Characterization of homogeneous atrial granule serine proteinase, a candidate processing enzyme of pro-atrial natriuretic factor

Natriuretic Peptides and the Dialysis Patient

The natriuretic peptide family consists of four structurally similar, but genetically distinct molecules with pronounced cardiovascular and renal actions. They are counterregulatory hormones playing an important role in fluid volume homeostasis. Atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) cause diuresis, natriuresis, and vasodilatation. C-type natriuretic peptide (CNP) has antimitogenic effects and causes vascular smooth muscle relaxation. Dendroaspis natriuretic peptide (DNP) shares many of the actions of ANP and BNP, but its function in humans is not yet fully understood. Natriuretic peptides have been extensively investigated as biochemical markers of the fluid state. Levels are elevated in disease conditions characterized by fluid overload and are closely related to survival in various cardiac disease states. In the dialysis population, BNP correlates significantly with cardiac function, whereas ANP is sensitive to volume changes during dialysis. However, changes in concentration do not predict achievement of euvolemia, and short half-life, combined with complicated assay techniques, make ANP a less than satisfactory tool for assessing hydration. BNP is a superior prognosticator for risk stratification in dialysis patients, and serial estimations will help in the identification of occult cardiac disease.

Atrial natriuretic peptide relation to plasma and heart zinc concentrations of Wistar rats exposed to cold and hot ambients

Plasma atrial natriuretic peptide (ANP) and zinc levels, as well as heart tissue zinc concentrations were determined, in male Wistar rats after the exposure of 114 rats at low temperature (4 degrees C) and 95 rats at high temperature (35-36 degrees C) for 28 d. Plasma ANP was estimated by radioimmunoassay and Zn2+ concentrations by atomic absorption spectrometry. Values were compared to a control group exposed at 20-22 degrees C (76 rats). Plasma ANP and Zn2+ levels, as well as heart tissue Zn2+ concentrations of control rats did not show statistically significant variations during the study, whereas rats exposed to cold and hot ambients showed significant variations of the parameters. A significant increase of plasma ANP and plasma zinc and heart tissue Zn2+ concentrations developed during cold exposure, whereas a gradual decrease of plasma ANP and Zn2+ levels was revealed during hot adaptation. Results also indicate that plasma ANP and zinc levels are proportionally related, whereas there is an inverse relationship between plasma ANP levels and heart Zn2+ concentrations, in both cold and hot exposed rats. In conclusion, our results show that ANP in relation to zinc probably play an important role in cold and hot acclimatization of rats.

The role of the circulation in processing pro-brain natriuretic peptide (proBNP) to amino-terminal BNP and BNP-32

Human proBNP (purified from cardiac tissue) was incubated at 37 degrees C in whole blood, serum and plasma and the products analyzed by size exclusion high pressure liquid chromatography and radioimmunoassay (RIA). In addition to RIAs for BNP-32 and NT-proBNP(1-13), a newly developed RIA for proBNP(62-76) was also used to identify the peptides. Incubation with serum resulted in the formation of a 9 kDa and a 3 kDa peptide, consistent with the N-terminal and the C-terminal peptides of the propeptide. Minimal processing of proBNP was seen in blood or plasma, suggesting that the circulation does not play a major role in the activation of proBNP. Analysis of degradation products of human proBNP using site directed specific antisera indicates that removal of N-terminal amino acids from proBNP occurs in serum. These findings support the view that the "high molecular weight BNP-32" previously identified in human plasma comprises amino-terminal deleted forms, and is unlikely to be intact proBNP(1-108).

N-terminal sequence analysis of atrial granule serine proteinase purified by affinity chromatography

Atrial granule serine proteinase is considered the leading candidate endoproteolytic processing enzyme of pro-atrial natriuretic factor. Its cleavage specificity is directed toward a monobasic amino acid processing site, and as such, the atrial enzyme is distinguished from the family of prohormone convertases which act at dibasic amino acid processing sites. To delineate the molecular mechanisms which distinguish monobasic from dibasic amino acid-directed processing enzymes, pure atrial enzyme is needed for sequence determination leading to molecular cloning, and for preparation of antisera. An affinity chromatography purification scheme seemed a logical modification of our established procedures to yield suitable amounts of enzyme for further studies [Damodaran and Harris (1995, J. Protein Chem., this issue] formed ineffective affinity ligands, even though these compounds contain essential residues on either side of what would be the scissile bond in a peptide substrate. On the other hand, tripeptide aldehydes (based on the substrate recognition sequence of the atrial enzyme) linked to Sepharose formed effective affinity matrices, permitting purification of the enzyme in a single step from a subcellular fraction enriched for atrial granules and lysosomes. Hence, the enzyme was purified 2000-fold in 90% overall yield, and subjected to N-terminal sequence analysis through 26 residues. The sequence determined, XXPEAAGLPG[R,L]GNPVP[F,G]R[Q,I]XY[G,E]XR(N,A]V, indicates that the atrial enzyme is unique, showing little sequence homology to other proteins in the database.

