C-terminally extended ocytocin and pro-ocytocin: neurophysin peptide converting enzyme in bovine corpus luteum

Conformational studies on synthetic peptides reproducing the dibasic processing site of pro-ocytocin-neurophysin

Synthetic peptides reproducing the proteolytic processing site of pro-ocytocin were studied by different spectroscopic techniques, including circular dichroism, Fourier transform infrared absorption, and mono and bidimensional nuclear magnetic resonance, in order to ascertain the possible role of three-dimensional structure in the recognition process by maturation enzymes. Experimental results were compared with energy minimization calculations and suggest that: (i) the region situated on the N-terminus of the Lys-Arg doublet may form a beta-turn; (ii) the sequential organization of the residues participating in the beta-turn determines the privileged relative orientation of the basic amino acid sidechains and the subtype of turn; and (iii) the peptide segment situated on the C-terminal side of the dibasic doublet may assume a helix arrangement. These findings, in spite of the limitations connected to the flexibility of linear peptides, seem to substantiate the hypothesis that structural motifs around the cleavage site could be important for recognition and processing. however, a straightforward correlation between details of the secondary structure and the in vitro reactivity toward a putative convertase is not yet possible.

COOH-terminally-extended processing forms of oxytocin in human ovary

Human granulosa cells synthesize and secrete the oxytocin hormone. We have already shown that oxytocin-Gly, the last post-translational maturation intermediate of pro-hormone, is largely secreted by cultured granulosa cells deprived of ascorbate (Plevrakis et al. (1990) J. Endocrinol. 124, R5-R8). Using a combination of high performance liquid chromatography and radioimmunoassay, the oxytocin-like material present in human granulosa cell extracts, in follicular fluid, in cultured granulosa cell supernatants and in corpora lutea extracts was identified. We have demonstrated the presence of oxytocin-Gly, oxytocin-Gly-Lys and oxytocin-Gly-Lys-Arg, the same post-translational maturation intermediates as those we identified in bovine corpus luteum secretory granules. Thus we conclude that post-translational maturation of pro-oxytocin/neurophysin in human ovary proceeds by the same proteolytic events as those we described in bovine post-pituitary gland and corpus luteum.

Prostaglandin-induced secretion of oxytocin and prolactin in red (Cervus elaphus) and Pere David's (Elaphurus davidianus) deer hinds: evidence for oxytocin of luteal origin

Peripheral plasma concentrations of oxytocin in female red deer during the luteal phase of the oestrous cycle (9.3 +/- 2.1 fmol/ml) exceeded those in the follicular phase (3.1 +/- 1.4) or during seasonal anoestrus (3.2 +/- 1.3). In both red and Père David's deer hinds during the mid-luteal phase of the cycle, systemic administration of a luteolytic dose of the prostaglandin F2 alpha analogue, cloprostenol, caused the concentration of oxytocin in the peripheral circulation to rise. Mean (+/- SEM) concentrations increased from 8.1 +/- 0.7 to 97 +/- 8 fmol/ml in red and from 6.2 +/- 0.7 to 153 +/- 30 fmol/ml in Père David's hinds within 5 min of treatment. During seasonal anoestrus oxytocin secretion in response to cloprostenol was reduced to less than 10% of that during the breeding season, in both species. Cloprostenol treatment raised circulating concentrations of prolactin in both species during the breeding season, and during anoestrus in red deer only. The concentration of oxytocin in a single corpus luteum removed at laparotomy from one red deer hind at the mid-luteal phase of the cycle was 66 nmol/g wet wt; identification was authenticated by HPLC. These results suggest that the corpus luteum secretes oxytocin in the Cervidae, as established previously in the Bovidae, and that luteal oxytocin secretion is stimulated by prostaglandin.

Synthesis and processing of pro-ocytocin in bovine corpus luteum and granulosa cells

Bovine corpus luteum is the site of intense production of pro-ocytocin-neurophysin mRNA at day 1 after estrus (Ivell et al. (1985) FEBS Lett. 190, 263-267) which is followed by apparent delayed production of ocytocin. Therefore it is a good model to study both the translational and post-translational production of this neuropeptide in non-hypothalamic tissues and its regulation. In order to assess if this mRNA is translated during the lag period we have analyzed the neurophysin-like species produced in this organ. As early as day 2 after estrus one neurophysin species (pI approximately 4.7) could be detected and was unequivocally identified as pro-ocytocin-neurophysin. In primary cultures of luteinizing granulosa cells, biosynthetic intermediates were characterized, i.e. ocytocin-Gly, ocytocin-Gly-Lys and ocytocin-Gly-Lys-Arg, whereas amidated, fully mature, ocytocin was undetectable. We conclude that translation of pro-ocytocin-neurophysin mRNA takes place soon after transcription and we propose that incomplete processing could be responsible for the low level of ocytocin in the early bovine corpus luteum.

