PAM family. The protein sequence contains the contiguous sequence of the catalytic domains of PHM and PAL, clearly demonstrating the existence of bifunctional Aplysia PAM, the first invertebrate PAM zymogen with an organization similar to that in vertebrates. None of the characterized clones encoded the so-called exon A domain between the PHM and PAL domains. Furthermore, in a specific search by reverse transcription-polymerase chain reaction of RNA from multiple tissues we could only detect exon A-less transcripts. PAM expression was detected in the central nervous system, and in several endocrine and exocrine organs. Aplysia PAM is a candidate prohormone processing enzyme that plays an important role in the processing of Aplysia prohormones in the secretory pathway.

Breeding stock-specific variation in peptidylglycine alpha-amidating monooxygenase messenger ribonucleic acid splicing in rat pituitary

Peptidylglycine alpha-amidating monooxygenase (PAM) is a bifunctional enzyme that catalyzes the carboxyl-terminal amidation of glycine-extended peptides in a two-step reaction involving a monooxygenase and a lyase. Several forms of PAM messenger RNA result from alternative splicing of the single copy PAM gene. The presence of alternately spliced exon A between the two enzymatic domains allows endoproteolytic cleavage to occur in selected tissues, generating soluble monooxygenase and membrane lyase from integral membrane PAM. While using an exon A antiserum, we made the unexpected observation that Charles River Sprague Dawley rats expressed forms of PAM containing exon A in their pituitaries, whereas Harlan Sprague Dawley rats did not. Forms of PAM containing exon A were expressed in the atrium and hypothalamus of both types of Sprague Dawley rat, although in different proportions. PAM transmembrane domain splicing also differed between rat breeders, and full-length PAM-1 was not prevalent in the anterior pituitary of either type of rat. Despite striking differences in PAM splicing, no differences in levels of monooxygenase or lyase activity were observed in tissue or serum samples. The splicing patterns of other alternatively spliced genes, pituitary adenylate cyclase-activating polypeptide receptor type 1 and cardiac troponin T, did not vary with rat breeder. Strain-specific variations in the splicing of transcripts such as PAM must be taken into account in analyzing the resultant proteins, and knowledge of these differences should identify variations with functional significance.

A molluscan peptide alpha-amidating enzyme precursor that generates five distinct enzymes

Mechanisms underlying the specificity and efficiency of enzymes, which modify peptide messengers, especially with the variable requirements of synthesis in the neuronal secretory pathway, are poorly understood. Here, we examine the process of peptide alpha-amidation in individually identifiable Lymnaea neurons that synthesize multiple proproteins, yielding complex mixtures of structurally diverse peptide substrates. The alpha-amidation of these peptide substrates is efficiently controlled by a multifunctional Lymnaea peptidyl glycine alpha-amidating monooxygenase (LPAM), which contains four different copies of the rate-limiting Lymnaea peptidyl glycine alpha-hydroxylating monooxygenase (LPHM) and a single Lymnaea peptidyl alpha-hydroxyglycine alpha-amidating lyase. Endogenously, this zymogen is converted to yield a mixture of monofunctional isoenzymes. In vitro, each LPHM displays a unique combination of substrate affinity and reaction velocity, depending on the penultimate residue of the substrate. This suggests that the different isoenzymes are generated in order to efficiently amidate the many peptide substrates that are present in molluscan neurons. The cellular expression of the LPAM gene is restricted to neurons that synthesize amidated peptides, which underscores the critical importance of regulation of peptide alpha-amidation.

N-acylglycine amidation: implications for the biosynthesis of fatty acid primary amides

Bifunctional peptidylglycine alpha-amidating enzyme (alpha-AE) catalyzes the O2-dependent conversion of C-terminal glycine-extended prohormones to the active, C-terminal alpha-amidated peptide and glyoxylate. We show that alpha-AE will also catalyze the oxidative cleavage of N-acylglycines, from N-formylglycine to N-arachidonoylglycine. N-Formylglycine is the smallest amide substrate yet reported for alpha-AE. The (V/K)app for N-acylglycine amidation varies approximately 1000-fold, with the (V/K)app increasing as the acyl chain length increases. This effect is largely an effect on the KM,app; the KM,app for N-formylglycine is 23 +/- 0.88 mM, while the KM,app for N-lauroylglycine and longer chain N-acylglycines is in the range of 60-90 microM. For the amidation of N-acetylglycine, N-(tert-butoxycarbonyl)glycine, N-hexanoylglycine, and N-oleoylglycine, the rate of O2 consumption is faster than the rate of glyoxylate production. These results indicate that there must be the initial formation of an oxidized intermediate from the N-acylglycine before glyoxylate is produced. The intermediate is shown to be N-acyl-alpha-hydroxyglycine by two-dimensional 1H-13C heteronuclear multiple quantum coherence (HMQC) NMR.