Preparations of psi-peptide bond and peptide-aldehyde inhibitors of atrial granule serine proteinase, a candidate processing enzyme of pro-atrial natriuretic factor

Pseudo-peptide bond inhibitors (psi-bond inhibitors) and peptide-aldehyde inhibitors of atrial granule serine proteinase, the candidate processing enzyme of pro-atrial natrieuretic factor, are prepared in high yield and purity by novel synthetic routes. The psi-bond compounds retain essential residues for enzyme binding, but place the enzyme inhibition site in the midst of the peptide sequence. Thus, Bz-APR-psi-LR and Bz-APR-psi-SLRR can be considered "readthrough inhibitors" of atrial granule serine proteinase. The most potent psi-peptide, Bz-APR-psi-SLRR (IC50=250 microM), is about fivefold less potent than the best peptide-aldehyde inhibitor (EACA-APR-CHO), and both the psi-bond and peptide-aldehyde compounds are competitive, reversible inhibitors of the enzyme. The psi-bond peptides containing two C-terminal Arg residues are three- to tenfold more potent than the analogous compounds containing only one C-terminal Arg residue, confirming the importance of both Arg residues in the enzyme processing recognition site. As expected, because of their moderate potencies, the psi-peptides are not useful affinity ligands for purification of atrial granule serine proteinase, but both peptide aldehydes are effective affinity ligands.

Pro-rat atrial natriuretic peptide-mimicking peptides as substrates for rat kallikreins rK2 (tonin) and rK9

Investigation of the substrate specificity of rat tissue kallikreins has shown the importance of an extended site of interaction, and that the proform of rat natriuretic peptides, pro-ANP, could be a substrate for two members of the family, rK2 (tonin) and rK9 (Moreau et al. (1992) J. Biol. Chem. 267, 10045-10051). Synthetic peptide substrates that reproduce the sequence of rat pro-ANP in the region of the activation sites were used to further assess the specificity of these two proteinases. Peptides 95-107 (AGPRSLRRSSCFG) and 91-107 (RALLAGPRSLRRSSCFG) of the rat pro-ANP sequence, which include all the cleavage sites for generating natriuretic peptides (R98, R101, R102), were synthesized and assayed as kallikrein substrates. Despite their homology, the two peptides had different susceptibilities to cleavage by rK2 and rK9. Peptide 91-107 was rapidly and specifically cleaved by both kallikreins, with a single cleavage site at the R98-S99 bond, which is the primary cleavage site in pro-ANP for generating ANP[1-28]. The kcat/Km values were 289,000 M-1 s-1 for rK2 and 39,000 M-1 s-1 for rK9. The N-terminally truncated peptide (95-107) was also cleaved at that bond by both proteinases, but far less rapidly than peptide 91-107, and additional cleavages appeared at secondary sites i.e those generating atriopeptin III (R101) and auriculin (R102) in rat pro-ANP. A commercial fluorogenic tetrapeptide substrate reproducing the sequence of rat pro-ANP was slowly hydrolysed under the same conditions. The kinin-releasing kallikrein rK1 did not cleave synthetic peptides at the R98-S99 bond, further demonstrating the different specificities of tissue kallikreins. The results indicate that residues in positions P5 to P8 with respect to the cleavage site in the substrate, are essential for the substrate binding and specificity of kallikreins rK2 and rK9. They also show that long peptide substrates should be used to identify biological substrates of kallikreins from the investigation of their kinetic properties. The biological significance of pro-ANP processing by these proteinases, remains, however, to be proven.

Brefeldin A defines distinct pathways for atrial natriuretic factor secretion in neonatal rat atrial and ventricular myocytes

The intracellular pathways for basal atrial natriuretic factor (ANF) secretion from the heart and their correlation with ANF processing to the active form were characterized in cultured neonatal rat atrial and ventricular myocytes. Brefeldin A, a fungal antimetabolite that blocks transport of newly synthesized proteins from the endoplasmic reticulum, was used to inhibit nascent protein trafficking. Thus, release of newly synthesized hormone was blocked, but release of stored hormone was unaffected. Whereas brefeldin A inhibited basal ventricular ANF release to 10% of the control value, basal ANF release from atrial cells was enhanced. Furthermore, basal atrial ANF secretion was inhibited by agents preventing myocyte depolarization, Ca2+ influx, release of Ca2+ from intracellular stores, or activation of protein kinase C, whereas ventricular ANF secretion was unaffected by these agents. Brefeldin A did not alter maturational processing of pro-ANF to ANF-(99-126) in either atrial or ventricular cultures. These findings indicate that (1) basal secretion of ANF from ventricular cells relies largely on newly synthesized hormone and is probably constitutive, (2) basal secretion of ANF from atrial cells is independent of transport of newly synthesized protein and occurs via a regulated pathway controlled at least in part by signaling changes associated with myocyte beating, and (3) processing of pro-ANF occurs either with constitutive or regulated secretion of hormone, which may indicate multiple cellular locations for the processing enzyme.