Endopeptidases and prohormone processing

Peptide hormones and peptide transmitters are generated from polypeptide precursors by specific cleavage reactions which take place principally at sites formed by single or paired basic residues. Not all the possible cleavage sites are utilised, however, and the degree of processing of many propeptides has been found to vary according to the tissue of origin. The restricted nature of processing reactions could point to the existence of a series of enzymes with stringent specificities, recognising regions of structure in addition to the single or paired basic residues. Alternatively the action of processing enzymes may be directed by conformation of the pro-peptide which could focus the action of a protease onto or away from a particular site. In addition certain post-translational modifications such as glycosylation or phosphorylation may influence the accessibility of a site to the approach of a processing enzyme. In this review we describe recent advances that have been made in the characteristization of proteolytic processing enzymes, we examine the relevance of the various factors that could account for restricted processing and discuss new approaches that may lead to better understanding of the mechanisms involved.

Functional diversity of bioactive peptides in the nervous system itself: "how the brain may understand"

The interactions involving cells of the nervous system are a complex form of intercellular communication. Biosynthesis of peptide hormones or active neuropeptides is generally through a precursor which provides increased product choices as a function of the processing pathway. Proteolytic processing as well as other molecular modifications lead to a wide range of mature products which may vary in different tissues even though they are derived from the same precursor. Also the same neuropeptide may exhibit different bioactivities for different target cells. Finally, by means of collective packaging in secretory organelles, a cell may be able by synergism to further broaden its biologic effects. In these ways, what is seen as added complication in the CNS, may be from the point of view of the cell, a successful attempt to increase its survival ability to adapt and influence its bioenvironment.

Proocytocin/neurophysin convertase from bovine neurohypophysis and corpus luteum secretory granules: complete purification, structure-function relationships, and competitive inhibitor

Structure-function relationship studies were conducted on the proocytocin/neurophysin endoprotease previously characterized in both bovine neurohypophyseal and corpus luteum granules, using as a reference substrate a synthetic peptide reproducing the entire (1-20) NH2-terminal domain of the precursor. The [D-Arg12] derivative of proocytocin/neurophysin (1-20) was found to be a good competitive inhibitor of the enzyme (Ki = 30 microM), while the [D-Lys11] derivative was not. This allowed the complete purification of two isoforms of the endoprotease (Mr 58,000 and 52,000, respectively) by affinity chromatography using covalently immobilized [D-Arg12] proocytocin/neurophysin (1-20) as the affinity adsorbent. The use of selectively modified or truncated forms of the reference substrate or of the [D-Arg12] competitive inhibitor of the endoprotease established clearly that this basic pair specific convertase is sensitive to modification of the substrate structure either at the basic residues of the cleavage locus or at amino acids around this site (i.e., Pro7 and Gly9). It is concluded that longer distance interactions between amino acids situated on both the NH2 and COOH sides of the basic doublet Lys11Arg12 may contribute to the stabilization of a preferred substrate conformation allowing recognition by the enzyme subsites.

Synthetic peptide substrates as models to study a pro-ocytocin/neurophysin converting enzyme

The selectivity and mechanism of processing at paired basic amino acids in hormone precursors was studied on several analogues of the (1-20)-aminoterminal domain of the ocytocin/neurophysin precursor in a cleavage assay by an endoprotease partially purified from bovine pituitary secretory granules. Peptide analogues with amino acid substitutions in, and around, the basic doublet were synthesized and used as substrates. The data obtained demonstrate the strict requirement of the processing enzyme for basic amino acids in tandem within a possibly preferred conformation which may be highly conserved in the aminoterminal domain of this hormone precursor.