Reaction versus subsite stereospecificity of peptidylglycine alpha-monooxygenase and peptidylamidoglycolate lyase, the two enzymes involved in peptide amidation

Carboxyl-terminal amidation, a required post-translational modification for the bioactivation of many neuropeptides, entails sequential enzymatic action by peptidylglycine alpha-monooxygenase (PAM, EC 1.14.17.3) and peptidylamidoglycolate lyase (PGL, EC 4.3.2.5). The monooxygenase, PAM, first catalyzes conversion of a glycine-extended pro-peptide to the corresponding alpha-hydroxyglycine derivative, and the lyase, PGL, then catalyzes breakdown of this alpha-hydroxyglycine derivative to the amidated peptide plus glyoxylate. We have previously established that PAM and PGL exhibit tandem reaction stereospecificities, with PAM producing, and PGL being reactive toward, only alpha-hydroxyglycine derivatives of absolute configuration (S). We now demonstrate that PAM and PGL exhibit dramatically different subsite stereospecificities toward the residue at the penultimate position (the P2 residue) in both substrates and inhibitors. Incubation of Ac-L-Phe-Gly, Ac-L-Phe-L-Phe-Gly, or (S)-O-Ac-mandelyl-Gly with PAM results in complete conversion of these substrates to the corresponding alpha-hydroxylated products, whereas the corresponding X-D-Phe-Gly compounds undergo conversions of < 1%. The KI of Ac-D-Phe-Gly is at least 700-fold higher than that of Ac-L-Phe-Gly, and the same pattern holds for other substrate stereoisomers. This S2 subsite stereospecificity of PAM also holds for competitive inhibitors; thus, the KI of 45 microM for Ac-L-Phe-OCH2CO2H increases to 2,247 microM for the -D-Phe- enantiomer. In contrast, incubation of PGL with Ac-L-Phe-alpha-hydroxy-Gly, Ac-D-Phe-alpha-hydroxy-Gly, (S)-O-Ac-mandelyl-alpha-hydroxy-Gly, or (R)-O-Ac-mandelyl-alpha-hydroxy-Gly results in facile enzymatic conversion of each of these compounds to their corresponding amide products. The simultaneous expression of high reaction stereospecificity and low S2 subsite stereospecificity in the course of PGL catalysis was illustrated by a series of experiments in which enzymatic conversion of the diastereomers of Ac-L-Phe-alpha-hydroxy-Gly and Ac-D-Phe-alpha-hydroxy-Gly was monitored directly by HPLC. Kinetic parameters were determined for both substrates and potent competitive inhibitors of PGL, and the results confirm that, in sharp contrast to PAM, the configuration of the chiral moiety at the P2 position has virtually no effect on binding or catalysis. These results illustrate a case where catalytic domains, which must function sequentially (and with tandem reaction stereochemistry) in a given metabolic process, nevertheless exhibit sharply contrasting subsite stereospecificities toward the binding of substrates and inhibitors.

Tissue-specific molecular diversity of amidating enzymes (peptidylglycine alpha-hydroxylating monooxygenase and peptidylhydroxyglycine N-C lyase) in Xenopus laevis

We investigated the molecular diversity of the paired enzymes, peptidylglycine alpha-hydroxylating monooxygenase (PHM) and peptidylhydroxyglycine N-C lyase (PHL), involved in peptide C-terminal amidation. Three kinds of amidating enzyme (AE) cDNAs (AE-I, AE-II and AE-III) have previously been isolated from Xenopus laevis skin. While AE-I cDNA encodes only PHM, AE-III cDNA encodes a protein containing both PHM and PHL sequences and a transmembrane domain. On the other hand, the translated product of AE-II has not been detected yet. Endoproteolytic cleavage of the AE-III protein generates separated forms of PHM and PHL that are purified from X. laevis skin. Expression of AE-III in insect cells using a baculovirus expression vector system indicated that PHM and PHL exist as a membrane-associated, bifunctional enzyme without endoproteolysis in insect cells. Both PHM and PHL activities were detected in all the X. laevis tissues examined. Particularly, the highest levels of both activities were found in skin, brain and heart. We identified basically three types of enzymes in X. laevis; soluble PHM, soluble PHL and a membrane-associated, bifunctional enzyme that has both PHM and PHL domains. While the skin contained soluble types of PHM and PHL, the brain and heart predominantly contained the membrane-associated, bifunctional type. Analysis of mRNA levels by the reverse-transcript polymerase chain reaction method and Western blot analysis using PHM-specific antibody revealed that such molecular diversity of PHM and PHL among the tissues are produced by changing the ratio of AE-I mRNA/AE-III mRNA, and by endoproteolytic processing of the membrane-associated precursor protein.