Conformational analysis and proteolytic processing of synthetic pre-pro-GnRH/GAP protein

Homogeneous pre-pro-GnRH/GAP protein was recently synthesized in 100 mg quantities by solid-phase methods and surprisingly, the synthetic pre-pro-protein, which normally does not escape the endoplasmic reticulum, was found to inhibit the release of prolactin from cultured pituitary cells. This is the first demonstration of significant biological activity associated with a precursor protein and provides the rationale for its further study. We now report the results of our initial examination of the conformational properties of pre-pro-GnRH/GAP protein as a prelude to solving its solution phase conformation by homonuclear 1H-NMR protocols. Thermal and pH titration fluorescence and circular dichroism spectroscopies reveal that the protein is resistant to thermal-induced conformational changes but is particularly sensitive to pH-induced conformational changes; while Asp/Glu and Arg residues may contribute to structural stability, His and Lys residues predominate. Pre-pro-GnRH/GAP is about 30% helix in the range of 2-40 degrees C; however, even at 90 degrees C, the peptide retains nearly 50% of its helix character. There is no evidence for a cooperative transition; for this reason, differential scanning calorimetry failed to yield a defined transition thermogram. Pre-pro-GnRH/GAP apparently does not pass through a transition state as a function of temperature but appears to flex and retain a high percentage of helix structure, resulting in subtle changes in secondary structure. There is no discernible isodichroic point. On either side of the neutral pH range, however, there are dramatic changes in structure that result in nonreversible denaturation of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Potential role of two novel elastase-like enzymes in processing pro-transforming growth factor-alpha

Transforming growth factor-alpha (TGF-alpha) is a mitogenic peptide produced by tumor cells and by virally and chemically transformed cells in culture. TGF-alpha is almost certainly derived from its precursor protein (pro-TGF-alpha) by limited proteolysis, but the physiologically relevant processing enzyme(s) is(are) unknown. We now report that oncogenically transformed rat liver epithelial cells (known to secrete TGF-alpha) and Schwann cells in culture transfected with SV40 T-antigen (which are now reported to express mRNA encoding pro-TGF-alpha) contain membrane associated, neutral pH, serine proteinases which are elastase-like in their substrate specificity, but elastase is not known to be associated with these cell types. In both cell types, the enzyme is associated with a subcellular fraction enriched for microsomes and plasma membranes. Furthermore, the enzyme appears to be specifically induced 4-fold in the transformed epithelial cells as compared with the level of enzyme present in the nontransformed parental cells. The enzymes have been purified approximately 20,000-fold to near homogeneity (50-60 units/mg) and are virtually identical with regard to their molecular weights (38,000) and other physiochemical properties. Results obtained with numerous synthetic peptide substrates show the enzymes prefer nonpolar residues such as Ala and Val in the P1 and P2 positions, but promiscuity of cleavage specificity observed with long-chain peptide substrates is attributed to the absence of structure in these peptides. Thus, although these enzymes may be involved in processing pro-TGF-alpha at the plasma membrane of the cell, it is just as likely that these enzymes play other physiological roles in the parental and/or transformed cells and that there is no specific endoproteolytic processing enzyme of pro-TGF-alpha.

GAP-releasing enzyme is a member of the pro-hormone convertase family of precursor protein processing enzymes

The recent discovery of mammalian endoproteinases which show extensive sequence homology with the yeast Kex 2 gene product (kexin) has lead to the hypothesis that processing enzymes of pro-hormone precursor proteins belong to a family of calcium dependent, subtilisin-like serine proteinases. We previously showed that hypothalamic GAP-releasing enzyme shares these characteristics and possesses the requisite specificity to be considered as a processing enzyme of progonadotropin releasing hormone (pro-GnRH) precursor protein. Thus, GAP-releasing enzyme (and other non-related proteins) were tested for their immunological reactivity with antisera raised against pituitary pro-hormone convertase 1/3 (PC1/3) and insulinoma PC2. On the basis of indirect enzyme-linked immunosorbent (ELISA) and Western blot assays, GAP-releasing enzyme is now shown to be immunologically related to PC1/3. We can conclude that GAP-releasing enzyme is also likely to be a member of the pro-hormone convertase family and should be considered the physiologically relevant processing enzyme of pro-GnRH. It is possible that GAP-releasing enzyme represents bovine hypothalamic PC1/3.