Structural and immunological homology of human and porcine pituitary and plasma IRCM-serine protease 1 to plasma kallikrein: marked selectivity for pairs of basic residues suggests a widespread role in pro-hormone and pro-enzyme processing

IRCM-serine protease 1 (SP1), originally isolated from porcine pituitaries and exhibiting preference for cleavage at pairs of basic residues has now been isolated in sufficient quantities to be structurally characterized from both porcine and human pituitaries and plasmas. Whereas the porcine protease shows a high degree of amino acid sequence homology to human plasma pre-kallikrein, the human homologue exhibits an identity of sequence in the first 25 residues of each chain (regulatory and catalytic chains). In addition, human plasma and pituitary IRCM-SP1 and human plasma pre-kallikrein show virtually identical immunological and molecular properties. These data strongly suggest that IRCM-SP1 and plasma pre-kallikrein originate from the same gene product. Purified extracts from perfused rat pituitaries show that 32% of the IRCM-SP1 activity found in normal rat pituitaries, still remain. These data together with the demonstrated association of IRCM-SP1 with particulate fractions of the pituitary suggest that IRCM-SP1 represents a tissue form of plasma pre-kallikrein. The characterization of the digestion products obtained upon reaction of IRCM-SP1 with pro-insulin, ACTH1-39, pro-dynorphin and pro-enkephalin-derived peptides, somatostatin-28, and a pro-renin-like peptide confirmed the high degree of cleavage selectivity of this enzyme for pairs of basic residues.

Expression of the bovine hypothalamic hormone oxytocin precursor in Escherichia coli

The bovine oxytocin precursor was expressed in Escherichia coli as a fusion protein by cloning the hormone encoding cDNA in frame behind the replicase gene of the RNA phage MS2. By step-wise extraction with different urea concentrations, the fusion protein was enriched in the 7 M urea fraction and further purified by Sephacryl S-300 chromatography. The oxytocin precursor was cleaved off the fusion protein by cyanogen bromide treatment, chromatographed on FPLC columns and identified by Western blot analysis, using antibodies raised against neurophysin.

Pro-opiomelanocortin and pro-vasopressin converting enzyme in pituitary secretory vesicles

Peptide hormones are synthesized from larger precursors by cleavages at paired basic residues. We have isolated a pro-hormone converting enzyme from bovine neural and intermediate lobe secretory vesicles that cleaves pro-vasopressin and pro-opiomelanocortin at Lys-Arg residues to yield vasopressin, and adrenocorticotropin/endorphin-related peptides, respectively. The enzyme from both lobes is an aspartyl protease of approximately 70,000 Da, is a glycoprotein and has an optimum pH range of 4.0-5.0. Present within the same secretory vesicles is an aminopeptidase B-like enzyme which is a metalloprotease that is inhibited by Co2+ and Zn2+. This enzyme may play a role in trimming off the N-terminal extended basic residues from peptides liberated by the pro-hormone converting enzyme.

Somatostatin-28 and pro-ocytocin/neurophysin convertases: basic pair selective endoproteases involved in pro-hormone processing in the rat brain cortex and bovine corpus luteum

Two neuropeptide precursor processing enzyme systems were characterized in the rat brain cortex and bovine neurohypophysis and corpus luteum. The first one combines the action of a 90 kDa endoprotease which cleaves somatostatin-28 before the Arg-Lys doublet and that of an aminopeptidase B-like enzyme. The second system associates the action of a 58 kDa endoprotease cleaving pro-ocytocin/neurophysin (1-20) after the Lys-Arg dibasic moiety and a carboxypeptidase B-like activity. Both systems appear to be located in membrane-limited secretory vesicles of the producing organs, and to exhibit the properties of metallo-enzymes sensitive to divalent cation chelators. In contrast, they do not show the characteristics of serine-proteases and of trypsin-like enzymes. Studies with substrate analogs selectively modified at the basic doublet indicated that the integrity of both basic amino acids is essential but that conformational parameters, probably governed by the amino acid sequences flanking the basic doublet, play an important role. These data will be discussed in relation to a hypothesis on the predicted preferred secondary structure of these restriction loci.

Proteolytic enzymes in the post-translational processing of polypeptide hormone precursors

Selective and limited proteolysis is a key step in the post-translational modification of peptide hormone precursors. This process appears to involve a proteolytic machinery including highly specific endoproteases. Some of the enzyme systems possibly involved in the processing of pro-neuropeptides will be described and their mechanism of action discussed. Special emphasis will be on the following: i) the physico-chemical characteristics of proteolytic enzymes which are believed to be involved in the processing of some of these polypeptide hormone precursors; ii) the bio-specificity of these enzymes toward the substrates; iii) the importance of both secondary and tertiary structures of the cleavage domain in recognition by the selective proteases. These properties will be discussed in connection with the possible importance of the maturation enzymes in the in vivo regulation of hormone biosynthesis.