Peptidylglycine alpha-amidating monooxygenase and other processing enzymes in the neurointermediate pituitary

Studies on the mRNAs encoding PAM and on the various PAM proteins have begun to reveal some of the intricate mechanisms used to optimize the ability of this enzyme to carry out the alpha-amidation of peptides. Comparison of the regulatory elements governing expression of the various enzymes involved in peptide processing should reveal common elements. Knowledge of the processing enzymes themselves should help us to understand how these enzymes function in the secretory granule environment. In addition to their catalytic domains, other processing enzymes, like PAM, may well have processing domains and routing domains designed to optimize their ability to function in secretory granules.

Peptidylglycine alpha-amidating monooxygenase: a multifunctional protein with catalytic, processing, and routing domains

Peptide alpha-amidation is a widespread, often essential posttranslational modification shared by many bioactive peptides and accomplished by the products of a single gene encoding a multifunctional protein, peptidylglycine alpha-amidating monooxygenase (PAM). PAM has two catalytic domains that work sequentially to produce the final alpha-amidated product peptide. Tissue-specific alternative splicing can generate forms of PAM retaining or lacking a domain required for the posttranslational separation of the two catalytic activities by endoproteases found in neuroendocrine tissue. Tissue-specific alternative splicing also governs the presence of a transmembrane domain and generation of integral membrane or soluble forms of PAM. The COOH-terminal domain of the integral membrane PAM proteins contains routing information essential for the retrieval of PAM from the surface of endocrine and nonendocrine cells. Tissue-specific endoproteolytic processing can generate soluble PAM proteins from integral membrane precursors. Soluble PAM proteins are rapidly secreted from stably transfected nonneuroendocrine cells but are stored in the regulated secretory granules characteristic of neurons and endocrine cells.

Functional expression and characterization of a Xenopus laevis peptidylglycine alpha-amidating monooxygenase, AE-II, in insect-cell culture

The alpha-amidating reaction of peptide hormones is a two-step process which is catalyzed by peptidylglycine alpha-hydroxylating monooxygenase (PHM) and peptidylhydroxyglycine N-C lyase (PHL). There are three types of mRNA for these amidating enzymes in Xenopus laevis, namely AE-I, AE-II and AE-III. AE-I encodes only PHM and AE-III encodes both PHM and PHL. AE-II seems to encode subtypes of both PHM and PHL. While AE-II mRNA is present in high amounts in frog skin, the actual enzymes originating from AE-II have not been detected. When we expressed AE-II in cultured insect-cells using the baculovirus expression vector system, the expressed enzyme was specifically localized to the membrane fraction due to its hydrophobic transmembrane domain. Alternatively, when the transmembrane-domain-deleted AE-II (Met1-Ile731) was expressed, the enzyme was secreted into the culture medium; this secreted enzyme was purified to homogeneity by a simple two-step procedure. We have verified that the reaction product of the purified enzyme was the amidated peptide, indicating that AE-II has the ability to catalyze the entire amidating reaction.

Efficient amidation of C-peptide deleted NPY precursors by non-endocrine cells is affected by the presence of Lys-Arg at the C-terminus

Post-translational processing of peptide precursors producing amidated, biologically active peptides generally occurs in specially differentiated endocrine or neural cells. However, we have previously shown that a C-peptide-deleted precursor of neuropeptide Y (NPY1-39) in which the precursor terminates in the sequence Gly-Lys-Arg was partially amidated by the non-endocrine cell line, CHO. In the present study we show that two other non-endocrine cell lines, NIH 3T3 and BHK, also possess amidating activities and that the NPY1-39 precursor was completely converted to NPY1-36 amide by the NIH 3T3 cell line. The role of the two basic residues (Lys-Arg) in the C-terminus was studied by transfection of a construct encoding a NPY precursor terminating with glycine alone. Both the CHO and NIH 3T3 cell lines, transfected with this construct, secreted a significantly smaller fraction of NPY reactive material as amidated NPY compared to the fraction of amidated NPY secreted by the cells transfected with the NPY1-39 precursor. It is concluded that the capacity to perform C-terminal amidation appears to be a universal feature of eukaryotic cells and that the carboxypeptidase E-like enzyme influences the amidation process, beyond its known ability to remove the C-terminal basic residues.

Characterization of a bifunctional peptidylglycine alpha-amidating enzyme expressed in Chinese hamster ovary cells

Peptidylglycine alpha-amidating enzyme (alpha-AE) catalyzes the conversion of glycine-extended prohormones to their biologically active alpha-amidated forms. We have derived a clonal Chinese hamster ovary cell line that secretes significant quantities of active alpha-AE. Enzyme production was increased by selection for methotrexate-resistant cells expressing a dicistronic message. Amplification of the alpha-AE gene was monitored by Southern blot analysis, enzyme activity, and immunoreactive protein throughout the selection process. The soluble enzyme is bifunctional as determined by the ability to convert either the glycine-extended substrate, dansyl-Tyr--Val--Gly, or the intermediate, dansyl-Tyr--Val--alpha-hydroxyglycine, to the dansyl-Tyr--Val--NH2 product. The recombinant alpha-AE was purified by a simple two-step chromatographic process. The purified enzyme is partially glycosylated and the glycosylated and nonglycosylated forms of the enzyme were separated on a Con A-Sepharose column. The kinetic constants for dansyl-Tyr--Val--Gly, dansyl-Tyr--Val--alpha-hydroxyglycine, ascorbate, and catechol were the same for both forms of alpha-AE. In addition, mimosine is competitive vs ascorbate with K(is) = 3.5 microM for the nonglycosylated alpha-AE and K(is) = 4.2 microM for the glycosylated alpha-AE. Therefore, the presence or absence of asparagine-linked oligosaccharide does not affect the catalytic efficiency of the enzyme. Overexpression of the recombinant enzyme in CHO cells greatly enhances expression of the endogenous gene, implicating a feedback mechanism on the alpha-AE gene.

Characterization of a Xenopus laevis skin peptidylglycine alpha-hydroxylating monooxygenase expressed in insect-cell culture

The C-terminal amide structure of peptide hormones and neurotransmitters is synthesized via a two-step reaction catalyzed by peptidylglycine alpha-hydroxylating monooxygenase (PHM) and peptidylhydroxyglycine N-C lyase. A Xenopus laevis PHM expressed in insect-cell culture by the baculovirus-expression-vector system was purified to homogeneity and characterized. Using a newly established assay system for PHM, the kinetic features of this enzyme were investigated. As expected, the enzyme required copper ions, L-ascorbate and molecular oxygen for turnover. Salts like KI and KCl, and catalase stabilized the enzyme in the presence of L-ascorbate. The optimum pH value for the enzyme reaction was around six when Mes buffer was used and around seven when phosphate buffer was used under the same assay condition. Below pH 6, acetate, iodide and chloride ions activated the reaction. The kinetic analysis is consistent with a ping-pong mechanism with respect to peptide and L-ascorbate, and the peptide showed substrate inhibition. The substrate specificity of the enzyme at the penultimate position was examined by competitive assay using tripeptides with glycine at the C-termini and the inhibitory potency of these peptides in descending order was methionine > aromatic > non-polar amino acids.

Purification and cDNA cloning of Xenopus laevis skin peptidylhydroxyglycine N-C lyase, catalyzing the second reaction of C-terminal alpha-amidation

The alpha-amidation of glycine-extended peptides is a two-step reaction catalyzed by peptidylglycine alpha-hydroxylating monooxygenase (PHM) and peptidylhydroxyglycine N-C lyase (PHL). PHL was purified to homogeneity from Xenopus laevis skin and its partial amino acid sequence (including the N-terminal 35 residues) was determined. It was found that the cDNA codes for a 935-residue precursor protein (AE-III protein), containing the PHM and PHL sequences at its N terminus and C terminus, respectively. The PHM sequence in AE-III protein is completely identical to that deduced from the nucleotide sequence of X. laevis AE-I cDNA, which encodes only PHM, except that the AE-I protein has an extra 10 residues at its C terminus. It is suggested that AE-I and AE-III mRNA are encoded by the same gene and produced by alternative splicing.

A deletion polymorphism in the human alpha-2-macroglobulin (A2M) gene

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Recombinant type A rat 75-kDa alpha-amidating enzyme catalyzes the conversion of glycine-extended peptides to peptide amides via an alpha-hydroxyglycine intermediate

The amidation of C-terminal glycine-extended peptides has been analyzed by the use of a truncated type A peptidylglycine alpha-amidating enzyme (alpha-AE) encoded by cDNA prepared with RNA from rat medullary thyroid carcinoma (MTC) cells. Mouse C127 cells transfected with the rat MTC cDNA encoding the truncated type A alpha-AE secrete the expected 75-kDa enzyme into the culture medium. Medium conditioned with the transfected C127 cells converts both dansyl-Tyr-Val-Gly and dansyl-Tyr-Val-alpha-hydroxyglycine to dansyl-Tyr-Val-NH2 at levels which are approximately 1000 times higher than the levels found in medium conditioned with untransfected C127 cells. This result indicates that rat type A alpha-AE alone catalyzes a two-step reaction involving an initial hydroxylation of peptidyl-Gly followed by conversion of the peptidyl-alpha-hydroxyglycine intermediate to the amidated product. The involvement of a separate, second enzyme to convert peptidyl-alpha-hydroxyglycine to peptidyl-NH2 is not necessary in this system. The initial hydroxylation step is rate-determining at infinite substrate concentration and requires a reducing equivalent, molecular oxygen, and copper.